Edexcel A-Level Chemistry Algebra Skills

Practise Edexcel A-Level Chemistry algebra skills through equation rearrangement, rate constants, electrode-potential relationships and multi-step mole calculations.

Exam points

rearrange rate equations or gradient relationships to calculate k or enthalpy change
solve electrode-potential equations for concentration using E, E° and constants
combine density, percentage volume and molar mass steps to calculate amount of substance

Covered syllabus points

B.2.1—Mathematical symbols B.2.2—Changing the subject of an equation B.2.3—Substitution into algebraic equations B.2.4—Solving algebraic equations B.2.5—Logarithmic quantities

[Maximum number: 2]

This question is about the reaction between nitrogen monoxide and oxygen.

\mathrm{NO}(\mathrm{~g})+1 / 2 \mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons \mathrm{NO}_{2}(\mathrm{~g})

The results of a series of kinetics experiments are shown.

Table for Question B.2 Algebra question 1 — Edexcel A-Level Chemistry A2

The data for experiments 1-5 were plotted on a graph.

Figure for Question B.2 Algebra question 1 — Edexcel A-Level Chemistry A2

Calculate the rate constant for this reaction using the data from experiment 1 and your rate equation. Include units in your answer.

substitution or rearrangement

- answer and units

Example of a calculation:
$\begin{aligned} 0.040=\mathrm{k}(0.010)^{2}(0.05) \mathrm{k}=\frac{0.040}{(0.010)^{2}(0.05)} \end{aligned}$ $=8000 \mathrm{dm}^{6} \mathrm{~mol}^{-2} \mathrm{~s}^{-1}$
Allow units in any order
Allow $\mathrm{s}^{-}$
Allow $\mathrm{dm}^{6} / \mathrm{mol}^{2} \mathrm{~s}$ but not $\mathrm{dm}^{6} / \mathrm{mol}^{2} / \mathrm{s}$
Allow TE on a(iv) for both marks, units must match
order for M2
Correct answer with units scores 2, even if expression
in Q18(a)(iv) is incorrect

(2)

[Maximum number: 1]

This question is about electrochemical cells.

The Direct Methanol Fuel Cell (DMFC) uses methanol and oxygen to generate electricity. When the cell operates, the methanol reacts with water at one electrode to produce carbon dioxide and hydrogen ions.
At the other electrode the oxygen reacts with hydrogen ions to form water.

The reduction half-cell in the DMFC has a standard electrode potential $E^{\ominus}=+1.23 \mathrm{~V}$ .
When the solution concentration changes, the electrode potential changes so that $E=+1.20 \mathrm{~V}$ .
The relationship between these values is given by the equation

E=E^{\ominus}+4.277 \times 10^{-3} \ln \mathbf{X}

where X is the concentration of the solution in $\mathrm{mol} \mathrm{dm}^{-3}$ .
Calculate the value of X in the non-standard half-cell.

calculation of concentration X

Example of calculation:
$\begin{aligned} & c(X)=\text { inverse } \ln \left(\left((1.20-1.23) \div 4.277 \times 10^{-3}\right)\right) & =8.9897 \times 10^{-4}\left(\mathrm{~mol} \mathrm{dm}^{-3}\right) \end{aligned}$

Ignore SF except 1SF

Do not award $9 \times 10^{-4}$

Ignore any units given with the numerical value

[Maximum number: 4]

This question is about the homogeneous system

\mathrm{H}_{2}(\mathrm{~g})+\mathrm{I}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{HI}(\mathrm{~g})

(a)

Chemist B determined the value of K_c at five different temperatures.

Table for Question (a) — Edexcel A-Level Chemistry A2

[ 4 ]

(i)

The gradient of the graph from (c)(iii) is related to the enthalpy change of the reaction, H.

\text { gradient }=-\frac{\Delta H}{R}

Calculate H, in J mol^-1, using your value for the gradient.

[ 2 ]

(ii)

The free energy change for the reaction, G, is related to entropy and can be calculated from K_c.

\Delta G=-R T \ln K_{c}

Calculate G at 620 K, using this equation and data from the table.
Include units.

[ 2 ]

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Covered syllabus points

B.2 Algebra question 1

B.2 Algebra question 2

B.2 Algebra question 3

Question (a)

Question (i)

Question (ii)