Topic 8 – Energetics - Questions

Q1. This question is about the enthalpy change of combustion of methanol.

A teacher asked two students to carry out a practical task to determine the enthalpy change of combustion of methanol. Both students were provided with the same apparatus and chemicals. The following procedure was provided for the students.

Procedure

Measure out 150 cm3 of distilled water, using a 250 cm3 measuring cylinder. Transfer the water to a copper calorimeter and note the initial temperature of the water (to the nearest 0.5°C) in Table 1. Weigh the spirit burner containing methanol and record its mass in Table 1. Place the spirit burner under the copper calorimeter, as shown in the diagram. Ignite the spirit burner and burn the methanol, whilst stirring the water with the thermometer. After heating the water for three minutes, extinguish the flame and immediately record the highest temperature reached by the water. As soon as possible, reweigh the spirit burner containing the methanol and record its mass in Table 1.

The results of Student 1 are recorded in Table 1.

Table 1

(a)  Complete Table 1, giving the values to an appropriate number of decimal places.

(2)(b)  Write the equation that represents the reaction that occurs when the standard enthalpy change of combustion of methanol, CH3OH(l), is measured. Include state symbols.

(2)

(c)  Use Student 1's result to calculate the enthalpy change of combustion of methanol in kJ mol–1. Give your answer to an appropriate number of significant figures.

Specific heat capacity of water = 4.18 J g–1 °C–1

Density of water = 1.00 g cm–3

(4)

(d)  Student 1 compared the experimental value for the enthalpy change of combustion of methanol obtained in part (c) with the standard value given on the internet.

The student's value was less exothermic than the standard value.

Student 1 decided to evaluate the uncertainty in the measurements made in this experiment.

(i)  Student 1 used a 250 cm3 measuring cylinder to measure the volume of 150 cm3 distilled water. The uncertainty in this volume measurement is ±1 cm3.

Calculate the percentage uncertainty in the volume of distilled water that Student 1 measured in the experiment.

(1)

(ii)  Compare and contrast the use of a 250 cm3 measuring cylinder to measure out the 150 cm3 distilled water with the use of a 25 cm3 measuring cylinder (uncertainty ±0.2 cm3 for each volume measurement) six times to measure the same volume.

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(iii)  Student 1 calculated the uncertainties in the remaining measurements. However, Student 1 realised that the measurement uncertainties did not explain the difference between the experimental value for the enthalpy change of combustion of methanol calculated in part (c) and the value obtained from the internet.

Other than human error, give three reasons for the difference in the values.(3)

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(e)  Student 1 decided to repeat the experiment.

Student 1 used the copper calorimeter and water from the first experiment and recorded the initial temperature as 60.0°C. Student 1 burned exactly the same mass of methanol as in the first experiment.

Explain, with a reason, how the value for the enthalpy change of combustion of methanol from this experiment would differ, if at all, from the value obtained in the first experiment.

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(f)  Student 2 followed the original instructions provided, but extinguished the flame after four minutes rather than after three minutes.

Explain how the value calculated by Student 2 for the enthalpy change of combustion of methanol compared with that obtained in Student 1's first experiment.

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(g)  Another student, Student 3, used the results from Student 1's first experiment to find the enthalpy change of combustion of methanol. Student 3 incorrectly used a value of 46.0 g mol–1 for the molar mass of methanol.

State and justify how this mistake would affect the calculated value for the enthalpy change of combustion of methanol.

(2) .............................................................................................................................................

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(Total for question = 21 marks) 

Q2.Which equation represents the reaction for which the enthalpy change, ΔH, is the mean bond energy of the C-F bond?

   A                CF4(g) → C(g) + 4F(g)

   B              ¼CF4(g) → ¼C(g) + F(g)

   C    C(g) + 4F(g) → CF4(g)

   D    ¼C(g) + F(g) → ¼CF4(g)

(Total for Question = 1 mark)

Q3. 

This question is about the gas ethane, C2H6, and its reactions.(a)  Write the equation, including state symbols, which represents the reaction taking place when the standard enthalpy change of combustion of ethane is measured.

(2)

(b)  Ethane can react with chlorine to form chloroethane and hydrogen chloride.

Rewrite this equation using displayed formulae.

Use the equation you have written, together with the bond enthalpy data, to calculate the enthalpy change for the reaction.

(4)

(c)  This reaction takes place in a number of steps, some of which are shown below.

(i)   State the type of reaction occurring in step 1 and the conditions needed for this step.(2)

Type

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Conditions

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(ii)   Complete the equation below for the third step of the reaction, and show the movement of electrons using the appropriate arrows.

(3)

(iii)   Write equations for two termination steps in this reaction.(2)

(d)  Ethane can be cracked in industry.  Write an equation for the cracking of ethane.(1)

(e)  Suggest two reasons why cracking of larger alkane molecules is important in industry.(2)

Reason 1:

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Reason 2:

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(Total for question = 16 marks)

Q4. 

This question is about enthalpy changes and entropy changes.

Propan-1-ol is dehydrated to form propene.

The relevant mean bond enthalpies are given in the table.

Calculate the C—O mean bond enthalpy, using the mean bond enthalpies given in the table and the enthalpy change of reaction.

(3)

 

(Total for question = 3 marks) 

Q5. 

In an experiment, 1.000 g of a hydrocarbon, A, was burned completely in oxygen to produce 3.143 g of carbon dioxide and 1.284 g of water.

In a different experiment, the molar mass of the hydrocarbon, A, was found to be 84.0 g mol–1.

A spirit burner was filled with the liquid hydrocarbon, A. The burner was weighed, lit and then used to raise the temperature of a quantity of water in a beaker, as shown in the diagram. The burner was then reweighed.

(i)  Use these results to calculate the enthalpy change of combustion of hydrocarbon A in kJ mol–1.

Give your answer to an appropriate number of significant figures and include a sign.(3)

(ii)  The beaker used in this experiment was made of copper rather than glass. Give a reason for this.

(1) .............................................................................................................................................

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(Total for question = 4 marks) 

Q6. 

In acid-base neutralisation reactions, there is a temperature change.

The enthalpy change when hydrochloric acid reacts with aqueous ammonia is –53.4 kJ mol–1.

HCl(aq) + NH3(aq) → NH4Cl(aq)

Calculate the temperature change you would expect when 25.0 cm3 of 1.00 mol dm–3 hydrochloric acid is mixed with 25.0 cm3 of 1.00 mol dm–3 aqueous ammonia.

Give your answer to an appropriate number of significant figures.

(3)

 

(Total for question = 3 marks) 

Q7. 

Propane is a saturated hydrocarbon with molecular formula C3H8.

*(a)  Explain the meaning of the terms saturated and hydrocarbon.

(2)Saturated

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Hydrocarbon

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(b)  Propane is sold in small cylinders for use as a fuel in camping stoves. The enthalpy change of combustion of propane can be measured by experiment using one of these cylinders.

A known mass of propane is burned to heat a container of water, and the temperature rise of the water is measured.The results of the experiment are shown below.

(i)  How would the mass of propane which was burned be measured?(1)

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(ii)  Calculate the energy transferred in the experiment, using the results above and the following expression.

Energy transferred (J) = mass × specific heat capacity × temperature changeThe specific heat capacity of water is 4.18 J g−1 °C−1.

(1)

(iii)  Calculate the enthalpy change of combustion of propane, ΔHc, in kJ mol−1.Give your answer to three significant figures and include a sign.

(3)

(iv)  The results of this experiment are inaccurate due to heat loss.Suggest one other source of error, other than measurement errors and limitations of the equipment.

(1) .............................................................................................................................................

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(c)  Another way of calculating the enthalpy change of combustion for propane is to use mean bond enthalpy data.

(i)  Complete the equations in the Hess cycle below. The enthalpy change of +6490 kJ mol−1 is the total energy required to break the bonds in propane and in oxygen.

(1)

(ii)  Use the data in the table to calculate the enthalpy change, Z, in kJ mol−1.

(1)

(iii)  Use the cycle in (c)(i), and your answer to (c)(ii), to calculate the enthalpy change, ΔHx, in kJ mol−1, for the combustion of propane.

(1)

(iv)  The data book value for the standard enthalpy change of combustion, , for propane is −2219.2 kJ mol−1. This value is more exothermic than that calculated using mean bond enthalpy data. Give one reason for this.

(1) .............................................................................................................................................

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(Total for question = 12 marks)

Q8. 

Ethanol burns completely in excess oxygen.

C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l)

(i)   The table shows some mean bond enthalpy data.

Calculate the enthalpy change, in kJ mol−1, for the complete combustion of 1 mol of ethanol.(3)

(ii)  Complete the reaction profile diagram for the combustion of ethanol and fully label the diagram.

(2)

(iii)  A data book value for the standard enthalpy change of combustion of ethanol is −1367.3 kJ mol−1.

Give the main reason why the value you calculated in (i) is different from this data book value.(1)

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(Total for question = 6 marks) 

Q9. 

This question is about the thermal decomposition of ammonia.

This reaction is catalysed by platinum and is represented by the equation:

2NH3(g) → N2(g) + 3H2(g)     = + 92 kJ mol−1

The table shows the bond enthalpies for the N≡N and H–H bonds.

Use this data, together with the standard enthalpy change of reaction, , for the decomposition of ammonia, to calculate a value, in kJ mol−1, for the mean bond enthalpy of the N–H bond in ammonia.

(3)

Q10. 

A student carries out two experiments to determine the enthalpy change that occurs when anhydrous sodium carbonate reacts to form hydrated sodium carbonate.

(a)   In the first experiment, the student determines the enthalpy change of solution for anhydrous sodium carbonate.

50.0g of distilled water is placed in a polystyrene cup and the temperature is recorded.A sample of anhydrous sodium carbonate is added to the water, the mixture is stirred and the final temperature recorded.The results for this experiment are shown in the table.

Calculate the enthalpy change of solution, in kJ mol−1, for anhydrous sodium carbonate.Give your answer to an appropriate number of significant figures and include a sign.[Use 4.18 J g–1 °C–1 as the specific heat capacity of water]

(4)

(b)   In the second experiment, the student determines the enthalpy change of solution for hydrated sodium carbonate.

Complete the Hess cycle and, together with your answer to (a) calculate the enthalpy change when anhydrous sodium carbonate reacts to form hydrated sodium carbonate. Include a sign in your answer.

(2)

(c)   Hydrated sodium carbonate slowly loses some water of crystallisation when left in air.

Explain how the enthalpy change in the second experiment would compare with the data book value if an old sample of hydrated sodium carbonate had been used.

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(Total for question = 8 marks) 

Q11.*(a)  Define the term enthalpy change of neutralization.

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(b)  The enthalpy change of the neutralization reaction between hydrochloric acid, HCl(aq), and sodium hydroxide, NaOH(aq), can be determined by the following procedure.

Procedure:

50.0 cm3 of 2.00 mol dm−3 hydrochloric acid is transferred to a polystyrene cup and its temperature recorded 50.0 cm3 of 2.00 mol dm−3 sodium hydroxide solution is placed in another polystyrene cup and its temperature recorded The two solutions are mixed, with stirring, and the maximum temperature is recorded

Results:

Initial temperature of both the HCl(aq) and NaOH(aq)         = 19.0 °C                        

Maximum temperature reached after mixing         = 32.5 °C                        

Assumption:

The specific heat capacity of all aqueous solutions is 4.18 J g−1 °C−1

The density of all aqueous solutions is 1.00 g cm−3

(i)  Calculate the heat energy released (in joules) on mixing the hydrochloric acid and the sodium hydroxide solutions.

Use the expression

energy released (J) = mass of solution × 4.18 × temperature change

(2)

(ii)  Calculate the number of moles of hydrochloric acid used in the experiment.

(1)

(iii)  Give the ionic equation, including state symbols, for the reaction between hydrochloric acid and sodium hydroxide solution.

(1)

(iv)  Use your answers to (b)(i), (ii) and (iii) to calculate the enthalpy change of neutralization for the above reaction. Include a sign and units in your answer.

(3)

(v)  Explain why the enthalpy change of neutralization for the reaction between dilute nitric acid, HNO3(aq), and potassium hydroxide solution, KOH(aq), is predicted to be the same as the enthalpy change of neutralization for the reaction carried out in part (b).

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(Total for Question = 10 marks)

Q12. 

Ethanethiol (CH3CH2SH) is a compound with a structure similar to that of ethanol.

Unlike ethanol, it has an unpleasant smell that can be detected in very low concentrations by humans.

For safety reasons it is added to gaseous, odourless fuels such as propane so that fuel leaks can be detected.

When the fuel is burned, the gaseous ethanethiol undergoes complete combustion to form the oxides CO2(g), SO2(g) and H2O(g).

(a)  Table 1 shows some mean bond enthalpy data.

(i)  Write the equation for the complete combustion of ethanethiol.State symbols are not required.

(1)

(ii)  Use your equation and data from Table 1 to calculate the enthalpy change for the combustion of ethanethiol.

(3)

(b)  Table 2 shows some enthalpy changes of formation.

(i)  Why is the value for ΔfH of O2(g) zero?(1)

   A    O2(g) is an element

   B    O2(g) is a gas

   C    O2(g) is a molecule

   D    O2(g) is a non-metal

(ii)  Use Hess's Law, and data from Table 2, to calculate another value for the enthalpy change for the combustion of ethanethiol.

(2)

(c)  Give a reason why the values for the enthalpy change of combustion of ethanethiol in (a)(ii) and (b)(ii) are different.

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(Total for question = 8 marks) 

Q13. 

An experiment is carried out to determine the enthalpy change of the reaction between copper(II) sulfate solution and zinc.

CuSO4(aq) + Zn(s) → Cu(s) + ZnSO4(aq)

The procedure is:

measure out 25.0 cm3 of 0.720 mol dm−3 copper(II) sulfate solution, using a pipette, into a polystyrene cup weigh out an excess of zinc granules place a thermometer into the cup. Stir and record the temperature to the nearest 0.1°C every 30 seconds, for 2½ minutes at 3 minutes, add the zinc granules to the cup gently stir the mixture and record the temperature every 30 seconds until a total time of 10 minutes is reached.

The data collected is shown on the graph.

(a)  Extrapolate the graph and hence determine the maximum temperature change for the reaction.

(2)

Maximum temperature change = ........................................................... °C

(b)  (i)  Explain why an extrapolation is required to determine an accurate measure of the maximum temperature change.

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(ii)  Explain how the experiment could be amended to allow a value for the maximum temperature change to be determined without the need for an extrapolation.

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(c)  Calculate the enthalpy change for the reaction, ΔrH. Give your answer to an appropriate number of significant figures.

(4)

(d)  The accepted value of ΔrH for this reaction is −217 kJ mol−1.

(i)  Calculate the percentage error in this experiment.(1)

(ii)  Give reasons why the experimental value calculated in (c) is different from the accepted value.(2)

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(iii)  In the same experiment, another student used a polystyrene cup that contained some distilled water left over from rinsing the cup.Explain what effect, if any, this would have on the value for the enthalpy change calculated by this student, compared to the answer in (c).

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(Total for question = 6 marks) 

Q14. 

The diagram shows apparatus that can be used, with appropriate safety precautions, to find the enthalpy change of combustion, ΔcH, of a liquid fuel.

The fuels used in an investigation are

two alkanes – pentane and octane two alcohols – methanol and ethanol

The lid should be removed from the spirit burner just before igniting the fuel, and replaced after the flame is extinguished.

(a)  These are the results of the experiment using pentane.

(i)  Calculate the heat energy, in kJ, transferred to the water.Give your answer to 3 significant figures.

(3)

(ii)  State the reason why using the lid increases the accuracy of this experiment.(1)

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(b)  In the experiment using octane, a black solid formed on the underside of the glass beaker.

Write an equation for the combustion of octane that shows the formation of this black solid.State symbols are not required.

(2)

(c)  In the experiment using methanol, the heat energy transferred to the water was 8960J, and the mass of methanol burned was 0.717g.

Calculate the enthalpy change of combustion of methanol, in kJ mol−1, using these data, giving your answer to an appropriate number of significant figures.

(3)

(d)  In the experiment involving ethanol, a value of −747kJ mol−1 was calculated for ΔcH.

Justify how, if at all, using a copper can in place of the glass beaker would affect the calculated value of ΔcH.

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(e)  Data book values for the enthalpy changes of combustion, in kJmol−1, of the compounds in the investigation are

Give two reasons why the combustion of 1 mol of each of the alcohols releases less energy than the combustion of 1 mol of each of the alkanes.

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(Total for question = 13 marks) 

Q15. 

Ammonia is used in the manufacture of nitric acid.

The equation for one step in this manufacturing process is:

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)   = −900 kJ mol−1

*(a)   A manufacturer carries out this reaction at a temperature of 1200 K and a pressure of 10 atm. A scientist proposes that a temperature of 1000 K should be used at the same pressure.

Evaluate the effects of making this change on the rate and yield of this reaction.(6)

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*(b)   When this reaction is used in industry, the catalyst is an alloy of platinum and rhodium.

The diagram shows the reaction profile for the uncatalysed reaction.

(i)   On the diagram, draw the reaction profile for the catalysed reaction.(1)

(ii)   Label the diagram to show

the enthalpy change, H the activation energy, Eafor the catalysed reaction.

(2)(c)   Write the expression for the equilibrium constant, Kc, for this reaction.

(1)

Q16. 

The equations for three reactions are:

reaction 1   C(s) + O2(g) → CO2(g)                               = −394 kJ mol−1

reaction 2   CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)     = −890 kJ mol−1

reaction 3   2H2(g) + O2(g) → 2H2O(l)                          = −572 kJ mol−1

(a)   Give two reasons why all three reactions are classified as examples of combustion.

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(b)   Only reaction 1 represents a standard enthalpy change of formation.

Give reasons why reactions 2 and 3 do not.(2)

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(c)   Which physical property should be kept constant when measuring an enthalpy change?

(1)  A   concentration  B   pressure  C   temperature  D   volume

(d)   A reaction occurs under standard conditions. Which of these represents a possible standard condition?

(1)  A   1 g cm−3

  B   4.18 J g−1 K−1

  C   24 dm3

  D   298 K

Q17. 

This question is about the oxidation of ammonia.

Nitric acid is made from ammonia. One of the stages in nitric acid production involves the oxidation of

ammonia to produce nitrogen(II) oxide, NO. In this process, a mixture of ammonia and oxygen is passed over a platinum-rhodium catalyst. One manufacturer uses a pressure of 5 atm and a temperature of 850 °C.

(i)  Use this equation, and the enthalpy changes of formation of nitrogen(II) oxide and water, to calculate the enthalpy change of formation of ammonia in kJ mol−1.

You may find it helpful to draw a Hess cycle first. You must show your working.

(3)

(ii)  Calculate the atom economy by mass for the formation of NO in this reaction.

Give your answer to an appropriate number of significant figures.(2)

 

(Total for question = 5 marks) 

Q18. 

Nitric acid reacts with sodium hydroxide solution in a neutralisation reaction.

In an experiment to determine the enthalpy change of neutralisation, the following results were obtained.

Volume of 1.00 mol dm−3 HNO3 = 25.0 cm3

Volume of 1.05 mol dm−3 NaOH = 25.0 cm3

Temperature rise = 6.8 °C

Calculate the enthalpy change of neutralisation for the reaction between nitric acid and sodium hydroxide

solution, using the results of the experiment.

Give your answer to an appropriate number of significant figures.

(4)

 

(Total for question = 4 marks) 

Q19.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)

(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)

(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.

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(Total for Question = 11 marks)

Q20. 

Some reactions involving copper(II) sulfate are investigated. The diagram shows the apparatus used to measure the temperature change when anhydrous copper(II) sulfate, CuSO4(s), is dissolved in water.

The chemical change can be represented by this equation:

CuSO4(s) + aq → CuSO4(aq)

The method is:

Pour some water into the plastic cup and record the steady temperature Add some anhydrous copper(II) sulfate and stir until it has all dissolved Record the maximum temperature reached in the reactionThese results are recorded.

Mass of water used: 50.0 gInitial temperature of water: 18.2 °CFinal temperature of water: 25.4 °CMass of anhydrous copper(II) sulfate used: 4.70 g

(a)   Calculate the standard enthalpy change, in kJ mol−1, for the reaction, giving your answer to an appropriate number of significant figures.

(5)

(b)   The method used in this experiment leads to an inaccurate value for the temperature change.

Describe an alternative method to find a more accurate value for the temperature change.

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(c)   The experiment is repeated using the same method but with hydrated copper(II) sulfate, CuSO4.5H2O(s), as well as with anhydrous copper(II) sulfate.

The same method is used to calculate the standard enthalpy changes for these reactions:

CuSO4.5H2O(s) + aq → CuSO4(aq)              1 = +8.5 kJ mol−1

 

CuSO4(s) + aq → CuSO4(aq)                 2 = −41 kJ mol−1

 

Calculate the standard enthalpy change for this reaction:

 

CuSO4(s) + aq → CuSO4.5H2O(s)

(2)

(d)   The experimental value in part (c) of 2 = −41 kJ mol−1 for the reaction involving anhydrous copper(II) sulfate was different from a data book value.

The anhydrous copper(II) sulfate used in this experiment was pale blue, rather than white.

Explain how this observation partly accounts for the difference between the experimental value and the data book value.

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Q21. 

One of the stages in the production of sulfuric acid from sulfide ores involves the oxidation of sulfur dioxide to sulfur trioxide. The equation for the reaction is

2SO2(g) + O2(g) 2SO3(g)      ΔrH = –197 kJ mol–1

The conditions used in one industrial process are: 420°C and a pressure of 1.7 atm together with a vanadium(V) oxide catalyst.

It is proposed to change the conditions to 600°C and 10 atm pressure, while still using the same catalyst.

(i)  On the axes provided, sketch the reaction profiles for the uncatalysed and catalysed reaction.

2SO2(g) + O2(g) 2SO3(g)      ΔrH = –197 kJ mol–1

Label the uncatalysed reaction, A, and the reaction catalysed by vanadium(V) oxide, B.(3)

(ii)  On your reaction profile, identify and label both the enthalpy change and the activation energy for the catalysed reaction.

(2)(Total for question = 5 marks)

 Q22.

The reaction of calcium oxide with hydrochloric acid is an exothermic reaction.

CaO(s) + 2HCl(aq) → CaCl2(aq) + H2O(l)

In an experiment to investigate this reaction, the following procedure was carried out.

1.    50.0 cm3 of hydrochloric acid, concentration 2.0 mol dm−3 (an excess), was pipetted into a polystyrene cup and the initial temperature measured using a thermometer with 0.5°C graduations.

2.    1.46 g of calcium oxide powder was weighed out and added to the acid. The mixture was stirred and the maximum temperature measured.

(a)  Calculate the enthalpy change, in joules, for the quantities in this experiment. Assume that the specific heat capacity of the solution is 4.18 J g−1 C−1.

Use the expression:energy transferred in joules = 50.0 × specific heat capacity × temperature change

(1)

(b)  Using your answer from (a), calculate the molar enthalpy change for the reaction between calcium oxide and hydrochloric acid. Include a sign and units in your answer.

(2)

(c)  The standard molar enthalpy change for the reaction between calcium oxide and hydrochloric acid is −196.8 kJ mol−1.

(i)  Suggest three reasons why the calculated value in part (b) is different from this value.(3)

Reason 1

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Reason 2

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Reason 3

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(ii)  Using the standard enthalpy change of −196.8 kJ mol−1, calculate the minimum mass of calcium oxide that would be needed to raise the temperature of 250 cm3 of hydrochloric acid (an excess) by 25.0°C.

(3)

(d)  The reaction of calcium carbonate with hydrochloric acid has the following standard molar enthalpy change.

ΔH = −18.8 kJ mol−1

This value can be used, with the enthalpy change for the reaction of calcium oxide with hydrochloric acid, to determine the enthalpy change for the thermal decomposition of calcium carbonate. This cannot be measured directly.

(i)  Complete the Hess energy cycle below by adding the missing arrow and entities.

Use the cycle, and the standard enthalpy change for the reaction of calcium oxide and hydrochloric acid (−196.8 kJ mol−1), to determine the standard enthalpy change for the decomposition of calcium carbonate.

(4)

Enthalpy change = ........................................................... kJ mol−1

(ii)  Complete and label the enthalpy level diagram below, for the series of reactions in (d)(i).

Your diagram does not have to be to scale.(1)

(Total for question = 14 marks)Q23. Write the equation to represent the standard enthalpy change of formation of ethanol. Include state symbols.

(2)

 

(Total for question = 2 marks)

 

Q24. 

In an experiment to measure the enthalpy change of a reaction involving gases, which of the following conditions must always be kept constant?

   A     Pressure   B     Temperature   C     Volume   D     Temperature and pressure

(Total for question = 1 mark)

Q25. 

The reaction for which the enthalpy change is the standard enthalpy change of formation of water, is

   A    H2(g) + ½O2(g) → H2O(l)

   B    H2(g) + ½O2(g) → H2O(g)

   C    2H2(g) + O2(g) → 2H2O(l)

   D    2H2(g) + O2(g) → 2H2O(g)

 

(Total for question = 1 mark)

Q26. 

The standard enthalpy change of neutralization when an acid reacts with an alkali is the number of kilojoules released by the

   A    formation of one mole of salt.

   B    formation of one mole of water

   C    neutralization of one mole of acid.

   D    neutralization of one mole of alkali.

 

(Total for question = 1 mark)

Q27.Given the following information

CH4(g) → C(g) + 4H(g)          ΔH = +Q kJ mol−1

the mean bond enthalpy for the C—H bond in methane is

   A    +Q

   B    +Q/4

   C    −Q

   D    −Q/4

(Total for Question = 1 mark)

Q28. 

This question is about enthalpy changes and entropy changes.

Which is the equation for the standard enthalpy change of formation, , of aluminium oxide?

(1)

   A    4Al(s) + 3O2(g) → 2Al2O3(s)

   B    4Al(s) + 6O(g) → 2Al2O3(s)

   C    2Al(s) + 11⁄2O2(g) → Al2O3(s)

   D    2Al(s) + 3O(g) → Al2O3(s)

 

(Total for question = 1 mark) 

Q29. 

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)

Q30.Consider the following information:

For the reaction

H2(g) + I2(g) → 2HI(g)

the enthalpy change, in kJ mol−1, is

   A    +288

   B    +144

   C    −11

   D    −5.5

(Total for Question = 1 mark)

Q31.The equation for the complete combustion of butanone, C2H5COCH3, is

C2H5COCH3(l) + 5½O2(g) → 4CO2(g) + 4H2O(l)         ΔH = −2440 kJ mol−1

From the above data, the standard enthalpy change of formation of butanone, in kJ mol−1, is

   A    −280

   B    +280

   C    −1760

   D    +1760

(Total for Question = 1 mark)

Q32. 

When 10 cm3 of 2 mol dm−3 hydrochloric acid is reacted with 10 cm3 of 2 mol dm−3 sodium hydroxide solution, the temperature change is ΔT.

When the reaction is repeated with 50 cm3 of each solution, the temperature change is

   A     ΔT

   B     5 × ΔT

   C      × ΔT

   D     10 × 2 × ΔT(Total for question = 1 mark)

Q33. 

In an endothermic reaction in aqueous solution, which of the following is correct?

(Total for question = 1 mark)