IBDP Chemistry - Reactivity 1.1 Measuring enthalpy changes- IB Style Questions For SL Paper 1A - FA 2025

Question

Which pair of elements would react with one another most vigorously?

(A) \(K(s)\) and \(Br_{2}(g)\)
(B) \(K(s)\) and \(Cl_{2}(g)\)
(C) \(Na(s)\) and \(Br_{2}(g)\)
(D) \(Na(s)\) and \(Cl_{2}(g)\)

▶️ Answer/Explanation

Detailed solution

1. Reactivity of Alkali Metals (Group 1):
Reactivity increases down the group because ionization energy decreases (easier to lose an electron). Therefore, Potassium (\(K\)) is more reactive than Sodium (\(Na\)).

2. Reactivity of Halogens (Group 17):
Reactivity increases up the group because electronegativity increases (easier to gain an electron). Therefore, Chlorine (\(Cl_2\)) is more reactive than Bromine (\(Br_2\)).

3. Combine for Most Vigorous Reaction:
The most vigorous reaction will occur between the most reactive metal and the most reactive non-metal.
Most reactive metal: \(K\).
Most reactive non-metal: \(Cl_2\).
Pair: \(K(s)\) and \(Cl_2(g)\).
✅ Answer: (B)

Question

Which processes have a negative enthalpy change?

I. \(2\text{CH}_3\text{OH}(l) + 3\text{O}_2(g) \rightarrow 2\text{CO}_2(g) + 4\text{H}_2\text{O}(l)\)

II. \(\text{HCl}(aq) + \text{NaOH}(aq) \rightarrow \text{NaCl}(aq) + \text{H}_2\text{O}(l)\)

III. \(\text{H}_2\text{O}(g) \rightarrow \text{H}_2\text{O}(l)\)

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

▶️ Answer/Explanation

Detailed solution

A negative enthalpy change (\(\Delta H < 0\)) means the process is exothermic, releasing heat to the surroundings.

Process I: Combustion of methanol. Combustion reactions are strongly exothermic, so \(\Delta H\) is negative.

Process II: Neutralization of a strong acid by a strong base. Neutralization reactions \(\text{acid} + \text{base} \rightarrow \text{salt} + \text{water}\) are exothermic, so \(\Delta H\) is negative.

Process III: Condensation of water vapor to liquid water. When a gas condenses to a liquid, intermolecular forces form and energy is released, so this is also exothermic (\(\Delta H < 0\)).

Therefore, all three processes I, II and III have negative enthalpy changes.

✅ Answer: (D)

Question

Which combination is correct for the exothermic reaction that occurs between zinc and copper sulfate solution.

	Temperature of solution	Heat released to surroundings	Enthalpy of products greater than enthalpy of reactants
A	increases	yes	yes
B	decreases	no	no
C	increases	yes	no
D	decreases	no	yes

▶️ Answer/Explanation

Detailed solution

The reaction between zinc and copper(II) sulfate solution is exothermic. For an exothermic process:

The system releases heat to the surroundings, so heat released to surroundings: yes.
The temperature of the solution (surroundings) therefore increases.
The enthalpy of the products is lower than that of the reactants, so the statement “enthalpy of products greater than enthalpy of reactants” is no.

This combination (temperature increases, heat released to surroundings yes, products not higher in enthalpy) corresponds to option C.

✅ Answer: (C)

IBDP Chemistry - Reactivity 1.1 Measuring enthalpy changes- IB Style Questions For SL Paper 1A - FA 2025

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