Chapter 1: Chemical Reactions and Equations
What is a chemical reaction?
When a substance changes into one or more new substances with different properties, it is called a chemical reaction. Milk turning into curd and iron rusting are everyday examples.
Signs of a chemical reaction
A change in colour, release of a gas, formation of a precipitate (an insoluble solid), a change in temperature, or a change in the state of the substance.
Chemical equations and balancing
Writing a reaction with symbols and formulae gives a chemical equation. By the law of conservation of mass, the number of atoms of each element must be equal on both sides of the arrow, so an equation must be balanced.
Types of chemical reactions
1. Combination: two or more substances join to form one new substance.
2. Decomposition: one substance breaks into two or more substances, using heat, light or electricity.
3. Displacement: a more reactive metal pushes a less reactive metal out of its compound.
4. Double displacement: two compounds exchange their ions. A precipitate usually forms.
5. Oxidation-reduction (redox): one substance gains oxygen (oxidation) while another loses oxygen (reduction).
Corrosion and rancidity
Corrosion: slow damage of a metal by air and moisture, such as rusting of iron. Rancidity: oils and fats react with oxygen in air and develop a bad smell and taste. Prevention: antioxidants, airtight containers, and flushing chip packets with nitrogen gas.
Important questions
Q1. Why must a chemical equation be balanced?
By the law of conservation of mass, mass is neither created nor destroyed in a reaction. So the number of atoms of each element must be the same on both sides.
Q2. Balance: Fe + H2O โ Fe3O4 + H2
3Fe + 4H2O โ Fe3O4 + 4H2. Both sides now have 3 Fe, 8 H and 4 O.
Q3. What is the difference between displacement and double displacement reactions?
In displacement, a more reactive element pushes another element out of its compound. In double displacement, two compounds exchange ions, and a precipitate usually forms.
Q4. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Iron is more reactive than copper, so it displaces copper. The blue solution turns pale green (FeSO4 forms) and a brown layer of copper deposits on the nail.
Q5. What is rancidity and how can it be prevented?
When oils or fats are oxidised by oxygen in air, they develop a bad smell and taste. This is rancidity. It can be prevented by adding antioxidants, using airtight containers, and flushing with nitrogen.
Q6. What is a precipitate?
An insoluble solid formed during a reaction, for example the white BaSO4 formed when Na2SO4 reacts with BaCl2.
Q7. What is an exothermic reaction? Give an example.
A reaction in which heat is released, such as CaO + H2O โ Ca(OH)2 + heat, or the burning of natural gas.
Q8. What is an endothermic reaction?
A reaction in which heat is absorbed, such as heating CaCO3 to form CaO and CO2.
Q9. Why is a magnesium ribbon cleaned before it is burnt?
To remove the protective layer of MgO on it, so that the ribbon burns easily.
Q10. What is seen when a magnesium ribbon burns?
A dazzling white flame, and a white powder of MgO is formed: 2Mg + O2 โ 2MgO.
Q11. Why is silver chloride kept in dark-coloured bottles?
In light it breaks down into Ag and Cl2: 2AgCl โ 2Ag + Cl2. A dark bottle protects it from light.
Q12. How is slaked lime made?
When water is added to quicklime (CaO), Ca(OH)2 (slaked lime) forms and heat is released.
Q13. What is oxidation?
Gain of oxygen or loss of hydrogen by a substance.
Q14. What is reduction?
Loss of oxygen or gain of hydrogen by a substance.
Q15. In CuO + H2 โ Cu + H2O, which substance is oxidised and which is reduced?
H2 is oxidised (it gains oxygen) and CuO is reduced (it loses oxygen).
Q16. Which gas is released when dilute H2SO4 is added to zinc granules?
Hydrogen gas: Zn + H2SO4 โ ZnSO4 + H2.
Q17. Write the balanced equation for the burning of methane.
CH4 + 2O2 โ CO2 + 2H2O
Q18. Why is corrosion harmful?
It weakens and damages iron objects such as bridges, ships and machines.
Q19. What is the chemical name of quicklime?
Calcium oxide (CaO).
Q20. Why are combination and decomposition reactions called opposites?
In combination, several reactants join to form one product; in decomposition, one reactant breaks into several products.
Quiz
1. CaO + H2O โ Ca(OH)2 is which type of reaction?
Two substances combine to form one new substance, so it is a combination reaction.
2. A balanced equation is based on which law?
Mass neither increases nor decreases in a chemical reaction.
3. Fe + CuSO4 โ FeSO4 + Cu is which type of reaction?
Iron displaces copper from its compound.
4. Why is nitrogen gas filled in chip packets?
Nitrogen removes oxygen, so the oil does not get oxidised.
5. Which is an example of corrosion?
Iron slowly turns into rust due to air and moisture.