Chemical Reactions and Equations

Whenever a chemical change occurs, a chemical reaction has taken place.

For example, magnesium ribbon burns with a dazzling white flame and changes into a white powder. This powder is magnesium oxide. It is formed due to the reaction between magnesium and oxygen present in the air.

Chemical Equations

When a magnesium ribbon is burnt in oxygen, it gets converted to magnesium oxide.

Word Equation

The word-equation for the reaction is

$$ \underbrace{\text{Magnesium + Oxygen}}_{\text{Reactants}} \longrightarrow \underbrace{\text{Magnesium oxide}}_{\text{Product}} $$

The substances that undergo chemical change in the reaction (magnesium and oxygen) are the reactants. The new substance (magnesium oxide) formed during the reaction, is a product.

A word-equation shows change of reactants to products through an arrow placed between them. The reactants are written on the left-hand side (LHS) with a plus sign (+) between them. Similarly, products are written on the right-hand side (RHS) with a plus sign (+) between them. The arrowhead points towards the products, and shows the direction of the reaction.

Chemical Equation

Chemical equations can be made more concise and useful if we use chemical formulae instead of words. A complete chemical equation represents the reactants, products and their physical states symbolically.

A chemical equation represents a chemical reaction.

$$ \ce{Mg + O2 -> MgO} $$

Count and compare the number of atoms of each element on the LHS and RHS of the arrow. If the number of atoms of each element the same on both the sides, then the equation is balanced. If not, then the equation is unbalanced because the mass is not the same on both sides of the equation. Such a chemical equation is a skeletal chemical equation for a reaction.

Balanced Chemical Equations

Mass can neither be created nor destroyed in a chemical reaction. That is, the total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants.

The number of atoms of each element remains the same, before and after a chemical reaction. Hence, you need to balance a skeletal chemical equation.

Step I: To balance a chemical equation, first draw boxes around each formula. Do not change anything inside the boxes while balancing the equation.

Step II: List the number of atoms of different elements present in the unbalanced equation.

Step III: Start balancing with the compound that contains the maximum number of atoms. It may be a reactant or a product. In that compound, select the element which has the maximum number of atoms.

Step IV: Pick other elements to proceed further.

Types of Chemical Reactions

During a chemical reaction atoms of one element do not change into those of another element. Nor do atoms disappear from the mixture or appear from elsewhere. Chemical reactions involve the breaking and making of bonds between atoms to produce new substances.

Combination Reaction

In a combination reaction two or more substances combine to form a new single substance.

(i) Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.

$$ \ce{CaO(s) + H2O(l) -> Ca(OH)2(aq) + Heat} $$

(ii) Burning of coal

$$ \ce{C(s) + O2(g) -> CO2(g)} $$

(iii) Formation of water from H₂(g) and O₂(g)

$$ \ce{2H2(g) + O2(g) -> 2H2O(l)} $$

Exothermic Reactions

Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.

Decomposition Reaction

Decomposition reactions are opposite to combination reactions. In a decomposition reaction, a single substance decomposes to give two or more substances.

(i) Ferrous sulphate crystals (FeSO₄.7H₂O) lose water when heated and the colour of the crystals changes. It then decomposes to ferric oxide (Fe₂O₃), sulphur dioxide (SO₂) and sulphur trioxide (SO₃). Ferric oxide is a solid, while SO₂ and SO₃ are gases.

$$ \ce{2FeSO4(s) ->[\text{Heat}] Fe2O3(s) + SO2(g) + SO3(g)} $$

The green colour of the ferrous sulphate crystals changes to reddish-brown due to Fe₂O₃.

(ii) Decomposition of calcium carbonate (limestone) to calcium oxide and carbon dioxide on heating is an important decomposition reaction used in various industries. Calcium oxide is called lime or quick lime. It has many uses like in the manufacture of cement. When a decomposition reaction is carried out by heating, it is called thermal decomposition.

$$ \ce{CaCO3(s) ->[\text{Heat}] CaO(s) + CO2(g)} $$

(iii) Lead nitrate

$$ \ce{2Pb(NO3)2(s) ->[\text{Heat}] 2PbO(s) + 4NO2(g) + O2(g)} $$

The emission of brown fumes are of nitrogen dioxide (NO₂).

(iii) Silver chloride

White silver chloride turns grey in sunlight. This is due to the decomposition of silver chloride into silver and chlorine by light.

$$ \ce{2AgCl(s) ->[\text{Sunlight}] 2Ag(s) + Cl2(g)} $$

Silver bromide also behaves in the same way.

Endothermic Reactions

Decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants. Reactions in which energy is absorbed are known as endothermic reactions.

Displacement Reaction

When an element displaces another element from its compound, a displacement reaction occurs.

(i) Iron displaces or removes copper from copper sulphate solution.

$$ \ce{Fe(s) + CuSO4(aq) -> FeSO4(aq) + Cu(s)} $$

Iron nail become brownish in colour and the blue colour of copper sulphate solution fades.

(ii) Zinc displaces copper

$$ \ce{Zn(s) + CuSO4(aq) -> ZnSO4(aq) + Cu(s)} $$

(iii) Lead displaces copper

$$ \ce{Pb(s) + CuCl2(aq) -> PbCl2(aq) + Cu(s)} $$

Zinc and lead are more reactive elements than copper. They displace copper from its compounds.

Double Displacement Reaction

Reactions in which there is an exchange of ions between the reactants are called double displacement reactions.

(i) The reaction of SO₄^2– and Ba²⁺. The other product formed is sodium chloride which remains in the solution.

$$ \ce{Na2SO4(aq) + BaCl2(aq) -> BaSO4(s) + 2NaCl(aq)} $$

A white substance, which is insoluble in water, is formed. This insoluble substance formed is known as a precipitate. Any reaction that produces a precipitate can be called a precipitation reaction.

Oxidation and Reduction

Reactions also involve the gain or loss of oxygen or hydrogen by substances. Oxidation is the gain of oxygen or loss of hydrogen. Reduction is the loss of oxygen or gain of hydrogen.

(i) After heating copper powder, the surface of copper powder becomes coated with black copper(II) oxide. This is because oxygen is added to copper and copper oxide is formed.

$$ \ce{2Cu(s) + O2(g) ->[\text{Heat}] 2CuO(s)} $$

(ii) If hydrogen gas is passed over this heated material (CuO), the black coating on the surface turns brown as the reverse reaction takes place and copper is obtained.

$$ \ce{CuO(s) + H2(g) ->[\text{Heat}] Cu(s) + H2O(g)} $$

If a substance gains oxygen during a reaction, it is said to be oxidised. If a substance loses oxygen during a reaction, it is said to be reduced. During this reaction, the copper(II) oxide is losing oxygen and is being reduced. The hydrogen is gaining oxygen and is being oxidised.

One reactant gets oxidised while the other gets reduced during a reaction. Such reactions are called oxidation-reduction reactions or redox reactions.

(ii) Carbon is oxidised to CO and ZnO is reduced to Zn

$$ \ce{ZnO(s) + C(s) -> Zn(s) + CO(g)} $$

(iii) HCl is oxidised to Cl₂ whereas Mn_O2 is reduced to MnCl₂.

$$ \ce{MnO2(s) + 4HCl(aq) -> MnCl2(aq) + 2H2O(l) + Cl2(g)} $$

Corrosion

Iron articles are shiny when new, but get coated with a reddish brown powder when left for some time. This process is commonly known as rusting of iron.

Some other metals also get tarnished in this manner. When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion.

The black coating on silver and the green coating on copper are other examples of corrosion.

Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metals, specially those of iron.

Rancidity

When fats and oils are oxidised, they become rancid and their smell and taste change. Usually substances which prevent oxidation (anti-oxidants) are added to foods containing fats and oil. Keeping food in air tight containers helps to slow down oxidation.