energetics

Question 1

This question is about combustion.
(a) State the meaning of the term standard enthalpy of combustion.

Answer 1

Enthalpy change when one mole of a substance burns completely in
oxygen
Allow heat energy change / allow fully combust
1
With all substances in their standard states (at stated temperature and
100kPa)

Question 2

This question is about enthalpy changes.
(a) Define the term enthalpy change

Answer 2

Heat (energy) change at constant pressure

Question 3

Explain why the value given for the O=O bond enthalpy in part (b) is not a
mean value.

Answer 3

Oxygen / O2 is the only substance that has O=O bond

Question 4

The student considered using a glass beaker on a tripod and gauze instead
of the clamped copper calorimeter.
Suggest two disadvantages of using a glass beaker on a tripod and gauze.
Disadvantage 1 ______________________________________________
___________________________________________________________

Disadvantage 2 _

Answer 4

Glass is a poorer conductor than copper
M1
Tripod and gauze would reduce heat transfer

Tripod and gauze would have a fixed height above the flame

Question 5

Suggest two reasons why the value of enthalpy of combustion from this
experiment is less exothermic than a data book value.

Answer 5

Heat loss to surroundings or to copper/calorimeter
M1
Incomplete combustion

Question 6

v Suggest one addition to this apparatus that would improve the accuracy of
the enthalpy value obtained.

Answer 6

w use a lid

Question 7

A student determines the enthalpy change for the reaction between
calcium carbonate and hydrochloric acid.
CaCO3(s) + 2 HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)
The student follows this method:
* measure out 50 cm3 of 1.00 mol dm–3 aqueous hydrochloric acid
using a measuring cylinder and pour the acid into a 100 cm3 glass
beaker
* weigh out 2.50 g of solid calcium carbonate on a watch glass and tip
the solid into the acid
* stir the mixture with a thermometer
* record the maximum temperature reached.

The student uses the data to determine a value for the enthalpy change.
Explain how the experimental method and use of apparatus can be
improved to provide more accurate data.
Describe how this data from the improved method can be used to
determine an accurate value for the temperature change.

Answer 7

Stage 1: Apparatus
1a. Use a burette/pipette (instead of a measuring cylinder)
1b. Use a polystyrene cup (instead of a beaker) / insulate beaker
1c. Reweigh the watchglass after adding the solid 1d: Use powdered solid

Stage 2: Temperature Measurements
2a. Measure and record the initial temperature of the solution for a few
minutes before addition
2b. Measure and record the temperature after the addition at regular
intervals (eg each minute) for 8+ minutes/until a trend is observed
Stage 3: Temperature Determination

3a. Plot a graph of temperature against time
3b. Extrapolate to the point of addition
3c. Determine ΔT at the point of addition

Question 8

Suggest how, without changing the apparatus, the experiment in part (c)
could be improved to reduce the percentage uncertainty in the temperature
change.

Answer 8

Increase the concentration of the solutions

Question 9

) State why the heat change calculated from the bomb calorimeter
experiment is not an enthalpy change

Answer 9

pressure not constant in bomb calorimeter

Question 10

The thermometer used to measure the temperature change of 12.2 °C in
part (b) has an uncertainty of ± 0.1 °C in each reading.
Calculate the percentage uncertainty in this use of the thermometer.
Suggest one change to this experiment that decreases the percentage
uncertainty while using the same thermometer.

Answer 10

100 × 0.2 = 1.64%
12.2
Allow 1.6%
Allow 2% if working shown
NOT 2.0%
1
use bigger mass of fuel (so ΔT greater)

Question 11

) A data book value for the enthalpy of combustion of cyclohexane is –3920
kJ mol–1
The student concluded that the temperature rise recorded in the
experiment was smaller than it should have been.
Suggest a practical reason for thisthermal energy / heat loss or
or idea of heat being transferred to calorimeter

Answer 11

thermal energy / heat loss or
or idea of heat being transferred to calorimeter
incomplete combustion or
allow idea that it is not under standard conditions
evaporation
all

Question 12

) A student recorded the temperature of aqueous ethanoic acid in a
polystyrene cup for three minutes.
At the fourth minute, the student added sodium hydrogen carbonate.
The student stirred the mixture and carried on recording the temperature
every minute for several minutes.
The student’s measurements are shown in the graph.
A best-fit line showing the temperature before mixing has been drawn.
Draw an appropriate best-fit line on the graph and use it to find the
temperature change at the time of mixing

Answer 12

learn this for energetics

Question 13

This question is about energetics.
(a) Write an equation, including state symbols, for the reaction with an
enthalpy change equal to the enthalpy of formation for iron(III) oxide

Answer 13

2Fe(s) + O2(g) ⟶ Fe2O3(s) ONLY

Question 14

A 50.0 g sample of water was used in this experiment.
Explain how you could measure out this mass of water without using a
balance.

Answer 14

Water has a known density (of 1.0 g cm–3
)
1
Therefore, a volume of 50.0 cm3 could be measured out

Question 15

Write an equation, including state symbols, for the reaction with an
enthalpy change equal to the enthalpy of formation for iron(III) oxide.

Answer 15

2Fe(s) + 3/2O2(g) ⟶ Fe2O3(s) ONLy

Question 16

) State how your answer to part (b) is likely to differ from the value quoted in
reference sources.
Give one reason for your answer

Answer 16

Less negative than the reference
1
Heat loss OR incomplete combustion OR evaporation of alcohol OR
heat transferred to beaker not taken into accoun