IB Chemistry HL 2.1 Amount of Chemical Change Questions

Practise IB Chemistry HL 2.1 with HL questions on balanced equations, mole calculations, limiting reactants, yield and atom economy.

Exam points

Balance equations with state symbols, then use mole ratios to calculate masses, concentrations and gas volumes.
Identify limiting reactants and calculate theoretical, experimental and percentage yields from data.
Calculate atom economy and link a low value to waste in a green-chemistry judgement.

Covered syllabus points

2.1.1—Chemical equations 2.1.2—Mole ratio 2.1.3—Limiting reactant 2.1.4—Percentage yield 2.1.5—Atom economy

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Study 2.1 Amount of chemical change conceptsOpen the corresponding Concept lesson for explanations, examples and exam connections.

[Maximum number: 1]

Phosphoryl chloride, $\mathrm{POCl}_{3}$ , is a dehydrating agent.

State the balanced chemical equation for the reaction of $\mathrm{PCl}_{3}(1)$ with water.

$\mathrm{PCl}_{3}(\mathrm{l})+3\mathrm{H}_{2}\mathrm{O}(\mathrm{l}) \rightarrow \mathrm{H}_{3}\mathrm{PO}_{3}(\mathrm{aq})+3\mathrm{HCl}(\mathrm{aq})$ ;

[Maximum number: 6]

Two groups of students (Group A and Group B) carried out a project* on the chemistry of some group 7 elements (the halogens) and their compounds.

(a)

In the first part of the project, the two groups had a sample of iodine monochloride (a corrosive brown liquid) prepared for them by their teacher using the following reaction.

\mathrm{I}_{2}(\mathrm{~s})+\mathrm{Cl}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{ICl}(\mathrm{l})

The following data were recorded.

Table for Question (a) — IB Chemistry HL

[ 4 ]

(i)

The iodine used in the reaction was in excess. Determine the theoretical yield, in g , of ICl(l).

[ 3 ]

(ii)

Calculate the percentage yield of ICl(l).

[ 1 ]

$\quad\left(\frac{8.60}{10.3} \times 100=\right) 83.5 \%$ ;

(b)

The students reacted ICl(l) with CsBr(s) to form a yellow solid, $\mathrm{CsICl}_{2}(\mathrm{~s})$ , as one of the products. $\mathrm{CsICl}_{2}(\mathrm{~s})$ has been found to produce very pure CsCl(s) which is used in cancer treatment.

To confirm the composition of the yellow solid, Group A determined the amount of iodine in 0.2015 g of $\mathrm{CsICl}_{2}(\mathrm{~s})$ by titrating it with $0.0500 \mathrm{moldm}^{-3} \mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{aq})$ . The following data were recorded for the titration.

Table for Question (b) — IB Chemistry HL

[ 1 ]

(i)

The overall reaction taking place during the titration is:

Calculate the amount, in mol, of iodine atoms, I , present in the sample of $\mathrm{CsICl}_{2}(\mathrm{~s})$ .

[ 1 ]

$\left(0.5 \times 1.21 \times 10^{-3}\right)=6.05 \times 10^{-4} / 0.000605(\mathrm{~mol})$ ;

(c)

In this project the students explored several aspects of the chemistry of the halogens. In the original preparation of $\operatorname{ICl}(1)$ , they observed the yellow-green colour of chlorine gas, $\mathrm{Cl}_{2}(\mathrm{~g})$ , reacting with solid iodine, $\mathrm{I}_{2}(\mathrm{~s})$ .

[ 1 ]

(i)

Chlorine can also react with water. State the balanced chemical equation for the reaction of $\mathrm{Cl}_{2}(\mathrm{~g})$ with water.

[ 1 ]

$\mathrm{Cl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{HCl}(\mathrm{aq})+\mathrm{HOCl}(\mathrm{aq})$ ;

[Maximum number: 3]

Phosphine (IUPAC name phosphane) is a hydride of phosphorus, with the formula $\mathrm{PH}_{3}$ .

(a)

2.478 g of white phosphorus was used to make phosphine according to the equation:

\mathrm{P}_{4}(\mathrm{~s})+3 \mathrm{OH}^{-}(\mathrm{aq})+3 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{PH}_{3}(\mathrm{~g})+3 \mathrm{H}_{2} \mathrm{PO}_{2}^{-}(\mathrm{aq})

[ 2 ]

(i)

This phosphorus was reacted with $100.0 \mathrm{~cm}^{3}$ of $5.00 \mathrm{~mol} \mathrm{dm}^{-3}$ aqueous sodium hydroxide. Deduce, showing your working, which was the limiting reagent.

[ 1 ]

(ii)

Determine the excess amount, in mol , of the other reagent.

[ 1 ]

amount in excess $\boldsymbol{<}=0.500-(3 \times 0.02000) \underset{0}{0}=0.440$ «mol»

(b)

Impurities cause phosphine to ignite spontaneously in air to form an oxide of phosphorus and water.

[ 1 ]

(i)

State the equation for the reaction of this oxide of phosphorus with water.

[ 1 ]

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