Exam practice
Every question from the end of every unit, with hints and answers to open. After you check an answer, mark how you did, and come back to the ones you didn’t get.
17 questions
Unit 15 · Chemical Reaction
2. Give reasons
Hydrogen peroxide is not stored in a transparent vessel.
Answer
Light increases the rate at which hydrogen peroxide decomposes into water and oxygen (2H₂O₂ → 2H₂O + O₂). In a transparent bottle, light would make it break down and lose its strength. So it is kept in dark (brown or opaque) bottles, often with a little phosphoric acid added as a negative catalyst.
An acid–base reaction is also known as a neutralization reaction.
Answer
When an acid reacts with a base, they form a salt and water, and the acid and the base lose (cancel) each other’s properties: the H⁺ ions of the acid and the OH⁻ ions of the base combine into water. The products are neutral, neither acidic nor basic, so the reaction is called neutralization.
The rate of chemical reaction is different according to the type of reaction.
Answer
The rate depends on the nature of the reactants and on how many bonds must be broken. In reactions between ions in solution (such as acid + base, or a precipitation), the particles are already free and only need to meet, so these reactions are almost instant. Where strong bonds must be broken first, or where a solid reacts only at its surface, as in the rusting of iron, few collisions have enough energy, and the reaction is slow.
3. Write differences between
Single and double displacement reaction
Answer
| Single displacement | Double displacement |
|---|---|
| an element displaces another element (or radical) from its compound | two compounds exchange their atoms or radicals |
| the reactants are an element and a compound | both reactants are compounds |
| the products are an element and a compound | both products are compounds |
| happens only if the free element is more reactive than the one it displaces | often happens when one product is a precipitate, a gas or water |
| general form: AB + C → A + BC | general form: AB + CD → AD + CB |
| e.g. Zn + 2HCl → ZnCl₂ + H₂ | e.g. NaCl + AgNO₃ → AgCl + NaNO₃ |
Combination and decomposition reaction
Answer
| Combination | Decomposition |
|---|---|
| two or more reactants combine into one product | one reactant breaks into two or more products |
| general form: A + B → AB | general form: AB → A + B |
| often gives out heat | usually needs energy (heat, light or electricity) |
| e.g. 2Mg + O₂ → 2MgO | e.g. CaCO₃ → CaO + CO₂ |
4. Balance these equations and name their types
KClO₃ → KCl + O₂
Answer
Check: K 2 = 2, Cl 2 = 2, O 6 = 3 × 2. Decomposition (by heat, faster with MnO₂ as a catalyst).
CH₄ + O₂ → CO₂ + H₂O
Answer
Check: C 1 = 1, H 4 = 2 × 2, O 2 × 2 = 4 = 2 + 2. This is the burning of methane (biogas): a combustion reaction, in which a fuel reacts with oxygen giving heat and light. It doesn’t fit the four types neatly; it is an oxidation.
Fe + O₂ → Fe₂O₃
Answer
Check: Fe 4 = 2 × 2, O 3 × 2 = 6 = 2 × 3. Combination (iron and oxygen form one product).
Zn + H₂SO₄ → ZnSO₄ + H₂
Answer
It is already balanced: . Zn 1 = 1, H 2 = 2, S 1 = 1, O 4 = 4. Single displacement (zinc displaces hydrogen from the acid).
Al + H₂SO₄ → Al₂(SO₄)₃ + H₂
Answer
Check: Al 2 = 2, H 6 = 6, S 3 = 3, O 12 = 12. Single displacement.
5. Answer these questions
What do you mean by a chemical reaction? Explain with an example.
Answer
A chemical reaction is a process in which one or more substances (reactants) change into new substances (products) with different properties, as their atoms are rearranged by losing, gaining or sharing electrons. For example, when magnesium ribbon burns in air, magnesium and oxygen form a new white substance, magnesium oxide: 2Mg + O₂ → 2MgO.
What is the rate of chemical reaction? Write any four factors that affect it.
Answer
The rate of a chemical reaction is the amount of reactant changed into product per unit time. Four factors:
- i.Catalyst: a positive catalyst increases the rate, a negative one decreases it.
- ii.Temperature: a higher temperature increases the rate.
- iii.Surface area: a larger surface area of a solid reactant increases the rate.
- iv.Pressure (for gases): a higher pressure increases the rate.
Others: light (for reactions like photosynthesis) and concentration.
What do you mean by a displacement reaction? Write two examples with balanced equations.
Answer
A displacement reaction is one in which an atom or radical of one reactant is displaced (pushed out) by an atom or radical of another reactant; a more reactive element displaces a less reactive one from its compound. Examples:
Write two examples of decomposition reactions with balanced equations.
Answer
An iron nail hangs by a thread in copper sulphate solution in a test tube (Figure 15.3). What type of reaction takes place? Write its balanced equation.
Answer
A single displacement reaction. Iron is more reactive than copper, so it displaces copper from copper sulphate. The nail is coated with reddish-brown copper and the blue solution turns pale green (iron(II) sulphate):
Four students, A, B, C and D, were asked to carry out a decomposition reaction in the laboratory. A burnt magnesium ribbon; B put zinc into ferrous sulphate solution; C heated potassium chlorate (KClO₃) in a hard-glass test tube; D mixed zinc with hydrochloric acid. Which student followed the correct procedure? Write the balanced equation for that process.
Answer
Student C. Heating potassium chlorate breaks one reactant into two products:
The others did different reactions: A did a combination (2Mg + O₂ → 2MgO); B and D did single displacements (Zn + FeSO₄ → ZnSO₄ + Fe, and Zn + 2HCl → ZnCl₂ + H₂).
Samir burns a magnesium ribbon. It burns brightly and forms a white ash, A. Name substance A and write a balanced equation for the process.
Answer
A is magnesium oxide (MgO).
It is a combination reaction.