Which statements about a redox reaction are correct?
- Oxidation is the gain of oxygen.
- Both oxidation and reduction take place in a redox reaction.
- Reduction is the gain of oxygen.
A) 1 and 2
B) 1 only
C) 2 and 3
D) 3 only
▶️ Answer/Explanation
Ans: A
Let’s evaluate each statement:
1. Correct – Oxidation can be defined as gain of oxygen (though it’s more accurately loss of electrons).
2. Correct – Redox reactions always involve both oxidation and reduction occurring simultaneously.
3. Incorrect – Reduction is actually the loss of oxygen (or gain of electrons/hydrogen).
Therefore, only statements 1 and 2 are correct, making option A the right answer.
Zinc is formed when zinc oxide is heated with carbon.
\[ \text{zinc oxide + carbon} \rightarrow \text{zinc + carbon monoxide} \]
Which substance is oxidised in this reaction?
A) carbon
B) carbon monoxide
C) zinc
D) zinc oxide
▶️ Answer/Explanation
Ans: A
Oxidation involves loss of electrons or gain of oxygen. Let’s analyze the reaction:
\[ \text{ZnO + C} \rightarrow \text{Zn + CO} \]
- Zinc oxide (ZnO) is reduced to Zn (loses oxygen)
- Carbon (C) is oxidized to CO (gains oxygen)
The oxidation numbers change as follows:
- Carbon goes from 0 (in C) to +2 (in CO) – oxidation
- Zinc remains +2 throughout (in ZnO and Zn, though elemental Zn is 0)
- Oxygen remains -2 throughout
Therefore, carbon is the substance that is oxidized in this reaction (Option A).
The equation for the reaction of magnesium with copper(II) oxide is shown.
\[ \text{Mg} + \text{CuO} \rightarrow \text{MgO} + \text{Cu} \]
Which word describes this reaction?
A) combustion
B) decomposition
C) neutralisation
D) redox
▶️ Answer/Explanation
Ans: D
This is a redox (reduction-oxidation) reaction because:
- Magnesium is oxidized (loses electrons, going from Mg to Mg²⁺ in MgO)
- Copper is reduced (gains electrons, going from Cu²⁺ in CuO to Cu)
It’s not:
- Combustion (no oxygen or burning involved)
- Decomposition (one compound isn’t breaking down into simpler substances)
- Neutralisation (no acid-base reaction occurring)
The transfer of oxygen from copper to magnesium and the change in oxidation states clearly indicate a redox reaction.
The equation for the reaction between hydrogen and oxygen is shown.
\[ 2H_2(g) + O_2(g) \rightarrow 2H_2O(g) \]
Which statement explains why this is a redox reaction?
A) Both oxidation and reduction take place.
B) Heat energy is released to the surroundings.
C) Hydrogen is a reactant.
D) The reaction can be reversed.
▶️ Answer/Explanation
Ans: A
A redox reaction involves both reduction and oxidation occurring simultaneously:
1. Oxidation: Hydrogen is oxidized as its oxidation state increases from 0 in H2 to +1 in H2O (loss of electrons).
2. Reduction: Oxygen is reduced as its oxidation state decreases from 0 in O2 to -2 in H2O (gain of electrons).
Option B describes an exothermic reaction (which this is, but doesn’t define it as redox). Option C is true but irrelevant to redox. Option D describes reversibility, not redox nature. Therefore, option A is correct as it identifies the simultaneous oxidation and reduction.