Measuring Enthalpy Change (DP IB Chemistry: SL): Exam Questions

The reaction between solid ammonium nitrate and water is one which is often conducted in school laboratories.

i) Describe how the heat energy transferred (q) for the reaction between solid ammonium nitrate and water could be conducted in a school laboratory. Give the names of the apparatus that is needed and state the measurements that would be taken to find the enthalpy change of reaction.

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ii) Discuss one major source of error in the experiment and suggest how that error would impact on the calculation of the enthalpy change (q).

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State the difference between accuracy and precision in experimental results.

Write an equation for the reaction that occurs when ammonium nitrate is added to water.

A student dissolves 3.5 g of ammonium nitrate in water. The water loses 1828 J of heat.

Determine the enthalpy change for this reaction in kJ mol^-1.

Methanol is an important industrial alcohol which is mostly used to create fuel, solvents, and antifreeze. A colorless liquid, it is volatile, flammable, and unlike ethanol, poisonous for human consumption.

The equation to show the enthalpy of combustion of methanol is:

CH₃OH (l) + 1½O₂ (g) → CO₂ (g) + 2H₂O (g)

Use the equation and the information in Table 1 below, to determine the theoretical enthalpy of combustion of methanol.

Table 1

The enthalpy of combustion of an alcohol can be determined in a school laboratory using the following apparatus.

Figure 1

In an experiment, a spirit burner containing methanol was weighed and used to heat water in a beaker as shown above. The following results were obtained:

i) Calculate the amount of methanol combusted in moles.

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ii) Calculate the heat energy transferred to the water, q, in kJ. Take the specific heat capacity of water as 4.18 J g^-1 K^-1.

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iii) Find the enthalpy of combustion of methanol in kJ mol^-1.

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Suggest one improvement that could be made to the apparatus in part (b) that would give a more accurate result.

The accepted data book value for the enthalpy of combustion of methanol is -726 kJ mol^-1.

i) Suggest why the data value is different from the values calculated in part (a).

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ii) Suggest why the data value is different from the values calculated in part (b).

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A teacher instructs a class to complete a calorimetry practical, to calculate the enthalpy change that occurs when hydrochloric acid and sodium hydroxide react together.  

Each student was given:

• 60 cm³ of 0.35 mol dm^-3 hydrochloric acid

• 60 cm³ of 0.35 mol dm^-3 sodium hydroxide

• A polystyrene cup

• Access to all standard laboratory equipment.

i) Draw a diagram to demonstrate the practical set up that the students would need to use to determine the enthalpy change during this neutralisation reaction.

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ii) State the key measurements that the students would have to make.

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The students then completed the practical from part (a), using their own method and measurements that they had chosen.

One student found that when they reacted 35.0 cm³ of the hydrochloric acid with 35.0 cm³ of the sodium hydroxide, the temperature rose from 19.6 ^oC to 22.3 ^oC.

Determine the enthalpy change, ΔH, for this reaction in kJ mol^-1. Assume that both solutions have a density of 1.00 g cm^-3 and a specific heat capacity of 4.18 J g^-1 K^-1.

State how the students’ practical could be improved to allow the students to calculate a more accurate value which is closer to the correct value given in data books.

Explain why the value that you have calculated for the students’ practical in part (b), might be different from the correct value given in a data book.

A group of students carried out a calorimetry experiment to determine the enthalpy change for the decomposition of hydrogen peroxide using manganese dioxide as a catalyst.

Each group added 1 g of manganese dioxide to 50.00 cm³ of 2.00 mol dm^-3 of hydrogen peroxide.

They recorded their data and drew a graph shown in Figure 1.

Figure 1

Use the graph to determine the temperature change, ΔT.

Using your answer to part (a) determine the enthalpy change, ΔH, for this reaction in kJ mol^-1.

 Assume the solution has a density of 1.00 g cm^-3 and a specific heat capacity of 4.18 J g^-1 K^-1 .

Define the term standard enthalpy of neutralisation , ΔH^ϴ_neut.

A student carried out a neutralisation reaction and recorded the temperature change.25.00 cm³ of 1.0 mol dm^-3 nitric acid, HNO₃ (aq) was neutralised by 50.00 cm³ of 1.0 mol dm^-3 of potassium hydroxide, KOH (aq).

Both solutions were initially at 20.5 ^oC and the reaction reached a maximum temperature of 24.5 ^oC.

Determine the enthalpy of neutralisation, ΔH^ϴ_neut, assuming the solutions has a density of 1.00 g cm^-3 and a specific heat capacity of 4.18 J g^-1 K^-1 . Give your answer in kJ mol^-1 to three significant figures.

Write an equation to demonstrate how nitric acid can behave as a Brønsted-Lowry acid when it reacts with water.

The accepted theoretical value from the literature of this enthalpy change is -57 kJ mol^-1. Calculate the percentage error to two significant figures.

Chemical reactions can be classified as endothermic or exothermic, a property that can be represented on an enthalpy level diagram.

An enthalpy level diagram for an endothermic reaction is shown below.

Figure 1

i) Give the sign of ΔH for the reaction.

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ii) State whether the bonds in the products or the reactants are stronger overall.

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The complete combustion of propan-1-ol is an exothermic reaction with ΔH꜀ө = -2021 kJ mol⁻¹.

i) Define the term standard enthalpy of combustion.

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ii) Draw a labelled enthalpy level diagram for the combustion of propan-1-ol.

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Alkanes can be used as fuels in internal combustion engines. When sufficient oxygen is present, they undergo complete combustion reactions.

Write an equation for the enthalpy of combustion of butane.

Define the term standard enthalpy of combustion, ΔH^ϴ_c.

Table 1 below contains bond enthalpy data for the reaction shown in part (a).

Table 1

Using the data in Table 1 and the equation in part (a), calculate the enthalpy change of combustion of butane.

In the absence of sufficient oxygen, butane will undergo incomplete combustion.

Write an equation for the incomplete combustion of butane.

A student measured the energy change when 1.35 g of zinc was added to 50 cm³ of 0.5 mol dm^-3 copper sulfate, CuSO₄ (aq), solution. The initial temperature of 21 ^oC was recorded before the addition of the zinc and a temperature reading was taken every 30 seconds.

Use the graph to determine the overall temperature change for the reaction

Calculate the enthalpy change for the reaction in kJ mol^-1.

Calculate the percentage error between your value for the enthalpy change of reaction and the literature value of -217 kJ mol^-1. Give your answer to two significant figures. 

Explain why your calculated value for the enthalpy change of reaction is different from the literature value of -217 kJ mol^-1. 

When anhydrous copper(II) sulfate is left in the atmosphere it will slowly turn to a blue pentahydrate solid. It is possible to measure the heat changes directly when both anhydrous and pentahydrated copper(II) sulfate are separately dissolved in water.

i) Write an equation for the reaction of anhydrous copper(II) sulfate with water to form pentahydrated copper(II) sulfate.

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ii) Construct an energy cycle which can be used to determine the enthalpy change indirectly. 

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To determine the enthalpy change a student placed 50 cm³ of water in a polystyrene cup and used a data logger to measure the temperature.

After two minutes she dissolved 6.30 g of anhydrous copper(II) sulfate in the water and continued to record the temperature while continuously stirring. She obtained the following results.

i) Using section 6 in the data booklet, determine the amount, in moles, of copper(II) sulfate.

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ii) Determine the temperature change, in °C, for the reaction assuming no heat had been lost to the surroundings. 

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iii) Using sections 1 and 2 in the data booklet, determine the heat change, in kJ mol^-1, for the reaction.

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The student repeated the experiment using 7.83 g of pentahydrated copper(II) sulfate and observed the temperature decreased by 2.5 °C. The student used the same volume of water.

i) Use section 6 of the data booklet to determine the amount, in moles, of pentahydrated copper(II) sulfate.

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ii) Use sections 1 and 2 in the data booklet to determine the heat change, in kJ mol^-1. 

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Using your answers to parts a), b) and c), determine the enthalpy change for the hydration of anhydrous copper(II) sulfate.

A student investigated the temperature change for the neutralisation of malonic acid, HOOCCH₂COOH, and sodium hydroxide solution. 

25.0 cm³ of 0.400 mol dm^-3 of malonic acid was added to a beaker and the temperature was recorded every minute for three minutes using a thermometer with an uncertainty of ±0.1° C. On the fourth minute, the student added 50.0 cm³ of 0.500 mol dm^-3 sodium hydroxide solution.

Finally, she recorded the temperature every minute for eight minutes. 

Determine the percentage uncertainty in the student’s 2.9 ^oC temperature rise.

Another student completed the same investigation and recorded a maximum temperature of 23.5 °C. The student calculated the heat energy, q, for the reaction to be 8.923 x 10^-1 kJ.

Use sections 1 and 2 in the data booklet and the information in part a) to estimate the initial temperature for this student’s investigation.

State the balanced symbol equation for the neutralisation of malonic acid with sodium hydroxide solution.

Using the data from part (b), determine the standard enthalpy of neutralization, ΔH_neut, for this reaction in kJ mol^-1.

Ethanol is a common biofuel. Its enthalpy of combustion can be determined theoretically using bond enthalpies and experimentally using calorimetry.

State the balanced chemical equation for the complete combustion of ethanol.

Using bond enthalpy values from section 12 of the data booklet, determine the theoretical enthalpy of combustion of ethanol.

A burner containing ethanol was weighed and used to heat water in a test tube as illustrated below.

The following data was collected when a spirit burner containing ethanol was used to heat 20.000 g of water.

Using this data, determine the experimental enthalpy change for the combustion of ethanol in kJ mol^-1.

The accepted data book value for the enthalpy of combustion of ethanol is -1367 kJ mol^-1 (section 14).

i) Suggest one reason why the theoretical value calculated from bond enthalpies in part (b) is different from the accepted data book value.

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ii) Suggest one reason why the experimental value calculated in part (c) is significantly different from the accepted data book value.

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The standard enthalpy of solution for lithium chloride is -37.0 kJ mol^-1. A student performs an experiment by dissolving 1.20 g of lithium chloride in 20.0 cm³ of water, initially at 19.5 ^oC.

Determine the amount, in moles, of lithium chloride dissolved.

Calculate the heat energy, q, in Joules, that would be released during this process

Calculate the theoretical temperature change, ΔT, of the water.

Assume the specific heat capacity and density of the solution are the same as pure water.

Hence, determine the final temperature of the solution.