Chemical Reactions and Equations Class 10 Notes

A chemical equation is the shorthand record of a chemical reaction. The reactants are written on the left, the products on the right, and the arrow between them shows the direction of the reaction. Where a state matters it is written in brackets, the solid as (s), the liquid as (l), the gas as (g) and the dissolved state as (aq).

• The reactants, the substances that take part in the reaction, written on the left of the arrow.
• The products, the substances formed, written on the right.
• The conditions, written over the arrow, such as a temperature for a decomposition or a catalyst.
• The states, in brackets: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
• The balance, the requirement that the atoms of each element match on both sides.

A chemical equation is balanced by placing a coefficient in front of a formula so that the atoms match. Coefficients go in front of the whole formula and are never written into a formula, so H₂O is never written as H₂O₂ by accident and never as H2O with a 2 on the oxygen.

• Write the skeleton equation with the correct formulas and no coefficients.
• Count the atoms of each element on the left and on the right.
• Place a whole-number coefficient before a formula to equalise the element that differs.
• Recheck every element, because fixing one may unbalance another.
• Reduce the coefficients to the smallest whole-number ratio at the end.

The practical file asks students to perform four reactions and classify them, so these five types are not theory to be memorised but categories to be recognised in an unfamiliar equation. The recognition test is always the same: count the products, and ask whether an element has moved from one compound to another.

• Combination — two or more substances join to form one product. 2Mg + O₂ → 2MgO, and CaO + H₂O → Ca(OH)₂.
• Decomposition — one substance breaks into two or more. 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ on heating, and 2Pb(NO₃)₂ → 2PbO + 2NO₂ + O₂.
• Displacement — a more reactive element takes the place of a less reactive one. Fe + CuSO₄ → FeSO₄ + Cu.
• Double displacement — two compounds exchange ions, giving two new compounds. Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl.
• Precipitation — a double displacement in which one product is an insoluble solid, the precipitate. It is the BaSO₄ in the reaction above, and the test is the visible solid.

Every reaction either releases energy to the surroundings or absorbs energy from them, and the distinction decides whether the surroundings feel warmer or cooler. In an exothermic reaction energy is released, so the surroundings warm up; in an endothermic reaction energy is absorbed, so the surroundings cool down.

• Exothermic — heat is released to the surroundings. Respiration, combustion, the burning of fuel, and the reaction of quicklime with water.
• Endothermic — heat is absorbed from the surroundings. The photosynthesis, the thermal decomposition of calcium carbonate, and the dissolving of ammonium chloride in water.
• The test in the practical — the temperature rise of the surroundings shows a release, and the fall shows an absorption.

Oxidation and reduction are the two halves of a single reaction and always occur together, which is why the pair is called a redox reaction. Oxidation is the gain of oxygen or the loss of hydrogen, and equivalently the loss of electrons; reduction is the loss of oxygen or the gain of hydrogen, and equivalently the gain of electrons.

• Oxidation — gain of oxygen, or loss of hydrogen, or loss of electrons. CuO + H₂ → Cu + H₂O: the copper loses oxygen, so the copper is reduced and the hydrogen is oxidised.
• Reduction — loss of oxygen, or gain of hydrogen, or gain of electrons. The same equation read from the hydrogen's side is the clearest illustration.
• The oxidising agent — the substance that supplies the oxygen and is itself reduced. CuO in the equation above.
• The reducing agent — the substance that removes the oxygen and is itself oxidised. H₂ in the equation above.
• Respiration and combustion, the two large everyday redox reactions, and the rusting of iron.

The questions in this chapter are short and classificatory, so the answer is nearly always an equation with a label attached. The skill being tested is the label, not the explanation.

• Write a balanced chemical equation for the action of water on quicklime.
• Give one example each of a combination, a decomposition, a displacement and a double displacement reaction.
• Define a precipitation reaction with an example.
• Distinguish between an exothermic and an endothermic reaction with an example of each.
• Define oxidation and reduction, and name the oxidising and the reducing agent in a given equation.
• Why is it necessary to balance a chemical equation?