Energetics (DP IB Chemistry: SL)

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

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 and determines q to be 1828 J. 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:

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

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

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 roughly 60 cm³ of 0.35 mol dm^-3 hydrochloric acid, roughly 60 cm³ of 0.35 mol dm^-3 sodium hydroxide, a polystyrene cup and access to all standard laboratory equipment.

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, and state the key measurements that the students would have to make.

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.

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.

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.

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 in the experiment.

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 .

Determine the enthalpy of reaction outlined in part (a) using the data in Table 1.

Table 1 

Draw the Lewis structure for hydrogen peroxide.

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).

The initial temperature of the potassium hydroxide was 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.

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.

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.

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.

Use your answers to parts a), b) and c) to determine the energy change for dissolving 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 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.

The student determined that the enthalpy change of neutralisation, ΔH_neut, was -35.7 kJ mol^-1. Deduce if the student is correct and justify your answer. 

Ethanol is made in large quantities via the hydration of ethene in the presence of a concentrated phosphoric acid catalyst or via the fermentation of glucose. Ethanol is widely used as a fuel.

The enthalpy of combustion of ethanol can also be determined experimentally in a school laboratory. A burner containing ethanol was weighed and used to heat water in a test tube as illustrated below.

The following data was obtained from the combustion of ethanol.

Use the information in part a) and sections 1, 2 and 6 in the data booklet to determine.

Compare the data book value in section 13 with your answer to part b) and suggest why these values differ.

The enthalpy change of solution for lithium chloride can be measured using calorimetry. 

1.20 g of lithium chloride is dissolved in 20.0 cm³ of water at 19.5 °C. 

Use section 6 in the data booklet to determine the amount, in moles, of the lithium chloride dissolved.

Use your answer to part a) and section 19 in the data booklet to determine the energy released, in J, when 1.20 g of lithium chloride is dissolved in 20.0 cm³ of water. 

Use your answer to part b) and sections 1 and 2 in the data booklet, determine the change in temperature, in °C, when the lithium chloride is dissolved.

Use your answer to part c) determine the maximum temperature, in °C, of the solution that was reached during the reaction.