Enthalpy Homework 1
1. The diagram shows the distribution of kinetic energies in a sample of
gas at 20°C.
number of
molecules
kinetic energy of the molecules (kJ)
a) Copy the diagram and add another line to show the kinetic energy
distribution of the molecules at 30°C.
1
b) Draw a line to represent the activation energy of a reaction which is
slow at 20°C.
1
c) With reference to the completed diagram explain why an increase of
temperature of 10°C can lead to a large increase in reaction rate.
2
2. What is meant by the term ‘activation energy”?
1
3. What is meant by the term ‘activated complex’?
1
4.
Potential
energy
(kJ)
reaction pathway
a)i) Is this an endothermic or an exothermic process?
ii) Explain your answer.
b) What is the value of:
i) the activation energy for the forward reaction?
ii) the enthalpy change for the reaction?
iii) the energy of the activated complex?
1
1
1
1
1
c) The reaction shown can be speeded up by the use of a suitable
catalyst.
What effect does a catalyst have on
i) the enthalpy change for the reaction?
ii) the activation energy for the forward reaction
2HI(g)
H2(g) + I2(g)
5.
1
1
The reaction above is reversible. The activation energy for the forward
reaction is 80 kJ and the reverse reaction is 50 kJ.
a) On the graph paper shown below how the potential energy varies as
the reaction proceeds.
160
140
120
Potential
100
energy
(kJ)
80
60
40
reactants
20
0
reaction pathway
b) Is the forward reaction is exothermic or endothermic.
c) Gold and platinum both catalyse the reaction. For the forward
reaction EA using gold is 30 kJ, while EA using platinum is 20 kJ.
1
i) using different dotted lines add this information to the graph. 2
ii) which is the better catalyst for the reaction? Explain your choice.
2
d) The gold and platinum catalysts are used in the solid state. Are the
catalysts heterogeneous or homogeneous catalysts? Explain your choice
2
Total = 20 marks
Enthalpy Homework 2
1. An oil company leaflet on “Unleaded Motoring” gives the following
advice.
‘Cars fitted with catalytic converters must use unleaded petrol only’
Explain the reason for this advice.
2
2. a) Describe in terms of adsorption how a heterogeneous catalyst
works.
b) Describe how a catalyst poison stops a catalyst working.
1
3.
800
Potential
energy
(kJ)
600
400
200
Reaction pathway
a) What is the activation energy for
i) the forward reaction
ii) the back reaction?
2
b) What is the enthalpy change for the formation of products?
1
c) Is this reaction endothermic or exothermic? Explain your answer. 1
d) On the diagram and add a dotted line to show the effect of using a
catalyst.
1
e) Mark with an X the position where the activated complex would be
formed.
1
4. The effect of temperature on reaction rate can be studied using the
reaction between oxalic acid and acidified potassium permanganate
solution.
(a) What colour change would indicate that the reaction was complete?1
(b) A student’s results are shown on the graph below.
(i) Use the graph to calculate the reaction time, in s, at 40oC.
1
(ii) Why is it difficult to obtain an accurate titration time when the
reaction is carried out at room temperature?
1
Total = 12 marks
Enthalpy Homework 3
1. a) Write a balanced equation for the complete combustion of
propane.
1
1 b) Using the enthalpy of combustion of butane given in the data booklet
calculate the amount of heat energy given out when 5.5 g of propane is
completely burned.
2
2. Calculate the temperature rise when 500 cm3 of water at 18°C is
heated by completely burning 2.2 g of propane. (Assume no heat losses.
Take the specific heat capacity of water to be 4.18 kJ kg–1 oC–1, and use
the heat of combustion of propane given in the data booklet).
3
3. When 0.1 g of ethanol (C2H5OH) is burned in oxygen the heat energy
given out raised the temperature of 500 cm3 of water from 20.3oC to
21.7oC. Calculate the enthalpy of combustion, in kJ mol-1, of ethanol. 3
4. When 10g of potassium bromide (KBr) was dissolved in water the
temperature of 100 cm3 of water fell from 20.1oC to 15.9oC. Using this
data along with the specific heat capacity of water given in the data
booklet, calculate the enthalpy of solution of potassium bromide.
3
5. When 100cm3 of 1 mol l-1 hydrochloric acid was reacted with 100cm3 of
1 mol l-1 potassium hydroxide the temperature of the mixture rose by
6.5oC. Calculate the enthalpy of neutralisation.
3
6. The enthalpies of combustion of methanol (CH3OH), ethanol (C2H5OH)
and propanol (C3H7OH) are –726, -1370 and –2020 kJ mol-1 respectively.
a) i) Why is there a regular increase in the enthalpies of combustion from
methanol to propanol?
1
ii) Estimate the enthalpy of combustion of butanol (C4H9OH).
1
b) Calculate the amount of heat energy given out by the complete
combustion of 1.6g of methanol.
2
c) i) Draw a labelled diagram of the apparatus you would use in the
laboratory to estimate the enthalpy of combustion of methanol.
2
ii) Give two reasons why the value obtained using this apparatus is less
than the accepted value given in the data booklet.
2
Total = 23
Enthalpy Homework 4
1. Sketch potential energy diagrams for the following reactions:
a. Enthalpy of reactants = 20kJ : ΔH = -30kJ : Ea = 15kJ
2
b. Enthalpy of products = 10kJ : ΔH = 10kJ : Ea = 35kJ
2
2. a. Explain why the enthalpy changes for many reactions may have a
negative or positive sign, but the enthalpies of combustion are
always negative.
2
b. In an experiment 0.7g of ethanol (CH3CH2OH) is used up when a
spirit burner heats 100g of water from 20.1oC to 35.4oC. Calculate
the enthalpy of combustion for ethanol (assume specific heat
capacity of water = 4200J/KgoC).
2
c. Suggest a reason why this value is much less than the value in
the Data Book which is -1371kJmol-1.
1
3.
a. Calculate the heat energy required to raise the temperature of
2.5Kg of water from 10oC to 50oC.
2
b. Find the heat energy required to raise the temperature of
500ml of water by 40oC.
2
4.
Calculate the enthalpy of combustion for methanol (CH3OH) if 8g
of the alcohol releases 181.5kJ when completely burned.
2
5.
5.8g of butane releases enough energy when completely burned to
heat 1Kg of water from 20oC to 90oC. Calculate the heat of
combustion for butane.
2
6.
a. Complete “the enthalpy of solution of a substance is the enthalpy
change when…”.
1
b. When 1g of sodium hydroxide was dissolved in 100ml of water,
the temperature increased by 10oC. Calculate the enthalpy of
solution for sodium hydroxide.
2
Total = 20
Enthalpy Homework 5
1. Calculate the enthalpy change for each of the following
experiments:
a. When 1g of potassium carbonate dissolves in 10cm3 of water the
temperature increased by 5.6oC.
2
b. When 1g of aluminium fluoride dissolves in 100cm3 of water the
temperature increased by 6oC.
2
2. a. Complete “the enthalpy of neutralisation of an acid is the
enthalpy change when …”.
1
Write balanced chemical equations for each of the following and
show the ΔH value with the appropriate sign in each case (show all
state symbols).
b. The neutralisation of 1 mole of sodium hydroxide by nitric acid
releases 57.3kJ of energy.
2
c. The neutralisation of 2moles of hydrochloric acid by barium
hydroxide releases 114.4kJ of energy.
2
d. The enthalpy of combustion of sulphur is -297kJmol-1.
2
e. When 1 mole of hydrogen burns in oxygen 286kJ of energy are
released.
2
3.
When 1g of ethanol was burned the heat produced warmed 5 litres
of water from 20.1oC to 21.5oC. Calculate the enthalpy of
combustion of ethanol.
2
4.
When 1g of butanol (C4H9OH) is burned, 36kJ of heat energy are
released. Calculate the enthalpy of combustion of butanol.
2
5.
When 1g of sodium hydroxide was dissolved in water 1.08kJ of heat
energy was released. Calculate the enthalpy of solution of sodium
hydroxide.
2
Total = 19
Enthalpy Homework 6
1. Sketch a potential energy diagram for a chemical reaction where
the activation energy for the reaction is 60kJmol-1 and the
enthalpy change is +20kJmol-1.
2
2. Consider the following graph:
Potential Energy
/kJ mol-1
80
60
40
20
Which of the following applies to this graph:
A Exothermic and Ea = 40kJmol-1
B Endothermic and Ea = 40kJmol-1
C Exothermic and Ea = 20kJmol-1
D Endothermic and Ea = 20kJmol-1
1
3. Which of the following is not a correct statement about this
effect of a catalyst? The catalyst:
A provides an alternative route to the products.
B lowers the energy which molecules need for successful collisions.
C provides energy so that more molecules have successful
collisions.
D forms bonds with reacting molecules.
1
4. Which of the following statements describes the effect of a
catalyst in a reaction?
A ΔH decreases and Ea increases
B ΔH does not change and Ea decreases
C ΔH decreases and Ea does not change
D ΔH increases and Ea decreases
1
5. Write the balanced chemical equation that illustrates the enthalpy
of combustion of pentane.
1
6. A pupil added 0.06mol of sodium nitrate to 100cm3 of water at
20oC. The enthalpy of solution of sodium nitrate is +20.5kJ mol-1.
After dissolving the solute calculate the temperature of the
solution
2
7. When 3.6g of glucose, C6H12O6, was burned, 56kJ of energy was
released. From this data, what is the enthalpy of combustion of
glucose, in kJmol-1?
2
Total = 10