Chemical Reactions and Equations

A chemical equation is the symbolic representation of a chemical reaction using formulae of reactants and products. Reactants are written on the left, products on the right, separated by an arrow pointing towards the products. According to the Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction, so the total mass of reactants equals the total mass of products. This means the number of atoms of each element must be equal on both sides. An equation that satisfies this is called a balanced chemical equation.

Consider the burning of hydrogen in oxygen. The skeletal (unbalanced) equation is H2 + O2 -> H2O. Counting atoms shows 2 oxygen atoms on the left but only 1 on the right, so it is unbalanced. We balance using the hit-and-trial method by placing coefficients (never changing subscripts):

2H2 + O2 -> 2H2O

Now both sides have 4 H atoms and 2 O atoms. We also add state symbols and conditions: 2H2(g) + O2(g) -> 2H2O(l).

Common mistake: Students often try to balance by changing subscripts (e.g. writing H2O2 instead of H2O). This is wrong because it changes the substance itself. Only coefficients in front of formulae may be adjusted while balancing.

Problem: Balance the equation for the burning of methane (natural gas) in oxygen.

Skeletal equation: CH4 + O2 -> CO2 + H2O

Step 1 — Count atoms on both sides.

Carbon is already balanced. Hydrogen and oxygen are not.

Step 2 — Balance hydrogen. There are 4 H on the left and 2 H on the right. Put coefficient 2 before H2O:

CH4 + O2 -> CO2 + 2H2O

Now H: 4 = 4. Recount oxygen on the right: 2 (in CO2) + 2 (in 2H2O) = 4.

Step 3 — Balance oxygen. Right side now needs 4 O atoms; left has 2. Put coefficient 2 before O2:

CH4 + 2O2 -> CO2 + 2H2O

Now O: 4 = 4.

Step 4 — Verify all elements. C: 1 = 1, H: 4 = 4, O: 4 = 4. The equation is balanced.

Step 5 — Add states and condition.

CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g)

This is a balanced chemical equation obeying the law of conservation of mass.

Chemical reactions and equations — key points:

• A chemical reaction changes reactants into new products; signs include change in colour, state, temperature, evolution of a gas, or formation of a precipitate.

• A chemical equation uses formulae to show reactants -> products. A balanced equation has equal numbers of atoms of each element on both sides, as required by the Law of Conservation of Mass. Balance using coefficients only (hit-and-trial); never change subscripts. Add state symbols (s, l, g, aq) and conditions over the arrow.

• Types of reactions:

  • Combination: two or more reactants -> one product (e.g. CaO + H2O -> Ca(OH)2).
  • Decomposition: one reactant -> two or more products using heat, light, or electricity (e.g. CaCO3 -> CaO + CO2). Electrolysis decomposes using electricity.
  • Displacement: a more reactive element displaces a less reactive one (e.g. Fe + CuSO4 -> FeSO4 + Cu).
  • Double displacement: two compounds exchange ions, often forming a precipitate (e.g. Na2SO4 + BaCl2 -> BaSO4 + 2NaCl).

• Reactions can be exothermic (release heat, e.g. respiration, combustion) or endothermic (absorb heat, e.g. decomposition of CaCO3).

• Oxidation and reduction (redox): Oxidation = gain of oxygen / loss of hydrogen; Reduction = loss of oxygen / gain of hydrogen. They occur together in a redox reaction. The oxidising agent gives oxygen; the reducing agent removes oxygen (e.g. CuO + H2 -> Cu + H2O).

• Effects of oxidation in daily life:

  • Corrosion: slow attack on metals by air and moisture; rusting of iron needs both oxygen and water. Prevented by painting, oiling, galvanisation, and alloying.
  • Rancidity: oxidation of fats and oils in food causing bad smell/taste; prevented by antioxidants, airtight packing, nitrogen flushing, and refrigeration.

Chemical reactions can be classified by what happens to the reactants. The main NCERT types are combination, decomposition, displacement, and double displacement.

1. Combination reaction: Two or more reactants combine to form a single product. Example — calcium oxide (quicklime) reacts with water:
CaO(s) + H2O(l) -> Ca(OH)2(aq). This is also exothermic (releases heat).

2. Decomposition reaction: A single reactant breaks down into two or more products. Heat, light, or electricity supplies the energy. Thermal decomposition of limestone:
CaCO3(s) --heat--> CaO(s) + CO2(g). Decomposition by electricity is called electrolysis (e.g. of water).

3. Displacement reaction: A more reactive element displaces a less reactive element from its compound. Iron displaces copper:
Fe(s) + CuSO4(aq) -> FeSO4(aq) + Cu(s). The blue colour of copper sulphate fades.

4. Double displacement reaction: Two compounds exchange ions, often forming a precipitate (insoluble solid):
Na2SO4(aq) + BaCl2(aq) -> BaSO4(s) + 2NaCl(aq). White BaSO4 precipitates.

Common mistake: Confusing displacement with double displacement. In displacement an element replaces part of a compound; in double displacement two compounds swap their ions. Always check whether free elements are involved.

Problem: Classify each reaction as combination, decomposition, displacement, or double displacement, and give a reason.

Reaction A: 2H2O(l) --electricity--> 2H2(g) + O2(g)
A single compound (water) splits into two simpler substances using electricity. One reactant -> two products. This is a decomposition reaction (specifically electrolytic decomposition).

Reaction B: Zn(s) + 2HCl(aq) -> ZnCl2(aq) + H2(g)
Zinc, a more reactive element, displaces hydrogen from hydrochloric acid. A free element replaces another element in a compound. This is a displacement reaction.

Reaction C: 2Mg(s) + O2(g) -> 2MgO(s)
Two reactants (magnesium and oxygen) combine to give a single product, magnesium oxide. This is a combination reaction. It is also exothermic and a redox reaction (Mg is oxidised).

Reaction D: AgNO3(aq) + NaCl(aq) -> AgCl(s) + NaNO3(aq)
Two compounds exchange ions; insoluble white silver chloride precipitates out. This is a double displacement reaction (a precipitation reaction).

Summary table:

Tip: First check the count of reactants and products, then look for free elements to decide between displacement and double displacement.

Oxidation is the gain of oxygen or loss of hydrogen by a substance. Reduction is the loss of oxygen or gain of hydrogen. When oxidation and reduction occur together in the same reaction, it is called a redox reaction (reduction-oxidation). In every redox reaction, one species is oxidised while another is reduced — they always happen simultaneously.

Consider the reaction of copper(II) oxide with hydrogen:
CuO(s) + H2(g) --heat--> Cu(s) + H2O(l)

Here CuO loses oxygen, so copper oxide is reduced to copper. Hydrogen gains oxygen, so hydrogen is oxidised to water. Both processes occur in one reaction, making it a redox reaction. The substance that gives oxygen (CuO) is the oxidising agent; the substance that takes oxygen (H2) is the reducing agent.

Redox reactions explain many everyday changes, including corrosion of metals and the rancidity of fats and oils.

Common mistake: Thinking a substance can be oxidised without anything being reduced. In a redox reaction, if one substance is oxidised, another must be reduced at the same time — oxidation and reduction are two halves of one process.

Problem: In the reaction below, identify the substance oxidised, the substance reduced, the oxidising agent, and the reducing agent.

ZnO(s) + C(s) --heat--> Zn(s) + CO(g)

Step 1 — Check the equation is balanced.
Zn: 1 = 1, O: 1 = 1, C: 1 = 1. It is balanced.

Step 2 — Track oxygen for each species.

• ZnO becomes Zn: it has lost oxygen.
• C becomes CO: it has gained oxygen.

Step 3 — Apply the definitions.

• Loss of oxygen = reduction. So ZnO is reduced to Zn.
• Gain of oxygen = oxidation. So carbon (C) is oxidised to CO.

Step 4 — Identify the agents.

• The oxidising agent supplies oxygen to another substance. ZnO gives oxygen to carbon, so ZnO is the oxidising agent.
• The reducing agent removes oxygen from another substance. Carbon removes oxygen from ZnO, so C is the reducing agent.

Step 5 — Conclusion. Because oxidation and reduction occur together, this is a redox reaction.

Memory aid — OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons); in terms of oxygen, oxidation is gain of oxygen and reduction is loss of oxygen.

Two important consequences of oxidation in everyday life are corrosion and rancidity.

Corrosion is the slow eating away of a metal by the action of air, moisture, or chemicals on its surface. The most common example is the rusting of iron, where iron reacts with oxygen and water (moisture) to form a brown flaky layer of hydrated iron(III) oxide (rust):
4Fe(s) + 3O2(g) + xH2O -> 2Fe2O3.xH2O(s)

Rusting requires both air (oxygen) and water; if either is absent, iron does not rust. Other examples: silver tarnishes to form a black coating (silver sulphide), and copper develops a green coating (basic copper carbonate). Corrosion can be prevented by painting, oiling, greasing, galvanisation (coating with zinc), chromium plating, and making alloys such as stainless steel.

Rancidity is the oxidation of fats and oils in food when exposed to air, producing an unpleasant smell and taste. To prevent rancidity we: add antioxidants, store food in airtight containers, flush packets with nitrogen gas (as in chips packets), and refrigerate food.

Common mistake: Believing rust forms with oxygen alone. Iron needs both oxygen and moisture to rust — that is why oiled or painted iron, which is shielded from air and water, does not corrode.

Problem: A packet of potato chips is flushed with nitrogen gas before sealing. Explain the chemistry, and state which everyday process this prevents.

Step 1 — Identify the problem being prevented.
Potato chips are fried in oil and contain fats. When fats and oils are exposed to oxygen in air, they undergo oxidation, which makes them smell and taste bad. This spoilage is called rancidity.

Step 2 — Explain the role of nitrogen.
Nitrogen (N2) is an unreactive (inert) gas under normal conditions. By flushing the packet with nitrogen, the manufacturer removes the oxygen from inside the packet and replaces it with nitrogen.

Step 3 — Connect cause and effect.
No oxygen inside the packet means the fats and oils cannot get oxidised. Therefore rancidity is delayed and the chips stay fresh and crisp for longer.

Step 4 — List other prevention methods (for comparison).

Conclusion: Flushing with nitrogen prevents rancidity, which is the oxidation of fats and oils in food. It works because nitrogen is inert and displaces the oxygen that would otherwise cause oxidation.