2022-may-Chemistry_paper_2__TZ2_HL Detailed Solution

Question-1(a) :2022-may-Chemistry_paper_2__TZ2_HL

Topic:

Given: Lithium reacts with water to form an alkaline solution.

Determine: the coefficients that balance the equation for the reaction of lithium with water.

$$
\ldots \mathrm{Li}(\mathrm{s})+\ldots \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \rightarrow \ldots \mathrm{LiOH}(\mathrm{aq})+\ldots \mathrm{H}_2(\mathrm{~g})
$$

Answer/Explanation

Solution:

The balanced equation for the reaction of lithium with water is:
$$
2 \mathrm{Li}(\mathrm{s})+2 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \rightarrow 2 \mathrm{LiOH}(\mathrm{aq})+\mathrm{H}_2(\mathrm{~g})
$$
In this equation, we need two lithium atoms and two water molecules to react and form two molecules of lithium hydroxide and one molecule of hydrogen gas. The coefficients in front of each reactant and product are used to balance the equation and ensure that the law of conservation of mass is satisfied.

Question-1[ (b) (i)] :2022-may-Chemistry_paper_2__TZ2_HL

Topic:

Given: A $\mathrm0.200 \mathrm{~g}$ piece of lithium was placed in $500.0 \mathrm{~cm}^3$ of water.

 Calculate: the molar concentration of the resulting solution of lithium hydroxide.

Answer/Explanation

Solution:

First, let’s calculate the number of moles of lithium used in the reaction:

moles of $\mathrm{Li}=\frac{\text { mass of } \mathrm{Li}}{\text { molar mass of } \mathrm{Li}}=\frac{0.200 \mathrm{~g}}{6.941 \mathrm{~g} / \mathrm{mol}} \approx 0.0288 \mathrm{~mol}$

$\mathrm{n}_{\mathrm{LiOH}}=\mathrm{n}_{\mathrm{Li}}=0.0288 \mathrm{~mol}$

The volume of the solution is given as $500.0~\mathrm{cm^3}$, which is equivalent to $0.5000~\mathrm{dm^3}$. Therefore, the molar concentration of the resulting solution of lithium hydroxide is:

Molarity $=\frac{\text { moles of } \mathrm{LiOH}}{\text { volume of solution }}=\frac{0.0144 \mathrm{~mol}}{0.500 \mathrm{dm^3}} \approx 0.0576 \mathrm{mol-dm^{-3}}$

So the molar concentration of the resulting solution of lithium hydroxide is approximately $0.0576~\mathrm{M}$.

Question-1[(b)(ii) ] :2022-may-Chemistry_paper_2__TZ2_HL

Topic:

Calculate: the volume of hydrogen gas produced, in $\mathrm{cm}^3$, if the temperature was $22.5^{\circ} \mathrm{C}$ and the pressure was $103 \mathrm{kPa}$. Use sections 1 and 2 of the data booklet.

Answer/Explanation

Solution:

We know from the balanced equation that 2 moles of lithium react with 1 mole of hydrogen gas. Therefore, the number of moles of hydrogen gas produced is:

$n_{H_2}=\frac{1}{2} \times 0.0288 \mathrm{~mol}=0.0144 \mathrm{~mol}$

Using the ideal gas law, we can calculate the volume of hydrogen gas produced at $22.5^{\circ} \mathrm{C}$ and $103~\mathrm{kPa}$:

$V=\frac{n R T}{P}=\left(\frac{0.0144 \mathrm{~mol} \times 8.31 \mathrm{JK}^{-1} \mathrm{~mol}^{-1} \times(22.5+273) \mathrm{K}}{103 \mathrm{kPa}}\right) \times 10^3 \approx 343 \mathrm{~cm}^3$

Therefore, the volume of hydrogen gas produced at $22.5^{\circ} \mathrm{C}$ and $103~\mathrm{kPa}$ is approximately $\boxed{$343~\mathrm{cm^3}$}$.

Question-1[(b) (iii)]:2022-may-Chemistry_paper_2__TZ2_HL

Topic:

Discuss: a reason why the volume of hydrogen gas collected was smaller than predicted.

Answer/Explanation

Solution:

One possible reason why the volume of hydrogen gas collected was smaller than predicted could be the presence of water vapor in the gas collected. When lithium reacts with water, it not only produces hydrogen gas, but also lithium hydroxide and heat. If the reaction is conducted in an open container, the hydrogen gas produced will mix with the surrounding air and water vapor, which will increase the total volume of gas collected. However, if the gas is collected in a closed container, any water vapor produced by the reaction will also be collected along with the hydrogen gas, which will decrease the volume of the collected gas. Therefore, the actual volume of hydrogen gas collected could be smaller than predicted due to the presence of water vapor in the collected gas.

Question-1(c) :2022-may-Chemistry_paper_2__TZ2_HL

Topic:

Discuss: The reaction of lithium with water is a redox reaction. Identify the oxidizing agent in the reaction giving a reason.

Answer/Explanation

Solution:

In the reaction of lithium with water, lithium is oxidized and water is reduced. The half-reactions for this reaction can be written as:

Oxidation half-reaction: $\mathrm{Li} \rightarrow \mathrm{Li}^+ + \mathrm{e}^-$

Reduction half-reaction: $\mathrm{2H_2O} + \mathrm{2e}^- \rightarrow \mathrm{H_2} + \mathrm{2OH}^-$

In the oxidation half-reaction, lithium loses an electron and is therefore oxidized to form lithium ions. In the reduction half-reaction, water gains electrons and is therefore reduced to form hydroxide ions and hydrogen gas.

The oxidizing agent is the substance that is reduced, because it gains electrons and causes another substance to lose electrons. In this reaction, the oxidizing agent is water, because it is reduced by gaining electrons to form hydroxide ions and hydrogen gas.

Question-1(d) :2022-may-Chemistry_paper_2__TZ2_HL

Topic:

Discuss: two observations that indicate the reaction of lithium with water is exothermic.

Answer/Explanation

Solution:

An exothermic reaction is a chemical reaction that releases heat to the surroundings. In the reaction of lithium with water, the following two observations can indicate that the reaction is exothermic:

  1. Temperature increase: When lithium is added to water, the temperature of the solution increases, which indicates that heat is being released by the reaction. This is because the reaction of lithium with water is highly exothermic, and the heat released by the reaction increases the temperature of the surrounding solution.

  2. Formation of bubbles: When lithium reacts with water, hydrogen gas is produced as a product. The hydrogen gas is formed as bubbles, which can be seen rising to the surface of the solution. The formation of bubbles indicates that gas is being produced, which is another sign that a chemical reaction is occurring. Additionally, the formation of bubbles can be accompanied by a hissing or popping sound, which is a further indication that heat is being released by the reaction.

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