Enthalpy changes

A student uses this apparatus to investigate the energy content of different fuels.

This is the student’s method.

• pour some water into the copper can

• record the mass of the spirit burner and fuel

• measure the initial temperature of the water

• place the spirit burner under the copper can and light the burner

• stop heating the water when the temperature reaches 30C

• record the new mass of the spirit burner and fuel

The student repeats the experiment with different fuels.

Explain two variables the student should control to make this a valid test.

In one of the experiments, the student uses ethanol as the fuel.

These are the student’s results for ethanol.

Calculate the value of heat energy change (Q) in joules.

[for water, c = 4.2 J/g/C 1.0 cm³ of water has mass = 1.0 g]

Calculate the enthalpy change (Δ H) in kJ/mol.

[for ethanol, M_r = 46]

Include a sign in your answer.

ΔH =_........ kJ/mol

A student uses this apparatus to investigate the heat energy released when a liquid fuel burns.

This is the student’s method.

• measure the mass of the spirit burner and fuel

• add 100 g of water to the aluminium can

• measure the temperature of the water

• use the spirit burner to heat the water until the temperature rises by 50 °C

• measure the new mass of the spirit burner and fuel

Give a reason why the student uses an aluminium can rather than a glass beaker.

When the fuel burns, a black solid forms on the aluminium can.

Identify the black solid.

Give a reason why the black solid forms.

Show that the heat energy needed to raise the temperature of 100 g of water by 50 °C is approximately 20 000 J.

[for water, c = 4.2 J/g/°C]

The student burns 1.84 g of ethanol.

Calculate the molar enthalpy change, Δ H, in kJ/mol, for the combustion of ethanol.

Include a sign in your answer.

[for ethanol, M_r = 46]

ΔH =_........ kJ/mol

In a different experiment, the student burns 3.7 g of butanol.

This is an equation for the incomplete combustion of butanol.

Complete the equation for the incomplete combustion of butanol.

Calculate the total amount, in moles, of gas produced by the incomplete combustion of 3.7 g of butanol.

A student uses this apparatus to find the heat energy released by the combustion of liquid fuels.

Explain what is meant by the term fuel.

In one experiment, the student uses liquid ethanol as the fuel.

The thermometer shows the temperature of the water at the start of the experiment.

Complete the table by giving the temperatures to the nearest 0.1 °C.

The metal can contains water of mass 150 g.

Show, by calculation, that the heat energy change (Q) for this reaction is approximately 36 000 J.

[for water, c = 4.2 J/g/°C]

In the experiment, 2.3 g of ethanol (M_r = 46) is burned.

Calculate the molar enthalpy change (Δ H), in kJ/mol, for the combustion of ethanol, C₂H₅OH

Include a sign in your answer.

Give your answer to two significant figures.

In this experiment, the calculated value of Δ H is less than the value given in a data book.

Give a possible reason for the difference in values.

A student uses this apparatus to find the molar enthalpy change (Δ H) of combustion for the liquid fuel, pentanol.

This is the student’s method.

• find the initial mass of the spirit burner and pentanol

• add 100 cm³ of water to the copper can

• record the initial temperature of the water

• light the wick of the spirit burner to heat the water

• stir the water until the temperature rises by 35.0 °C

• extinguish the flame and immediately find the final mass of the spirit burner and pentanol

State why the student stirs the water.

Suggest why it is important that the student immediately finds the final mass of the spirit burner and pentanol.

The diagram shows the initial temperature of the water.

Complete the table to show the temperature readings.

Give both values to the nearest 0.1 °C.

Show by calculation that the heat energy (Q) supplied by the pentanol is approximately 15 000 J.

[for water, c = 4.2 J / g / °C]

[for 1.0 cm³ of water, mass = 1.0 g]

The table gives the initial and final mass readings.

Use your answer to part (c)(i) and the information in the table to calculate the molar enthalpy change (Δ H) of combustion, in kJ / mol, for pentanol.

[for pentanol, M_r = 88]

Include a sign in your answer.

ΔH for pentanol =_........ kJ / mol

The formula of pentanol is C₅H₁₁OH

Write a chemical equation for the complete combustion of pentanol.

A student uses this method to investigate the temperature change when solid sodium hydrogencarbonate is added to ethanoic acid solution.

• pour 100 cm³ of ethanoic acid solution into a polystyrene cup

• record the temperature of the ethanoic acid solution

• add a 1 g portion of sodium hydrogencarbonate to the ethanoic acid solution and stir

• record the new temperature

• add further 1 g portions of sodium hydrogencarbonate, stirring and recording the temperature after each portion is added

The table shows the student’s results.

Plot the student’s results on the grid.

Draw a circle around the anomalous result.

Draw a curve of best fit.

Explain why using a polystyrene cup makes the results more accurate.

Suggest a mistake the student might have made to cause the anomalous result.

State how the results show the reaction is complete.

State how the results show that the reaction is endothermic.

Use the results to calculate the heat energy change, Q, in joules.

[for 1.0 cm³ of ethanoic acid solution, mass = 1.0 g]

[for ethanoic acid solution, c = 4.2 J / g / °C]

The student repeats the experiment starting with a different volume of ethanoic acid solution.

The student uses 7.0 g of sodium hydrogencarbonate to neutralise the ethanoic acid solution.

The heat energy change, Q, is 3200 J.

Calculate the molar enthalpy change, Δ H, in kJ / mol.

Include a sign with your answer.

[for sodium hydrogencarbonate, M_r = 84]

ΔH =_........ kJ / mol

A student uses this apparatus to record the maximum temperature in the reaction between solutions of hydrochloric acid and sodium hydroxide.

This is the student’s method.

• add 25 cm³ of hydrochloric acid to the beaker

• add 25 cm³ of sodium hydroxide solution to the beaker

• stir the mixture

• record the maximum temperature reached

Name a suitable piece of apparatus to add 25 cm³ of solution to the beaker.

Before the reaction, both solutions have a temperature of 21.0 °C.

The heat energy change, Q, for the reaction is 2880 J.

Calculate the theoretical maximum temperature reached by the mixture, which has a mass of 50 g.

[specific heat capacity of mixture, c = 4.2 J / g / °C]

Give a reason why the maximum temperature recorded by the student is lower than the theoretical maximum temperature calculated.

In the reaction, 0.0500 mol of hydrochloric acid completely react.

Calculate the molar enthalpy change, Δ H, in kilojoules per mole of hydrochloric acid.

Include a sign in your answer.

ΔH =_........ kJ / mol

A student uses this apparatus to find the heat energy change when ammonium nitrate dissolves in water to form a solution.

This is the student’s method.

• add 50 cm³ of distilled water to the polystyrene cup and record the initial temperature of the water

• add a known mass of ammonium nitrate to the polystyrene cup and stir the mixture until all the solid dissolves

• record the minimum temperature of the ammonium nitrate solution

Give two reasons why the student stirs the mixture.

The diagram shows the minimum temperature of the solution.

Complete the table by giving the missing information.

Give both values to the nearest 0.1 °C.

The student repeats the experiment, again using 50 cm³ water, and finds that the temperature change is 3.9 °C.

Show that the heat energy change, Q, in this experiment is about 800 J.

[c = 4.2 J / g / °C for the solution]

[mass = 1.0 g for 1.0 cm³ of solution]

The student uses 2.8 g of ammonium nitrate in this experiment.

Calculate the enthalpy change, Δ H, in kJ / mol.

Include a sign in your answer.

[for NH₄NO₃, M_r = 80]

ΔH =_........ kJ / mol

Describe a test to show that the ammonium nitrate solution contains ammonium ions.

In another experiment the student adds anhydrous copper(II) sulfate to distilled water.

The table shows the student’s results.

Explain what the results show about the type of energy change that occurs when anhydrous copper(II) sulfate dissolves in water.

When 10.0 kg of a solid fuel is burned, the heat energy produced is 3.28 × 10⁵kJ.

Calculate the heat energy, in kJ/mol, produced by the fuel.

You should assume that the fuel is pure carbon.

Give your answer to three significant figures.

[for carbon, A_r = 12]

heat energy =_........ kJ/mol

The fuel actually contains some impurities, including sulfur.

There is 600 g of sulfur in 20 kg of fuel.

Calculate the percentage of sulfur in the fuel.

When the fuel burns, the 600 g of sulfur produces sulfur dioxide gas.

Calculate the volume, in cm³, of sulfur dioxide gas that would be produced at room temperature and pressure (rtp) when the fuel burns.

[for a gas, molar volume = 24 dm³ at rtp]

State the name of the environmental problem caused by sulfur dioxide in the atmosphere.

This question is about exothermic reactions.

A student uses this apparatus to measure the temperature increase when an excess of zinc powder is added to copper(II) sulfate solution.

Complete the word equation for the reaction.

zinc + copper(II) sulfate _........ +_........

Give a reason why the student uses a polystyrene cup inside a glass beaker.

State why zinc reacts with copper(II) sulfate solution.

The temperature at the start of the reaction is 19.7C.

The temperature at the end of the reaction is 48.3C.

Calculate the heat energy change, in joules, for the reaction.

[for the mixture, c = 4.2 J/g/C]

The reaction between zinc and silver nitrate solution is exothermic.

A mass of 0.65 g of zinc is added to excess silver nitrate solution.

The heat energy change is 800 J.

Calculate the molar enthalpy change, Δ H, in kJ/mol.

Include a sign in your answer.

ΔH =_........ kJ/mol

This is the ionic equation for the reaction between zinc and silver nitrate solution.

Explain, in terms of electrons, why this is a redox reaction.

A student uses this apparatus to find the heat energy supplied by a liquid fuel.

The student burns some fuel to heat the water in the copper container and measures the change in temperature.

The student notices that the bottom of the container turns black.

Give the name of the black substance that forms on the bottom of the container.

In one experiment, the student burns 0.92 g of ethanol.

The student calculates that the heat energy absorbed by the water is 18.2 kJ.

Show that the results of this experiment give an approximate value for the enthalpy of combustion of ethanol of Δ H = –900 kJ/mol.

[M_r of ethanol = 46]

The data book value of Δ H for the combustion of ethanol is –1367 kJ/mol.

Give two reasons why the student’s value is much lower than the data book value.

Calculate the bond energy of the C H bond, using information from the equation and the table.

Complete the energy level diagram to show the products and Δ H.

Lithium, sodium and potassium are the first three elements in Group 1 of the Periodic Table.

Suggest why these three elements are all stored in paraffin oil.

Caesium, Cs, is below potassium in Group 1.

Give a similarity and a difference between the reactions of potassium with water and caesium with water.

Give the chemical equation for the reaction between caesium and water.

A student investigates the temperature change in the reaction between dilute acids and solutions of Group 1 hydroxides.

He uses this apparatus.

This is the student’s method.

measure the temperature of 50 cm³ of hydrochloric acid

pour the acid into a polystyrene cup

add 50 cm³ of sodium hydroxide solution to the acid

measure the maximum temperature of the mixture

Suggest what could be added to the apparatus to improve the experiment.

Explain a change to the method that would improve the accuracy of the experiment.

These are the student’s results.

temperature of hydrochloric acid = 19.9 °C

maximum temperature of mixture = 26.5 °C

Calculate the energy change, Q, in joules for this reaction.

[mass of 1.0 cm³ of mixture = 1.0 g]

[for the mixture, c = 4.2 J/g/°C]

In the student’s reaction between hydrochloric acid and sodium hydroxide, 0.050 mol of water forms.

Calculate the molar enthalpy change, Δ H, in kJ/mol for this reaction.

ΔH =_........ kJ/mol

A student uses this apparatus to investigate the heat energy change when a salt dissolves in water to form a solution.

This is the student’s method.

● add 50 cm³ of distilled water to a polystyrene cup

● record the initial temperature of the water

● add a known mass of solid anhydrous copper(II) sulfate to the polystyrene cup and stir the solution with the thermometer until all the solid has dissolved

● record the maximum temperature of the copper(II) sulfate solution

Name the piece of apparatus the student should use to add the distilled water to the polystyrene cup.

The student stirs the solution to help the solid dissolve more quickly.

Suggest another reason why the student stirs the solution.

State the colour of the copper(II) sulfate solution.

The diagram shows the temperatures in one experiment.

Complete the table, giving all values to the nearest 0.1C.

In a second experiment, when a student dissolves the anhydrous copper(II) sulfate in 50 cm³ of distilled water, the increase in temperature is 3.3C.

Show that the heat energy change (Q) in this second experiment is approximately 700 J.

[for water, c = 4.2 J/g/C]

[mass of 1.0 cm³ of water = 1.0 g]

In this experiment the student uses 1.70 g of the anhydrous copper(II) sulfate.

Calculate the molar enthalpy change (Δ H) in kJ/mol.

Include a sign in your answer.

[M_r of CuSO₄ = 159.5]

ΔH =_........ kJ/mol

Another student does a similar experiment but uses hydrated copper(II) sulfate instead of anhydrous copper(II) sulfate.

The table shows his results.

Explain what the results show about the type of energy change that occurs when hydrated copper(II) sulfate dissolves.

A student uses this apparatus to investigate the reaction between potassium hydroxide solution and dilute hydrochloric acid.

This is her method.

• pour 25 cm³ of potassium hydroxide solution into a polystyrene cup and record the temperature of the solution

• pour 25 cm³ of dilute hydrochloric acid into a measuring cylinder and record the temperature of the acid

• add the acid to the polystyrene cup and stir the mixture

• record the highest temperature reached

Give a word equation for the reaction between potassium hydroxide and hydrochloric acid.

Explain why the student needs to stir the mixture.

The table gives the temperatures of the solutions before the student mixes them.

Calculate the mean (average) temperature of the two solutions.

The student repeats the experiment on a different day, using 25 cm³ of potassium hydroxide solution and 25 cm³ of dilute hydrochloric acid.

The thermometer shows the highest temperature reached at the end of the experiment.

Complete the table by giving the missing information.

Give both temperatures to the nearest 0.1 °C.

Show that the heat energy change, Q, in the student’s experiment is about 1100 J.

[for the mixture, c = 4.2 J/g/°C]

[mass of 1.0 cm³ of mixture = 1.0 g]

The student uses 0.020 mol of potassium hydroxide in his experiment.

Calculate the enthalpy change (Δ H) in kJ/mol, for 1.0 mol of potassium hydroxide.

Include a sign in your answer.

ΔH =_........ kJ/mol

Ethanol, C₂H₅OH, can be oxidised to produce ethanoic acid, CH₃COOH, by heating it with potassium dichromate(VI).

Name one other reactant needed for this reaction to occur.

Which colour change occurs during this reaction?

When ethanol is burned in air, complete combustion can occur.

The equation for this reaction is

This equation can also be written using displayed formulae to show all the covalent bonds in the molecules.

H H

H H

The table gives the bond energies for these bonds.

Use values from the table to calculate the energy needed to break all the bonds in the reactants.

energy needed_........

Use values from the table to calculate the energy released when all the bonds in the products are formed.

energy released_........

Calculate the molar enthalpy change (Δ H) in kJ/mol, for the complete combustion of ethanol.

Include a sign in your answer.

ΔH =_........ kJ/mol

Ethanol reacts with methanoic acid, HCOOH, in the presence of an acid catalyst to form an ester.

The equation for the reaction is

Give the name of the ester that forms.

Draw the displayed formula for this ester.

When this reaction takes place in a sealed container, the reaction can reach dynamic equilibrium.

Give two characteristics of a reaction at dynamic equilibrium.

Methanoic acid reacts with sodium carbonate to form sodium methanoate, carbon dioxide and water.

The equation for the reaction is

Calculate the volume, in cm³, of carbon dioxide gas produced when 2.3 g of methanoic acid reacts completely with sodium carbonate.

[M_r of HCOOH = 46]

[molar volume of carbon dioxide at rtp = 24 dm³]

The reaction between magnesium and copper(II) sulfate solution is exothermic.

This apparatus is used to measure the temperature increase when excess magnesium is added to 100 cm³ of copper(II) sulfate solution.

State why a reaction occurs when magnesium is added to copper(II) sulfate solution.

Complete the word equation for this reaction.

magnesium + copper(II) sulfate _........ +_........

The temperature at the start of the reaction is 20.2 °C.

The maximum temperature recorded is 56.3 °C.

Calculate the heat energy change, in joules, for the reaction.

[mass of 1.00 cm³ of solution = 1.00 g]

[c for the solution = 4.2 J/g/ °C]

Explain why it is better to use a polystyrene cup rather than a glass beaker in this experiment.

The reaction between zinc and copper(II) sulfate solution is also exothermic.

A mass of 0.500 g of zinc is reacted with an excess of copper(II) sulfate solution.

The heat energy change is 1.67 kJ.

Calculate the molar enthalpy change, H, in kJ/mol.

Include a sign in your answer.

Give your answer to three significant figures.

H =_........ kJ/mol

The ionic equation for the reaction between zinc and copper(II) sulfate is

Explain why this is a redox reaction.

A student uses this apparatus to investigate the heat energy released when a liquid fuel is burned.

This is the student’s method.

● measure the mass of the spirit burner and fuel

● add 100 cm³ of water to the copper can

● record the temperature of the water

● use the spirit burner to heat the water until the temperature rises by 30 °C

● immediately measure the new mass of the spirit burner and fuel

Suggest why the student measures the mass of the spirit burner and fuel immediately after heating the water.

When the fuel is burned, the student notices that a black solid forms on the bottom of the copper can.

Identify the black solid.

Explain why the black solid forms.

Show that the heat energy change, Q, to raise the temperature of 100 cm³ of water by 30 °C is approximately 13 kJ.

[c for water = 4.2 J/g/ °C]

The student burns 0.96 g of methanol, CH₃OH

Calculate the molar enthalpy change, Δ H, in kJ/mol, for the combustion of methanol.

Include a sign in your answer.

[M_r of methanol = 32]

ΔH =_........ kJ/mol

The table shows data book values for the molar enthalpy change, Δ H, for the combustion of some alcohols with different numbers of carbon atoms per molecule.

Plot the data values from the table on the grid.

Draw a straight line of best fit.

Number of carbon atoms

Deduce the value of Δ H for an alcohol with six carbon atoms per molecule.

Show on the graph how you obtained your answer.

State the relationship between Δ H and the number of carbon atoms per molecule.