IB Chemistry HL 2.1 Amount of Chemical Change Questions

Practise IB Chemistry HL 2.1 by balancing equations, converting reacting amounts with mole ratios, identifying limiting reagents and calculating yield and atom economy.

Exam points

Balance chemical equations with molecular formulas, coefficients and state symbols.
Use equation coefficients to convert masses, concentrations or gas volumes into reacting amounts.
Compare reactant amounts with coefficients to identify limiting/excess reagents and calculate theoretical yield.
Calculate percentage yield from actual and theoretical product, then explain losses or yields below 100%.
Calculate atom economy from the balanced equation to compare desired-product use and waste.

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

[Maximum number: 1]

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

State the balanced chemical equation for the reaction of $\mathrm{PCl}_{3}(\mathrm{l})$ 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 $\operatorname{ICl}(l)$ , 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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Covered syllabus points

Question 1

Question 2

Question (a)

Question (i)

Question (ii)

Question (b)

Question (i)

Question (c)

Question (i)

Question 3

Question (a)

Question (i)

Question (ii)

Question (b)

Question (i)