Enthalpy 3 due 6/1/17
/24
%
NAME:
Questions
Q1.
The enthalpy change for the reaction between hydrochloric acid and sodium hydroxide is −56 kJ mol−1.
Therefore
A
the reaction is exothermic and the temperature rises.
B
the reaction is exothermic and the temperature falls.
C
the reaction is endothermic and the temperature rises.
D
the reaction is endothermic and the temperature falls.
(Total for Question = 1 mark)
Q2.
The enthalpy change for the reaction to form hydrated sodium thiosulfate crystals cannot be measured
directly.
The following Hess cycle can be used.
The enthalpy change for the reaction, ΔHr, is equal to
A ΔH1 + ΔH2
B ΔH1 − ΔH2
C −ΔH1 − ΔH2
D −ΔH1 + ΔH2
(Total for question = 1 mark)
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Enthalpy 3 due 6/1/17
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NAME:
Q3.
The molar enthalpy change of combustion of some alkanes is given below in kJ mol−1.
Another alkane was found to have an enthalpy change of combustion of −6125 kJ mol−1
The alkane is
A
C7H16
B
C8H18
C
C9H20
D
C10H22
(Total for question = 1 mark)
2
Enthalpy 3 due 6/1/17
/24
%
NAME:
Q4.
The standard enthalpy change, ΔH1 , for the decomposition of potassium hydrogencarbonate,
KHCO3, is impossible to determine directly.
The value of ΔH1
reactions below:
can be calculated from the standard enthalpy changes which accompany the
Procedure:
 The solids were added to separate 30 cm3 portions of dilute hydrochloric acid. The acid was in
excess for both solids.
 The maximum temperature change for each experiment was noted.
Results:
The following results were obtained with KHCO3(s).
Mass of KHCO3 used = 2.00 g
Temperature change = −4.9 °C
The experiment with K2CO3(s) gave a ΔH3
value of −34 kJ mol−1.Assumption:
The dilute hydrochloric acid solution has a density of 1 g cm−3.
(a) (i) Calculate the heat energy absorbed, in joules, by the reaction of the KHCO3(s) with the solution
of dilute hydrochloric acid.
Use the expression
energy absorbed (J) = mass of solution × 4.18 × temperature change
(1)
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Enthalpy 3 due 6/1/17
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NAME:
(ii) Calculate the number of moles of KHCO3(s) used. Assume that the molar mass
of KHCO3(s) is 100 g mol−1.
(1)
(iii) Use your answers to (a)(i) and (ii) to calculate, in kJ mol−1, the enthalpy change
when one mole of KHCO3(s) reacts completely with the acid (i.e. ΔH2 ).
Include a sign in your answer.
(2)
(b) A Hess Cycle based on these reactions is shown below.
(i) Apply Hess's Law to obtain an expression for ΔH1
in terms of ΔH2
and ΔH3
(1)
ΔH1 =
4
Enthalpy 3 due 6/1/17
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NAME:
(ii) Use your answers to (a)(iii) and (b)(i), and the ΔH3 value of −34 kJ mol−1, to
calculate a value for ΔH1 in kJ mol−1. Include a sign in your answer.
(2)
(c) The maximum errors for the apparatus used in the experiment with the KHCO 3(s)
were as follows:
Balance
0.01g
Measuring cylinder
0.5 cm3
(i) Calculate the maximum percentage error in using each of the following pieces
of apparatus in the KHCO3(s) experiment:
(2)
Balance
Measuring cylinder
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Enthalpy 3 due 6/1/17
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%
NAME:
(ii) Suggest a piece of apparatus that could have been used to measure the volume
of dilute hydrochloric acid more accurately in this experiment.
(1)
..............................................................................................................................................
(Total for question = 10 marks)
Q5.
Sodium hydrogencarbonate decomposes on heating to form sodium carbonate. It is difficult to measure
the enthalpy change of this reaction directly.
2NaHCO3(s) → Na2CO3(s) + CO2(g) H2O(l)
One method of determining this enthalpy change is to react known amounts of sodium
hydrogencarbonate and sodium carbonate, separately, with excess dilute hydrochloric acid.
(a) 0.010 mol of solid sodium hydrogencarbonate was added to 25 cm3 of dilute hydrochloric acid. A
temperature rise of 11 °C was measured using a thermometer graduated at 1 °C intervals.
(i) Calculate the heat energy produced by this reaction using the equation:
Energy transferred in joules = mass × 4.18 × change in temperature
(1)
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Enthalpy 3 due 6/1/17
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NAME:
(ii) Calculate the standard enthalpy change for the reaction when one mole of sodium
hydrogencarbonate reacts with hydrochloric acid.
Remember to include a sign and units with your answer which should be given to three significant figures.
(2)
*(b) The standard enthalpy change for the reaction between sodium carbonate and dilute hydrochloric
acid is found by a similar method to be
ΔH
= −321.6 kJ mol−1
Complete the Hess energy cycle below by adding the missing arrow and entities. Use it to calculate the
standard enthalpy change for the decomposition of two moles of sodium hydrogencarbonate as in the
equation below.
Remember to show your reasoning clearly.
(5)
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Enthalpy 3 due 6/1/17
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NAME:
(c) The uncertainty for each thermometer reading is ± 0.5 °C.
Calculate the percentage error in the temperature rise of 11 °C.
(1)
(d) Sodium hydrogencarbonate is used in cooking. Suggest what it is used for and how it works.
(2)
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(Total for Question = 11 marks)
8