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Types of Chemical Reactions
During a chemical reaction, the substances that react are known as reactants whereas the substances that are formed during a chemical reaction are known as products. Six common types of chemical reactions are discussed below.
Different Types of Chemical Reactions
The 5 primary types of chemical reactions are:
Combination reaction
Decomposition reaction
Displacement reaction
Double Displacement reaction
Precipitation Reaction
1. Combination Reaction
A reaction in which two or more reactants combine to form a single product is known as a combination reaction.
It takes the form of X + Y → XY
Combination reaction is also known as a synthesis reaction.
Example of combination reaction: 2Na + Cl2 → 2NaCl
2. Decomposition Reaction
A reaction in which a single compound breaks into two or more simpler compounds is known as a decomposition reaction.
It takes the form of XY → X + Y
A decomposition reaction is just the opposite of combination reaction.
Example of a decomposition reaction: CaCO3 → CaO + CO2
The reaction in which a compound decomposes due to heating is known as a thermal decomposition reaction.
3. Displacement Reaction
A chemical reaction in which a more reactive element displaces a less reactive element from its aqueous salt solution.
It takes the form X + YZ → XZ + Y
It is also called a substitution reaction
Example of displacement reaction: Zn + CuSO4 → ZnSO4 + Cu
4. Double Displacement Reaction
A chemical reaction in which ions gets exchanged between two reactants which form a new compound is called a double displacement reaction.
It takes the form of XY + ZA → XZ + YA
It is also called a metathesis reaction
Example of double displacement reaction: BaCl2 + Na2SO4 → BaSO4 + 2NaCl
5. Precipitation Reaction
A chemical reaction that involves the formation of an insoluble product (precipitate; solid) is called Precipitation reaction.
The reactants are soluble, but the product formed would be insoluble and separates out as a solid.
The chemical equation by which a chemical change is described is adequate for reaction in solutions, but for reactions of ionic compounds in aqueous solution (water), the typical molecular equation has different representations.
A molecular equation may indicate formulas of reactants and products that are not present and eliminate completely the formulas of the ions that are the real reactants and products.
If the substance in the molecular equation that is actually present as dissociated ions are written in the form of their ions, the result is an ionic equation.
A precipitation reaction occurs when a solution, originally containing dissolved species, produces a solid, which generally is denser and falls to the bottom of the reaction vessel.
The most common precipitation reactions occurring in aqueous solution involve the formation of an insoluble ionic compound when two solutions containing soluble compounds are mixed. Consider what happens when an aqueous solution of NaCl is added to an aqueous solution of AgNO3. The first solution contains hydrated Na+ and Cl− ions and the second solution, Ag+, and NO3− ions.
NaCl(s) → Na+(aq) + Cl−(aq)
AgNO3(s) → Ag+(aq) + NO3−(aq)
When mixed, a double displacement reaction takes place, forming the soluble compound NaNO3 and the insoluble compound AgCl. In the reaction vessel, the Ag+ and Cl− ions combine, and a white solid precipitated from the solution. As the solid precipitates, the Na+ and NO3− ions remain in solution.
The overall double displacement reaction is represented by the following balanced equation:
NaCl(aq) + AgNO3(aq) → AgCl(s) + NaNO3(aq)
Example Problem
When aqueous solutions of Pb(NO3)2 and KI are mixed, does a precipitate form?
Write a balanced equation for the precipitation reaction that occurs when aqueous solutions of copper(II) iodide and potassium hydroxide are combined.
Solution:
You are asked to predict whether a precipitate will form during a chemical reaction and to write a balanced equation for a precipitation reaction.
You are given the identity of two reactants.
1. Yes, a solid precipitate, PbI2, forms when these solutions are mixed:
Pb(NO3)2(aq) + KI(aq) → PbI2(s) + 2KNO3(aq)
2. The two products of the reaction are insoluble copper (II) hydroxide and soluble potassium iodide.
CuI2(aq) + 2 KOH(aq) → Cu(OH)2(s) + 2 KI(aq)
Classifying chemical reactions
Chemists classify reactions in a number of ways: (a) by the type of product, (b) by the types of reactants, (c) by reaction outcome, and (d) by reaction mechanism. Often, a given reaction can be placed in two or even three categories.
Classification by type of product
Gas-forming reactions
Many reactions produce a gas such as carbon dioxide, hydrogen sulfide (H2S), ammonia (NH3), or sulfur dioxide (SO2). An example of a gas-forming reaction is that which occurs when a metal carbonate such as calcium carbonate (CaCO3, the chief component of limestone, seashells, and marble) is mixed with hydrochloric acid (HCl) to produce carbon dioxide.
CaCO3(s) + 2 HCl(aq) → CaCl2(aq) + CO2(g) + H2O (l)
In this equation, the symbol (aq) signifies that a compound is in an aqueous, or water, solution.
Cake-batter rising is caused by a gas-forming reaction between an acid and baking soda, sodium hydrogen carbonate (sodium bicarbonate, NaHCO3). Tartaric acid (C4H6O6), an acid found in many foods, is often the acidic reactant.
C4H6O6(aq) + NaHCO3(aq) → NaC4H5O6(aq) + H2O (l) + CO2(g)
In this equation, NaC4H5O6 is sodium tartrate.
Most baking powders contain both tartaric acid and sodium hydrogen carbonate, which are kept apart by using starch as a filler. When baking powder is mixed into the moist batter, the acid and sodium hydrogen carbonate dissolve slightly, which allows them to come into contact and react. Carbon dioxide is produced, and the batter rises.
Precipitation reactions
Formation of an insoluble compound will sometimes occur when a solution containing a particular cation (a positively charged ion) is mixed with another solution containing a particular anion (a negatively charged ion). The solid that separates is called a precipitate.
Compounds having anions such as sulfide (S2−), hydroxide (OH−), carbonate (CO32−), and phosphate (PO43−) are often insoluble in water. A precipitate will form if a solution containing one of these anions is added to a solution containing a metal cation such as Fe2+, Cu2+, or Al3+.
Fe2+(aq) + 2 OH−(aq) → Fe(OH)2(s)
Al3+(aq) + PO43−(aq) → AlPO4(s)
Minerals are water-insoluble compounds. Precipitation reactions in nature can account for mineral formation in many cases, such as with undersea vents called “black smokers” that form metal sulfides.
Classification by types of reactants
Two types of reactions involve transfer of a charged species. Oxidation-reduction reactions occur with electron transfer between reagents. In contrast, reactions of acids with bases in water involve proton (H+) transfer from an acid to a base.
Oxidation-reduction reactions
Oxidation-reduction (redox) reactions involve the transfer of one or more electrons from a reducing agent to an oxidizing agent. This has the effect of reducing the real or apparent electric charge on an atom in the substance being reduced and of increasing the real or apparent electric charge on an atom in the substance being oxidized. Simple redox reactions include the reactions of an element with oxygen. For example, magnesium burns in oxygen to form magnesium oxide (MgO). The product is an ionic compound, made up of Mg2+ and O2− ions. The reaction occurs with each magnesium atom giving up two electrons and being oxidized and each oxygen atom accepting two electrons and being reduced.
Another common redox reaction is one step in the rusting of iron in damp air.
2Fe(s) + 2H2O(l) + O2(g) → 2Fe(OH)2(s)
Here iron metal is oxidized to iron dihydroxide (Fe(OH)2); elemental oxygen (O2) is the oxidizing agent.
Redox reactions are the source of the energy of batteries. The electric current generated by a battery arises because electrons are transferred from a reducing agent to an oxidizing agent through the external circuitry. In a common dry cell and in alkaline batteries, two electrons per zinc atom are transferred to the oxidizing agent, thereby converting zinc metal to the Zn2+ ion. In dry-cell batteries, which are often used in flashlights, the electrons given up by zinc are taken up by ammonium ions (NH4+) present in the battery as ammonium chloride (NH4Cl). In alkaline batteries, which are used in calculators and watches, the electrons are transferred to a metal oxide such as silver oxide (AgO), which is reduced to silver metal in the process.
Oxidation and reduction in terms of oxygen transfer
The terms oxidation and reduction can be defined in terms of the adding or removing oxygen to a compound. while this is not the most robust definition, as discussed below, it is the easiest to remember.
Oxidation is the gain of oxygen.
Reduction is the loss of oxygen.
For example, in the extraction of iron from its ore:
Because both reduction and oxidation are occurring simultaneously, this is known as a redox reaction.
An oxidizing agent is substance which oxidizes something else. In the above example, the iron(III) oxide is the oxidizing agent. A reducing agent reduces something else. In the equation, the carbon monoxide is the reducing agent.
Oxidizing agents give oxygen to another substance.
Reducing agents remove oxygen from another substance.
Oxidation and reduction in terms of hydrogen transfer
These are old definitions which are no longer used, except occasionally in organic chemistry.
Oxidation is the loss of hydrogen.
Reduction is the gain of hydrogen.
Notice that these are exactly the opposite of the oxygen definitions (#1).
For example, ethanol can be oxidized to ethanal:
An oxidizing agent is required to remove the hydrogen from the ethanol. A commonly used oxidizing agent is potassium dichromate(VI) solution acidified with dilute sulfuric acid. Ethanal can also be reduced back to ethanol by adding hydrogen. A possible reducing agent is sodium tetrahydridoborate, NaBH4. Again the equation is too complicated to consider at this point.
More precise definitionsof oxidizing and reducing agents are
Oxidizing agents add oxygen to another substance or remove hydrogen from it.
Reducing agents remove oxygen from another substance or add hydrogen to it.
Oxidation and reduction in terms of electron transfer
Oxidation is loss of electrons
Reduction is gain of electrons
Remembering these definitions is essential, and easily done using this convenient acronym:
The equation below shows an obvious example of oxygen transfer in a simple redox reaction:
CuO+Mg→Cu+MgO
Copper(II) oxide and magnesium oxide are both ionic compounds. If the above is written as an ionic equation, it becomes apparent that the oxide ions are spectator ions. Omitting them gives:
In the above reaction, magnesium reduces the copper(II) ion by transferring electrons to the ion and neutralizing its charge. Therefore, magnesium is a reducing agent. Another way of putting this is that the copper(II) ion is removing electrons from the magnesium to create a magnesium ion. The copper(II) ion is acting as an oxidizing agent.
Oxidant = it is a substance that has the ability to oxidize other substances , to cause them to lose electrons. examples are oxygen, hydrogen peroxide and the halogens.
Reductant= it is an element (such as calcium) or compound that loses (or donates) an electron to another chemical species in a redox chemical reaction. Since the reducing agent is losing electrons, it is said to have been oxidized.
In redox processes, the reductant transfers electrons to the oxidant. Thus, in the reaction, the reductant or reducing agent loses electrons and is oxidized, and the oxidant or oxidizing agent gains electrons and is reduced.
Oxidation Reaction
Oxidation reactions can be defined in terms of addition of oxygen or the removal of hydrogen. There are a number of reactions in which oxygen is added, or in which hydrogen is removed.
Examples: Addition of Oxygen:
(a). CuO(s) + H2(g) → Cu(s) + H2O(l)
CuO adds oxygen to H2; CuO is therefore the oxidizing agent or oxidant, it oxidizes the H2 to H2O.
(b). C(s) + H2O(g) → H2(g) + CO(g)
H2O is the oxidizing agent; it gives oxygen to carbon, which is oxidized to CO.
Removal of Hydrogen:
(a). H2S(g) + Cl2(g) → 2HCl(g) + S(s)
Chlorine is the oxidant, it removes hydrogen from H2S, thereby oxidizing it to sulphur.
(b). 2H2O(l) + 2F2(g) → 4HF(aq) + O2(g)
Fluorine removes hydrogen from the water, therefore, fluorine is the oxidant. The water is oxidized to oxygen.
Reduction Reaction
Reduction reactions can be defined as reactions that involve the removal of oxygen or the addition of hydrogen.
Example, Removal of Oxygen:
(a). CuO(s) + H2(g) → Cu(s) + H2O(l)
Hydrogen removes oxygen from CuO, thereby reducing it to Cu. Hydrogen is therefore the reducing agent or reductant.
(b). H2O(g) + C(s) → H2(g) + CO(g)
Carbon is the reductant, it removes oxygen from steam, thereby reducing it to hydrogen.
Addition of Hydrogen:
(a). H2S(g) + Cl2(g) → 2HCl(g) + S(s)
H2S adds hydrogen to Cl2(g). H2S is the reducing agent. It reduces Cl2(g) to HCl(g).
(b). 2H2O(l) + 2F2(g) → 4HF(aq) + O2(g)
H2O here adds hydrogen to F2, thereby reducing it to HF. H2O is the reductant in this reaction.
Summary
Confusion can result from trying to learn both the definitions of oxidation and reduction in terms of electron transfer and the definitions of oxidizing and reducing agents in the same terms.
The following thought pattern can be helpful:
An oxidizing agent oxidizes something else.
Oxidation is loss of electrons (OIL RIG).
Therefore, an oxidizing agent takes electrons from that other substance.
Therefore, an oxidizing agent must gain electrons.
Here is another mental exercise:
An oxidizing agent oxidizes something else.
Therefore, the oxidizing agent must be reduced.
Reduction is gain of electrons (OIL RIG).
Therefore, an oxidizing agent must gain electrons.
What are Acids, Bases, and Salts?
Many acids and bases occur naturally in nature, such as citric acid in fruits like orange, lemon etc, tartaric acid in tamarind, malic acid in apples and lactic acid in milk and milk products, hydrochloric acid in gastric juices.
Similarly, many bases are found such as lime water. We use many of these acids in our day-to-day life, such as vinegar or acetic acid in the kitchen, boric acid for laundry, baking soda for the purpose of cooking, washing soda for cleaning etc.
Many of the acids that we do not consume in the household are used in the laboratories and industries, which include an acid such as HCl, H2SO4 etc. and bases such as NaOH, KOH etc. When these acids and bases are mixed in the right proportions, the neutralization reaction thus results in the formation of salt and water. Some naturally occurring salts found in nature include NaCl and KCl etc in seawater and natural rock deposits. In this section, we will read more about acid, base and salt and their properties.
Definitions
Acid:- An acid is defined as a substance whose water solution tastes sour, turns blue litmus red and neutralizes bases.
Base:- A substance is called base if its aqueous solution tastes bitter, turns red litmus blue or neutralizes acids.
Salt:- Salt is a neutral substance whose aqueous solution does not affect litmus.
Acids
The term acid is derived from a Latin word ‘acidus’ or ‘acere’, which means sour. The most common characteristic is their sour taste. An acid is a substance that renders ionizable hydronium ion (H3O+) in its aqueous solution. It turns blue litmus paper red. These dissociate in their aqueous solution to form their constituent ions, as given by the following examples.
Natural Acids: These are obtained from natural sources, such as fruits and animal products. For e.g. lactic, citric, and tartaric acid etc.
Mineral Acids: Mineral acids are acids prepared from minerals. For example, Hydrochloric acid (HCl), Sulphuric Acid (H2SO4), and nitric acid (HNO3) etc.
Bases
The most common characteristic of bases is their bitter taste and soapy feel. A base is a substance that renders hydroxyl ion(OH–) in their aqueous solution. Bases turn the colour of red litmus paper to blue.
Salts
Salt is an ionic compound that results from the neutralization reaction of acids and bases. Salts are constituted of positively charged ions, known as cations and negatively charged ions, known as anions, which can either be organic or inorganic in nature. These ions are present in a relative amount, thus rendering the nature of the salt neutral.
Acid-base reactions
Acids and bases are important compounds in the natural world, so their chemistry is central to any discussion of chemical reactions. There are several theories of acid-base behaviour.
The Arrhenius theory
The Arrhenius theory, named after Swedish physicist Svante August Arrhenius, views an acid as a substance that increases the concentration of the hydronium ion (H3O+) in an aqueous solution and a base as a substance that increases the hydroxide ion (OH−) concentration. Well-known acids include hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid (HNO3), and acetic acid (CH3COOH). Bases includes such common substances as caustic soda (sodium hydroxide, NaOH) and slaked lime (calcium hydroxide, Ca(OH)2). Another common base is ammonia (NH3), which reacts with water to give a basic solution according to the following balanced equation.
NH3(aq) + H2O(l) → NH4+(aq) + OH−(aq)
(This reaction occurs to a very small extent; the hydroxide ion concentration is small but measurable.)
A large number of natural bases are known, including morphine, cocaine, nicotine, and caffeine; many synthetic drugs are also bases. All of these contain a nitrogen atom bonded to three other groups, and all behave similarly to ammonia in that they can react with water to give a solution containing the hydroxide ion.
Amino acids, a very important class of compounds, are able to function both as acids and as bases. Amino acid molecules contain both acidic (―COOH) and basic (―NH2) sites. In an aqueous solution, amino acids exist in both the molecular form and the so-called "zwitterionic" form, H3N + CH2CO2−. In this structure the nitrogen atom bears a positive charge, and the oxygen atom of the acid group bears a negative charge.
According to the Arrhenius theory, acid-base reactions involve the combination of the hydrogen ion (H+) and the hydroxide ion to form water. An example is the reaction of aqueous solutions of sodium hydroxide and hydrochloric acid.
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O (l)
The Brønsted-Lowry theory
A somewhat more general acid-base theory, the Brønsted-Lowry theory, named after Danish chemist Johannes Nicolaus Brønsted and English chemist Thomas Martin Lowry, defines an acid as a proton donor and a base as a proton acceptor. In this theory, the reaction of an acid and base is represented as an equilibrium reaction.
acid (1) + base (2) ⇌ base (1) + acid (2)
(The double arrows, ⇌, indicate that the products can re-form the reactants in a dynamic process.)
Acid (1) and base (1) are called a conjugate acid-base pair, as are acid (2) and base (2). The advantage of this theory is its predictive capacity. Whether the equilibrium lies toward the reactants (reactant-favoured) or the products (product-favoured) is determined by the relative strengths of the acids and bases.
The Brønsted-Lowry theory is often closely associated with the solvent water. Dissolving an acid in water to form the hydronium ion and the anion of the acid is an acid-base reaction. Acids are classified as strong or weak, depending on whether the equilibrium favours the reactants or products. Hydrochloric acid, a strong acid, ionizes completely in water to form the hydronium and chlorine (Cl−) ions in a product-favoured reaction.
HCl(aq) + H2O (l) → H3O+(aq) +Cl−(aq)
Using the Brønsted-Lowry theory, the reaction of ammonia and hydrochloric acid in water is represented by the following equation:
NH3(aq) + HCl(aq) → NH4+(aq) + Cl−(aq)
Hydrochloric acid and the chlorine ion are one conjugate acid-base pair, and the ammonium ion and ammonia are the other. The acid-base reaction is the transfer of the hydrogen ion from the acid (HCl) to the base (NH3). The equilibrium favours the weaker acid and base, in this case the products. Note that the hydroxide ion does not appear in this equation, a point differentiating the Arrhenius and Brønsted-Lowry theories.
The Lewis theory
A still broader acid and base theory was proposed by American physical chemist Gilbert Newton Lewis. In the Lewis theory, bases are defined as electron-pair donors and acids as electron-pair acceptors. Acid-base reactions involve the combination of the Lewis acid and base through sharing of the base’s electron pair.
Ammonia is an example of a Lewis base. A pair of electrons located on the nitrogen atom may be used to form a chemical bond to a Lewis acid such as boron trifluoride (BF3). (In the following equation, the colon represents an electron pair.)
H3N: + BF3 → H3N― BF3
Ammonia, water, and many other Lewis bases react with metal ions to form a group of species known as coordination compounds. The reaction to form these species is another example of a Lewis acid-base reaction. For example, the light blue colour of a solution of Cu2+ ions in water is due to the [Cu(H2O)6]2+ ion. If ammonia is added to this solution, the water molecules attached to copper are replaced by ammonia molecules, and the beautiful deep blue ion [Cu(NH3)4]2+ is formed.
Note:
pH paper is another medium for the more clear estimation of the acidity of the acid and the basicity of the bases. The compound is brought in contact with the damped pH paper. The paper changes its color based on the pH of the compound which is under consideration. Compared with litmus paper which shows only acidic and basic, the pH paper also shows the strength of acidity or basicity.
Frequently Asked Questions – FAQs
What is salt in acid base and salt?
In chemistry, a salt is a substance obtained by the reaction of an acid and a base. Salts are composed of positive ions (cations) of bases and negative ions (anions) of acids. The reaction of acid and base is called the neutralization reaction.
Is NH4Cl a basic salt?
Ammonium chloride (chemical formula NH4Cl) is an acid salt because it is a salt of a strong acid (i.e. hydrochloric acid) and a weak base (i.e. ammonium hydroxide).
What are the 2 types of acids?
There are two main types of acids: organic acids and inorganic acids. Inorganic acids are sometimes referred to as inorganic acids. Generally speaking, organic acids are not as strong as inorganic acids.
Is salt basic or acidic?
The salt is basic only when it contains a weak acid conjugate base. For example, sodium chloride contains chloride (Cl-), the conjugate base of HCl.
What happens when salt reacts with HCl?
In this case, the acid is dilute hydrochloric acid and the metal is iron. Dilute hydrochloric acid is added to the iron filings to generate iron (II) chloride and hydrogen. In this reaction, iron replaces hydrogen from hydrochloric acid to form iron chloride and hydrogen. Gas, this reaction is a simple displacement reaction.
Classification by reaction outcome
Chemists often classify reactions on the basis of the overall result. Here several commonly encountered reactions are classified. As previously noted, many reactions defy simple classification and may fit in several categories.
Decomposition reactions
Decomposition reactions are processes in which chemical species break up into simpler parts. Usually, decomposition reactions require energy input. For example, a common method of producing oxygen gas in the laboratory is the decomposition of potassium chlorate (KClO3) by heat.
2KClO3(s) → 2KCl(s) + 3O2(g)
Another decomposition reaction is the production of sodium (Na) and chlorine (Cl2) by electrolysis of molten sodium chloride (NaCl) at high temperature.
2NaCl (l) → 2Na (l) + Cl2(g)
A decomposition reaction that was very important in the history of chemistry is the decomposition of mercury oxide (HgO) with heat to give mercury metal (Hg) and oxygen gas. This is the reaction used by 18th-century chemists Carl Wilhelm Scheele, Joseph Priestley, and Antoine-Laurent Lavoisier in their experiments on oxygen.
2HgO(s) → 2Hg (l) + O2(g)
Substitution, elimination, and addition reactions
These terms are particularly useful in describing organic reactions. In a substitution reaction, an atom or group of atoms in a molecule is replaced by another atom or group of atoms. For example, methane (CH4) reacts with chlorine (Cl2) to produce chloromethane (CH3Cl), a compound used as a topical anesthetic. In this reaction, a chlorine atom is substituted for a hydrogen atom.
Substitution reactions are widely used in industrial chemistry. For example, substituting two of the chlorine atoms on chloroform (CHCl3) with fluorine atoms produces chlorodifluoromethane (CHClF2). This product undergoes a further reaction when heated strongly.
2CHClF2(g) → F2C=CF2(g) + 2HCl(g)
This latter reaction is an example of an elimination reaction, a hydrogen atom and a chlorine atom being eliminated from the starting material as hydrochloric acid (HCl). The other product is tetrafluoroethylene, a precursor to the polymer known commercially as Teflon.
Addition reactions are the opposite of elimination reactions. As the name implies, one molecule is added to another. An example is the common industrial preparation of ethanol (CH3CH2OH). Historically, this compound was made by fermentation. However, since the early 1970s, it has also been made commercially by the addition of water to ethylene.
C2H4+ H2O → CH3CH2OH
Polymerization reactions
Polymers are high-molecular-weight compounds, fashioned by the aggregation of many smaller molecules called monomers. The plastics that have so changed society and the natural and synthetic fibres used in clothing are polymers. There are two basic ways to form polymers: (a) linking small molecules together, a type of addition reaction, and (b) combining two molecules (of the same or different type) with the elimination of a stable small molecule such as water. This latter type of polymerization combines addition and elimination reactions and is called a condensation reaction .
An example of the first type of reaction is the union of thousands of ethylene molecules that gives polyethylene.
nH2C=CH2 → [―CH2CH2―]n
Other addition polymers include polypropylene (made by polymerizing H2C=CHCH3), polystyrene (from H2C=CH C6H5), and polyvinyl chloride (from H2C=CHCl).
Starch and cellulose are examples of the second type of polymer. These are members of a class of compounds called carbohydrates, substances with formulas that are multiples of the simple formula CH2O. Both starch and cellulose are polymers of glucose, a sugar with the formula C6H12O6. In both starch and cellulose, molecules of glucose are joined together with concomitant elimination of a molecule of water for every linkage formed.
nC6H12O6 → ―[―C6H10O5―]―n + nH2O
The synthetic material nylon is another example of this type of polymer. Water and a polymer (nylon-6,6) are formed when an organic acid and an amine (a compound derived from ammonia) combine.
The natural fibres of proteins such as hair, wool, and silk are also polymers that contain the repeating unit (-CHRCONH-), where R is a group of atoms attached to the main polymer. These form by joining amino acids with the elimination of a water molecule for each CONH or peptide linkage formed; for example, the structure of the tripeptide chain is formed from three units of the amino acid glycine (NH2CH2CO2H).
Solvolysis and hydrolysis
A solvolysis reaction is one in which the solvent is also a reactant. Solvolysis reactions are generally named after the specific solvent—for example, the term hydrolysis when water is involved. If a compound is represented by the formula AB (in which A and B are atoms or groups of atoms) and water is represented by the formula HOH, the hydrolysis reaction may be represented by the reversible chemical reaction
AB + HOH ⇌ AH + BOH.
Hydrolysis of an organic compound is illustrated by the reaction of water with esters. Esters have the general formula RCOOR′, R and R′ being combining groups (such as CH3). The hydrolysis of an ester produces an acid and an alcohol. The equation for the reaction of methyl acetate and water is
CH3COOCH3(aq) + H2O(l) → CH3COOH(aq) + CH3OH(aq).
Hydrolysis reactions play an important role in chemical processes that occur in living organisms. Proteins are hydrolyzed to amino acids, fats to fatty acids and glycerol, and starches and complex sugars to simple sugars. In most instances, the rates of these processes are enhanced by the presence of enzymes, biological catalysts.
Hydrolysis reactions are also important to acid-base behaviour. Anions of weak acids dissolve in water to give basic solutions, as in the hydrolysis of the acetate ion, CH3C OO−.
CH3COO−(aq) + H2O (l) → CH3COOH(aq) + OH−(aq)
Although this is a reactant-favoured reaction, it occurs to an extent sufficient to cause a solution containing the acetate ion to exhibit basic properties (e.g., turning red litmus paper blue).
Hydrolysis reactions account for the basic character of many common substances. Salts of the borate, phosphate, and carbonate ions, for example, give basic solutions that have long been used for cleaning purposes. Many food products also contain basic anions such as tartrate and citrate ions.
Classification by reaction mechanism
Reaction mechanisms provide details on how atoms are shuffled and reassembled in the formation of products from reactants. Chain and photolysis reactions are named on the basis of the mechanism of the process.
Chain reactions
Chain reactions occur in a sequence of steps, in which the product of each step is a reagent for the next. Chain reactions generally involve three distinct processes: an initiation step that begins the reaction, a series of chain-propagation steps, and, eventually, a termination step.
Polymerization reactions are chain reactions, and the formation of Teflon from tetrafluoroethylene is one example. In this reaction, a peroxide (a compound in which two oxygen atoms are joined together by a single covalent bond) may be used as the initiator. Peroxides readily form highly reactive free-radical species (a species with an unpaired electron) that initiate the reaction. There are a number of different ways to terminate the chain, only one of which is shown. (In the following equations, the dots represent unpaired electrons, and R is a generic organic group.)
Photolysis reactions
Photolysis reactions are initiated or sustained by the absorption of electromagnetic radiation. One example, the decomposition of ozone to oxygen in the atmosphere, is mentioned above in the section Kinetic considerations. Another example is the synthesis of chloromethane from methane and chlorine, which is initiated by light. The overall reaction is
CH4(g) + Cl2(g) + hυ → CH3Cl(g) + HCl(g),
where hυ represents light. This reaction, coincidentally, is also a chain reaction. It begins with the endothermic reaction of a chlorine molecule (Cl2) to give chlorine atoms, a process that occurs under ultraviolet irradiation. When formed, some of the chlorine atoms recombine to form chlorine molecules, but not all do so. If a chlorine atom instead collides with a methane molecule, a two-step chain propagation occurs. The first propagation step produces the methyl radical (CH3). This free-radical species reacts with a chlorine molecule to give the product and a chlorine atom, which continues the chain reaction for many additional steps. Possible termination steps include combination of two methyl radicals to form ethane (CH3CH3) and a combination of methyl and chlorine radicals to give chloromethane.
Frequently Asked Questions – FAQs
How do you identify a chemical reaction?
A chemical reaction is typically followed by physical signs that are readily detected, such as heat and light emission, precipitate formation, gas evolution, or a change of appearance.
How do you identify physical and chemical changes?
The shape or form of the matter varies through a physical transition, but the sort of matter in the material does not. In a chemical shift, however, the type of matter shifts and at least one new material with new properties is created. There is no straight cut of the gap between physical and chemical transition.
Why do we write a chemical equation?
The purpose of writing a balanced chemical equation is to explain the occurring reactants (starting material) and products (end results). The ratios in which they answer so that you can measure the amount of reactants you need and the amount of goods you can make.
What is chemical reaction and equation?
A chemical equation is the symbolic representation in the form of symbols and formulas of a chemical reaction in which the reactant entities on the left-hand side and the product entities on the right-hand side are given.
What is the skeleton equation?
A skeleton equation is when each product that takes part in the reaction is written with the chemical formulas describing the chemical reaction. Examples: The term equation: oxygen + methane. Dioxide with carbon + Vapour.
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