Energetics (Edexcel IGCSE Chemistry)

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

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

 (1)

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

 Suggest another reason why the student stirs the solution. 

(1)

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

(1)

The diagram shows the temperatures in one experiment.

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

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

i) 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] 

(2)

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

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

 Include a sign in your answer.

 [M_r of CuSO₄ = 159.5]

 (4)

 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 temperature change that occurs when ammonium nitrate is dissolved in water.

She uses this method.

• put 100 cm³ of water into the polystyrene cup and measure the initial temperature of the water

• add 8.00 g of ammonium nitrate and stir

• record the lowest temperature reached by the solution

The table shows her results.

 Use the results of the experiment to explain what type of reaction is taking place when ammonium nitrate is added to water.

Show that the heat energy change, Q, is about 2400 J.

 [mass of 1.00 cm³ of solution = 1.00 g]

 [for the solution, c = 4.185 J / g / °C]

 Q = .............................. J

Use your answer to part (b) to calculate the enthalpy change, ΔH, in kilojoules per mole of ammonium nitrate.

 [M_r of ammonium nitrate = 80.0]

 Include a sign in your answer.

 ΔH = .............................. kJ / mol

Sodium oxide reacts with water to form sodium hydroxide.

A scientist plans to measure the temperature change of the reaction using calorimetry in a lab.

This is the scientist's basic method:

• use 50 g of water

• record the temperature of the water

• add sodium hydroxide

• stir

• record the maximum temperature reached

Which apparatus list is the most appropriate for this experiment?

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.

i) Identify the black solid.

(1)

ii) Explain why the black solid forms.

(2)

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

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

(3)

ii) The student burns 0.96 g of methanol, CH₃OH Calculate the molar enthalpy change, ΔH, in kJ / mol, for the combustion of methanol. Include a sign in your answer. [M_r of methanol = 32]

(3)

ΔH = ...................................................................... kJ / mol

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

i) Plot the data values from the table on the grid. Draw a straight line of best fit.

(2)

ii) Deduce the value of ΔH for an alcohol with six carbon atoms per molecule. Show on the graph how you obtained your answer.

(2)

ΔH = ...................................................................... kJ/mol

iii) State the relationship between ΔH and the number of carbon atoms per molecule.

(1)

This question is about lithium oxide.

The diagram shows the electron configurations of an atom of lithium and an atom of oxygen.

Describe the changes in electronic configuration when lithium and oxygen react to form lithium oxide, Li₂O

Lithium oxide reacts with water to form lithium hydroxide as the only product. A scientist uses this apparatus to measure the temperature change of the reaction.

This is the scientist’s method.

• pour 100 g of water into a polystyrene cup

• record the temperature of the water

• add the lithium oxide and stir the mixture

• record the maximum temperature reached

The diagram shows the thermometer readings before and after adding the lithium oxide.

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

(2)

ii) Calculate the heat energy change in the reaction. Give your answer to two significant figures. [c = 4.2 J / g / °C]

(4)

heat energy change = .............................. J

iii) In another experiment the scientist obtains these results.

Calculate the molar enthalpy change (ΔH) in kJ / mol. Include a sign in your answer.

(3)

ΔH = ..............................kJ / mol

iv) Give a reason why the scientist does the experiment in a polystyrene cup.

(1)

Another student carries out the experiment to find the heat energy released in the reaction of sodium oxide with water:

Na₂O (s) + H₂O (l)  2NaOH (aq)

They found that 10,800 J of heat energy was released when 0.060 mol of sodium oxide reacts.

What is the value of ΔH, in kJ/mol, for the reaction of sodium oxide with water?

The actual molar enthalpy change, ΔH, for the reaction of sodium oxide with water, is -238 kJ/mol.

Experimental results from calorimetry in labs give less exothermic results.

Which of the following does not explain this difference?

A student investigates the reaction between zinc and copper(II) sulfate solution. The equation for the reaction is

Zn + CuSO₄ → Cu + ZnSO₄

This is his method.

• add exactly 25.0 cm³ of copper(II) sulfate solution to a polystyrene cup

• record the temperature of the solution

• add about 5 g of zinc powder (an excess) and stir the mixture

• record the highest temperature reached

i) Suggest why it is not important to add an exact mass of zinc powder.

(1)

ii) State the colour change of the solution. from ........................................................... to ...........................................................

(2)

The table shows the student’s results

i) Show that the heat energy change (Q) is about 1300 J. [for the solution, c = 4.18 J/g/°C] [mass of 1.00 cm³ of solution = 1.00 g]

(3)

ii) The mass of anhydrous copper(II) sulfate (CuSO₄) used to make 25.0 cm³ of solution is 2.00 g. Calculate the amount, in moles, of CuSO₄ in 2.00 g. [M_r of CuSO₄ = 159.5]

(1)

iii) Calculate the value of the enthalpy change (ΔH), in kilojoules per mole, for the reaction between zinc and copper(II) sulfate. Include a sign in your answer.

(3)

During the Second World War, engineers developed a rocket-powered aircraft.

The aircraft carried these two liquids

• hydrazine, N₂H₄

• hydrogen peroxide, H₂O₂

When these two liquids mix in the combustion chamber, they evaporate and then react rapidly to form nitrogen gas, N₂, and steam, H₂O

The reaction is exothermic.

The equation for the reaction is

                                        N₂H₄ + 2H₂O₂ → N₂ + 4H₂O

The displayed formulae for the reactants and products are

The tables give the bond energies for the bonds broken in the reactants and the bonds made in the products.

i) Use the data in the tables to calculate the total amount of energy required to break all of the bonds in the reactants.

(1)

energy required = .............................................................. kJ

ii) Use the data in the tables to calculate the total amount of energy released when all of the bonds in the products are made.

(1)

energy released = .............................................................. kJ

iii) Calculate the enthalpy change, ∆H, in kJ/mol, for the reaction.

Include a sign in your answer.

(3)

∆H = .............................................................. kJ/mol

Explain, in terms of bonds broken and bonds made, why this reaction is exothermic.

Draw an energy level diagram for the reaction between N₂H₄ and H₂O₂

Oxygen can be prepared from hydrogen peroxide using a catalyst.

Which is a correct statement about oxygen?

Explain how a catalyst increases the rate of a reaction.

The equation for the preparation of oxygen from hydrogen peroxide is

2H₂O₂ → 2H₂O + O₂

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

2H—O—O—H → 2H—O—H + O=O

The table gives the bond energies for these bonds.

i) Use the values in the table to calculate the enthalpy change, ΔH, for the reaction. Include a sign in your answer.

(3)

ii) Complete the energy level diagram to show the position of the products and the enthalpy change, ΔH, for the reaction.

(2)

Chlorine reacts with methane in sunlight.

 CH₄ + Cl₂  → CH₃Cl + HCl

The diagram below shows the displayed formulae for the reaction of chlorine with methane.

The table below shows the bond energies and the overall energy change in the reaction. 

What is the value of X?

The reaction between hydrogen and chlorine is exothermic. This is the equation for the reaction.

H₂ (g) + Cl₂ (g) → 2HCl (g)        ΔH = −184kJ

State the meaning of the term exothermic.

The table gives the bond energies for the H–H and H–Cl bonds.

Use the equation and information from the table to calculate the bond energy of the Cl–Cl bond.

bond energy = .............................................................. kJ/mol

Explain why this reaction is exothermic. Refer to bond-breaking and bond-making in your answer.

Complete the reaction profile diagram to show the position of the products, the enthalpy change (ΔH) and the activation energy (E_a) for the reaction.

When aqueous solutions of potassium hydroxide and nitric acid are mixed together, an exothermic reaction occurs. The diagram shows the apparatus used in an experiment to measure the temperature increase.

This is the student’s method:

• use the larger measuring cylinder to add 25cm³ of aqueous potassium hydroxide to the polystyrene cup

• record the steady temperature

• use the smaller measuring cylinder to add 5cm³ of dilute nitric acid to the cup, stir the mixture with the thermometer

• record the highest temperature of the mixture

• continue adding further 5cm³ portions of dilute nitric acid to the cup, stirring and recording the temperature, until a total volume of 35cm³ has been added.

A teacher advises the student to use a 50 cm³ burette instead of the 10 cm³ measuring cylinder. Suggest two reasons why it would be better to use a burette instead of a measuring cylinder to add the acid in this experiment.

The diagram shows the thermometer readings at the start and at the end of one experiment.

Complete the table to show:

• the thermometer reading at the start of the experiment

• the temperature rise in the experiment.

Another student uses the same method, adding the dilute nitric acid from a burette. The table shows his results.

This is the student’s graph.

The point where the lines cross represents complete neutralisation.

i) Identify the maximum temperature reached during the experiment.

(1)

maximum temperature = .............................................................. °C

ii) Identify the volume of dilute nitric acid that exactly neutralises the 25 cm³ of aqueous potassium hydroxide.

(1)
volume = .............................................................. cm³

Another student records these results.

Calculate the heat energy released in this experiment.

heat energy = .............................................................. J

In another experiment, the heat energy released is 1600 J when 0.040 mol of potassium hydroxide is neutralised. Calculate the value of ΔH, in kJ/mol, for the neutralisation of potassium hydroxide.

ΔH = .............................................................. kJ/mol

A student investigates the reaction between magnesium and hydrochloric acid.

He uses this method:

   Step 1: add 25 cm³ of dilute hydrochloric acid to a polystyrene cup

   Step 2: record the temperature of the acid

   Step 3: find the mass of a 10 cm strip of magnesium ribbon

   Step 4: add the magnesium ribbon to the hydrochloric acid

   Step 5: when all the magnesium has reacted, record the highest temperature reached

Complete the word equation for the reaction.

 magnesium + hydrochloric acid → ......................... + ...........................

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

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

(2)

ii) Show that the heat energy change (Q) for this reaction is about 2200 J.

 [mass of 1.0 cm³ of solution = 1.0 g]

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

 (2) 

iii) The mass of magnesium used by the student was 0.12 g.

 Calculate the value of the enthalpy change (ΔH), in kilojoules per mole of magnesium, for the reaction between magnesium and hydrochloric acid.

 Include a sign in your answer. 

(4)

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.

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

(1)

ii) Complete the word equation for this reaction.

(1)

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

The temperature at the start of the reaction is 20.2 °C. The maximum temperature recorded is 56.3 °C.

i) 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]

(2) 

heat energy change = .............................................................. J

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

(2)

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

i) 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, ΔH, in kJ/mol. Include a sign in your answer. Give your answer to three significant figures.

(3)

 ΔH = .............................................................. kJ/mol

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

 Zn (s) + Cu²⁺ (aq)  Zn²⁺ (aq) + Cu (s)

Explain why this is a redox reaction.

(3)

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

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

(1)

ii) Explain why the student needs to stir the mixture.

(2)

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

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

mean temperature = .............................................................. °C

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.

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

(1)

ii) Explain why the student needs to stir the mixture.

(2)

A student investigates the temperature change during the reaction between zinc metal and copper(II) sulfate solution.

The student considers two different methods.

Method 1

• Pour 50 cm³ of copper(II) sulfate solution into the polystyrene cup

• Record the temperature of the solution

• Add 3 g of zinc powder

• Stir using the thermometer and record the highest temperature reached

Method 2

• Record the temperature of 50 cm³ of copper(II) sulfate solution

• Pour the 50 cm³ of copper(II) sulfate solution into the glass bottle

• Add 3 g of zinc powder

• Push the bung and thermometer into the bottle and record the highest temperature reached

Discuss the advantages and disadvantages of each method.

The equation for the reaction is 

Zn (s) + CuSO₄ (aq) → ZnSO₄ (aq) + Cu (s)

 50cm³ of copper(II) sulfate solution contains 0.025 mol CuSO₄

 A mass of 3 g of zinc is used.

 Show that the zinc is in excess.

 [A_r of zinc = 65]

The student reacts a solution containing 0.025 mol CuSO₄ with an excess of zinc. 

These are the student’s results. 

• Temperature of 50 cm³ of copper(II) sulfate solution = 21.1 °C

• Highest temperature reached = 40.6 °C

i) Show that the energy change Q for this reaction is about 4000 J

 [mass of 1 cm³ of solution = 1.0 g]

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

 (3)

 ii) Calculate the molar enthalpy change (ΔH), in kJ / mol, for the reaction.

 (3)

 ΔH = .............................. kJ / mol

The ionic equation for the reaction is 

Zn (s) + Cu²⁺ (aq) → Zn²⁺ (aq) + Cu (s)

 Explain what is oxidised and what is reduced in this reaction.

How much energy is needed to break all the bonds in 0.025 g of nitrogen gas?

The relative atomic mass, Ar, of N is 14.