Exothermic & endothermic reactions

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

• pour 25 cm³ of dilute hydrochloric acid into a glass beaker

• measure the temperature of the acid

• add 5 cm³ of sodium hydroxide solution and stir the mixture

• record the highest temperature reached

• continue to add further 5 cm³ portions of sodium hydroxide solution until a total of 40 cm³ has been added

• record the temperature after adding each 5 cm³ portion of sodium hydroxide solution

State two factors that the student must keep constant to make this a valid investigation.

Explain how using a polystyrene cup, instead of a glass beaker, would increase the accuracy of the results.

The graph shows the student’s results.

Volume of sodium hydroxide in cm³

Use the graph to determine the maximum temperature change in °C.

Explain the shape of the graph.

The student repeats the experiment using a polystyrene cup.

These are the student’s results.

Calculate the heat energy change (Q) in kJ.

[for the solution, 1.0 cm³ has a mass of 1.0 g c = 4.2 J / g / °C]

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 teacher ignites a piece of magnesium ribbon and places it in a gas jar of oxygen.

This is the equation for the reaction.

Give two observations that would be made during this reaction.

The teacher demonstrates the reaction between aluminium and iron(III) oxide.

The teacher ignites the magnesium fuse. A very exothermic reaction then occurs between aluminium powder and iron(III) oxide.

The products of the reaction are iron and aluminium oxide.

State what is meant by the term exothermic.

Give a chemical equation for the reaction between aluminium and iron(III) oxide.

Explain why this reaction is a redox reaction.

A student uses this apparatus to determine the formula of magnesium oxide.

This is the student’s method.

• find mass of empty crucible and lid

• place some magnesium ribbon in crucible

• find mass of crucible, lid and magnesium ribbon

• heat crucible, lifting lid occasionally

• after five minutes, find mass of crucible, lid and magnesium oxide

Explain why the student lifts the lid.

The mass of magnesium oxide formed is less than expected.

Describe what the student should do next to obtain a more accurate value for the mass of magnesium oxide.

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.

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.

The reaction between hydrogen and chlorine is exothermic.

This is the equation for the reaction.

State the meaning of the term exothermic.

The table gives the bond energies for the H H and H l bonds.

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

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.

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 piece of magnesium ribbon is ignited and placed in a gas jar of oxygen.

The equation for the reaction is

Give two observations that would be made in this reaction.

State why this is an oxidation reaction.

A second piece of magnesium ribbon is ignited and placed in a gas jar of carbon dioxide.

A very exothermic reaction occurs, forming magnesium oxide and carbon.

State what is meant by the term exothermic.

Give the chemical equation for this reaction.

A fire starts in a warehouse where magnesium is stored.

Suggest why it would not be suitable to use a carbon dioxide fire extinguisher to put out this fire.

A student uses this apparatus to find the mass of magnesium oxide that forms when a known mass of magnesium is heated.

This is his method.

• find the mass of the crucible and lid

• place some magnesium ribbon in the crucible

• find the mass of the crucible, lid and magnesium

• heat the crucible with the lid on for a few minutes

• find the mass of the crucible, lid and magnesium oxide

Using this method, the mass of magnesium oxide formed is less than expected.

Explain two changes that the student should make to his method to obtain a mass of magnesium oxide closer to the expected mass.

The diagram shows two samples of iron, A and B.

Sample B is coated with a thin layer of zinc.

Name the process used to coat iron with zinc.

The two samples of iron are left outside for several weeks.

A brown solid containing hydrated iron(III) oxide forms on sample A.

Give the common name for the brown solid.

Give the names of the two substances that react with the iron to form the brown solid.

Iron can be formed by reacting aluminium powder with iron(III) oxide.

The diagram shows how this reaction can be demonstrated.

When the magnesium fuse is lit, a very exothermic reaction occurs.

State the meaning of the term exothermic.

The equation for the reaction between aluminium and iron(III) oxide is

Explain what this reaction shows about the relative reactivities of aluminium and iron.

Explain why the reaction between aluminium and iron(III) oxide is a redox reaction.

A student uses this method to investigate the reaction of dilute hydrochloric acid with zinc.

● pour some dilute hydrochloric acid into a glass beaker

● record the initial temperature of the acid

● add a piece of zinc and stir the mixture

● record the temperature of the mixture after one minute

Write a word equation for the reaction of dilute hydrochloric acid with zinc.

The diagram shows the thermometer readings for this reaction.

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

Another student repeats the method using five different metals to compare their reactivity.

This student uses a polystyrene cup instead of a glass beaker.

Explain why a polystyrene cup is better than a glass beaker in this investigation.

Give three factors that the student should keep constant in this investigation.

The table shows some of the student’s results.

State why there is no temperature change for copper.

Predict the temperature change for iron.

Deduce the order of reactivity of the five metals.

A student investigates the reaction between sodium hydroxide solution and hydrochloric acid.

He uses this method.

Step 1 add 50 cm³ of dilute hydrochloric acid to a conical flask

Step 2 add a 5 cm³ portion of sodium hydroxide solution to the conical flask

Step 3 test the pH of the mixture using both universal indicator paper and a pH meter

The student repeats step 2 and step 3 until a total of 50 cm³ of sodium hydroxide solution has been added.

State the piece of apparatus that should be used to measure 50 cm³ of hydrochloric acid.

Name the type of reaction that occurs between hydrochloric acid and sodium hydroxide.

Graph 1 shows how the pH of the mixture changes as the sodium hydroxide solution is added.

Volume of sodium hydroxide solution in cm³

Graph 1

Determine the pH after 40 cm³ of sodium hydroxide solution has been added.

Suggest the colour of the universal indicator paper when these volumes of sodium hydroxide solution have been added.

Give the formula of the ion that causes sodium hydroxide to be alkaline.

Another student investigates how the temperature changes when the sodium hydroxide solution is added to the hydrochloric acid.

The hydrochloric acid and the sodium hydroxide solution are at the same temperature at the start of the investigation.

The student records the temperature of the mixture after adding each 5 cm³ portion of sodium hydroxide solution.

Graph 2 shows her results.

Volume of sodium hydroxide solution in cm³

Graph 2

Explain the shape of graph 2.