OCR A-Level Chemistry Unified chemistry (03), June 2017: Question 2

Question Answer Marks Guidance

2 (a) Please refer to the marking instructions on page 5 of this 6 Indicative scientific points may include:

mark scheme for guidance on how to mark this question. 1. Masses and ∆T from raw results

Level 3 (5–6 marks) m(Na2O) = 1.24 (g)

A comprehensive conclusion, using all quantitative data, to m(solution) = 25.75 (g)

calculate the energy change and ∆H values for reactions ∆T = 35.0 (ºC)

3.1 and 3.2

AND linking ∆H data using Hess’ Law Energy change from mc∆T

energy released in J OR kJ

There is a well-developed line of reasoning which is clear = 25.75 × 4.18 × 35.0

and logically structured. The working throughout is clearly = 3767 (J) OR 3.767 (kJ)

shown. All values calculated with reasonable numbers of (3.767225 unrounded)

SF and correct signs mostly shown, allowing for ECF. ----------------------------------------------

2. ∆rH for reaction 3.2

Level 2 (3–4 marks) 1.24

Attempts to describe all three scientific points but n(Na2O) = 62.0 = 0.0200 (mol)

explanations may be incomplete. 3767

∆ H value – = –188 (kJ mol–1)

OR Explains two scientific points thoroughly with few r 0.0200

omissions. (–188.36125 unrounded)

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There is a line of reasoning with some logical structure. 3. ∆ H for reaction 3.1

r

There may be minor errors in energy change and errors in

∆H value for reaction 3.1 clearly linked to ∆H

the calculations of ∆H for reaction 3.1 or reaction 3.2. for reaction 3.2 and reaction 3.3 in energy

cycle or an expression:

Level 1 (1–2 marks)

∆H(3.1) = ∆H(3.2) + 2∆H(3.3)

Processes raw mass and temperature data and obtains a

∆H(3.1) = –188 + (2 –57.6)

calculated value for the energy change using mc∆T –1

= –188 – 115.2 = –303(.2) (kJ mol )

OR attempts to obtain values for two scientific points but

(–303.56125 unrounded)

explanations may be incomplete

Note

There is an attempt at a logical structure with a line of

Throughout, ALLOW ECF from previous value

reasoning to obtain a value for energy change. There may

ALLOW omission of trailing zeroes

be minor errors in calculation of energy change.

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0 marks – No response or no response worthy of credit.

Question Answer 9 Marks Guidance

(b) % uncertainties to at least 1 SF, rounded or truncated 2 ALLOW error for uncertainty

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ONE correct % uncertainty ALLOW ECF from mass and ∆T in 2(a)

BOTH correct % uncertainties IGNORE % uncertainty of mass of solution

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0.005 × 2 ALLOW one mark for:

mass: 1.24 100 = 0.8/0.81 OR 0.80 (truncated)

2 calculations with both 2 factors missing

0.1 × 2 i.e. mass 0.3% AND ∆T 0.4%

∆T: 35.0 100 = 0.6 / 0.57 (%)

Not converting to %s using 2 factors

Calculator values: i.e. 0.008 AND 0.006

mass: 0.8064516129

∆T: 0.5714285714

(c) ALLOW uncertainty OR error throughout 2 ALLOW up to 2 marks based on a single mass

measurement:

Greater mass of Na2O OR more Na2O one mass measurement

For mass, ALLOW amount/moles/quantity OR measure mass directly

e.g. tare balance

larger ∆T % uncertainty reduced by half

OR reduces % uncertainty in ∆T ------------------------------------------------------------

IGNORE

repeat and take average

read to more figures (same apparatus)

increase volume

(reduces mass error but increases ∆T error)

use a cooling curve

use a lid

(d) (i) sodium nitrate(III) 1 ALLOW sodium nitrite OR sodium nitrite(III)

(d) (ii) Sodium/Na oxidised from 0 to +1 2 ALLOW 1+ for +1 and 3+ for +3

Nitrogen/N reduced from +3 to 0 ALLOW N2 for nitrogen

ALLOW 1 mark for elements AND all oxidation

numbers correct, but N on oxidised line and Na

on reduced line

‘+’ is required in +3 and +1 oxidation numbers

(d) (iii 2NaNO2 + 6Na 4Na2O + N2 1 ALLOW multiples, e.g.

) NaNO2 + 3Na 2Na2O + ½N2

IGNORE state symbols

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