Exothermic & endothermic reactions

Metals react with acids to produce a salt and a gas.

Which salt is produced when magnesium reacts with hydrochloric acid?

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Which gas is produced when magnesium reacts with hydrochloric acid?

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A student investigated the temperature change in the reaction between metals and hydrochloric acid.

This is the method used.

1. Measure 25 cm³ of hydrochloric acid at 20 °C into a container.

2. Add 1 g of a metal.

3. Record the temperature change.

4. Repeat steps 1 to 3 using a different metal.

Draw one line from the type of variable to the name of the variable in the investigation.

Which container would be the most suitable to reduce energy transfer with the surroundings?

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Figure 1 shows part of the scale of the thermometer used in the investigation.

What is the resolution of the thermometer?

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The reaction between a metal and an acid is exothermic.

What happens to the temperature of the mixture during the reaction?

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Figure 2 shows the reaction profile for the reaction between a metal and an acid.

Which letter in Figure 2 represents the reactants?

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Which letter in Figure 2 represents the activation energy?

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Which letter in Figure 2 represents the overall energy change?

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This question is about energy changes of reactions.

Zinc reacts with copper sulfate solution.

The word equation for the reaction is:

zinc + copper sulfate → zinc sulfate + copper

What type of reaction is the reaction between zinc and copper sulfate solution?

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Calculate the percentage (%) by mass of copper in copper sulfate (CuSO₄).

Give your answer to 3 significant figures.

Relative atomic mass (A_r): Cu = 63.5

Relative formula mass (M_r): CuSO₄ = 159.5

A student investigated the energy change in the reaction between zinc and copper sulfate solution.

This is the method used.

1. Measure 25 cm³ of copper sulfate solution into a polystyrene cup.

2. Weigh 0.20 g of zinc powder.

3. Add the zinc powder to the copper sulfate solution.

4. Measure the highest temperature reached by the mixture.

5. Repeat steps 1 to 4 using different masses of zinc powder.

Control variables are used to make an investigation a fair test.

Which is a control variable in the investigation?

Tick () one box.

Figure 6 shows the results.

What is the minimum mass of zinc powder needed to react with all the copper sulfate solution?

Use Figure 6.

What is the maximum temperature change in the reaction between zinc powder and 25 cm³ of copper sulfate solution?

Use Figure 6.

25 cm³ of copper sulfate solution contained 6.75 g of copper sulfate.

Calculate the concentration of the solution in g/dm³.

You should:

calculate the volume of the solution in dm³ (1000 cm³ = 1 dm³)

use the equation:

Another student investigated the energy change of the reaction between sodium hydrogencarbonate and hydrochloric acid.

This is the method used.

1. Measure 25 cm³ of hydrochloric acid.

2. Weigh 1.0 g of sodium hydrogencarbonate.

3. Add the sample of sodium hydrogencarbonate to the hydrochloric acid.

4. Measure the lowest temperature reached by the mixture.

5. Repeat steps 1 to 4 using different masses of sodium hydrogencarbonate.

Figure 7 shows the results.

Draw two straight lines of best fit on Figure 7.

The lines should cross.

Which statement describes the energy change in the reaction shown in Figure 7 ?

Tick () one box.

A student investigated the energy change of the reaction between zinc and copper sulfate solution.

This is the method used.

1. Measure 25 cm³ of copper sulfate solution into a polystyrene cup.

2. Measure the temperature of the copper sulfate solution.

3. Add 0.20 g of zinc powder to the copper sulfate solution.

4. Stir the reaction mixture.

5. Record the highest temperature reached.

6. Repeat steps 1 to 5 with different masses of zinc powder.

Figure 3 shows the results.

Draw two lines of best fit on Figure 3.

The lines should cross.

Explain the results shown in Figure 3.

Do not refer to anomalous points.

Use data from Figure 3.

Explain why using a polystyrene cup gives more accurate results than using a glass beaker.

Complete the ionic equation for the reaction between zinc and copper sulfate solution.

Include state symbols.

A different student repeated steps 1 to 5 of the method four times using 0.50 g of zinc powder.

Table 2 shows the results.

Calculate the mean highest temperature reached.

Include the uncertainty in your answer.

The results show random errors.

The student did not make any measuring errors.

Suggest one reason for the random errors in this experiment.

This question is about temperature changes.

A student investigated the change in temperature of a solution when different masses of ammonium nitrate were dissolved in water. This is the method used.

1. Measure 200 cm³ of water into a polystyrene cup.

2. Measure the temperature of the water.

3. Add 4.0 g of ammonium nitrate to the water.

4. Stir the solution until all the ammonium nitrate has dissolved.

5. Measure the lowest temperature reached by the solution.

6. Repeat steps 1 to 5 with different masses of ammonium nitrate.

Give the independent variable and the dependent variable in the investigation. Independent variable Dependent variable

Table 6 shows the results.

Plot the data from Table 6 on Figure 11. Draw a line of best fit.

Determine the initial temperature of the water. You should extend your line of best fit on Figure 11.

How do the results show that dissolving ammonium nitrate in water is endothermic?

The student repeated the experiment three more times. Table 7 shows the results for 8.0 g of ammonium nitrate.

The student recorded the mean lowest temperature of the solution for 8.0 g of ammonium nitrate as 16.5 ± 0.3 ºC. Explain why the student included ± 0.3 ºC after the mean lowest temperature.

What type of error is shown by the results in Table 7 ? Tick (✓) one box.

This question is about temperature changes.

A student investigated the change in temperature of a solution when different masses of ammonium nitrate were dissolved in water. This is the method used.

1. Measure 200 cm³ of water into a polystyrene cup.

2. Measure the temperature of the water.

3. Add 4.0 g of ammonium nitrate to the water.

4. Stir the solution until all the ammonium nitrate has dissolved.

5. Measure the lowest temperature reached by the solution.

6. Repeat steps 1 to 5 with different masses of ammonium nitrate.

Give the independent variable and the dependent variable in the investigation. Independent variable Dependent variable

Table 2 shows the results.

Plot the data from Table 2 on Figure 2. Draw a line of best fit.

Determine the initial temperature of the water. You should extend your line of best fit on Figure 2.

How do the results show that dissolving ammonium nitrate in water is endothermic?

The student repeated the experiment three more times. Table 3 shows the results for 8.0 g of ammonium nitrate.

The student recorded the mean lowest temperature of the solution for 8.0 g of ammonium nitrate as 16.5 ± 0.3 ºC. Explain why the student included ± 0.3 ºC after the mean lowest temperature.

What type of error is shown by the results in Table 3 ? Tick (✓) one box.

This question is about hydrogen and compounds of hydrogen. Figure 3 shows the displayed formulae for the reaction between hydrogen and chlorine.

Figure 3

Table 4 shows the bond energies.

Which expression shows how to calculate the overall energy change for the reaction in Figure 3 ? Use Table 4. Tick (✓) one box.

The reaction between hydrogen and chlorine is exothermic.

Explain why this reaction releases energy to the surroundings.

Figure 4 shows part of a reaction profile for the reaction between hydrogen and chlorine. Complete the reaction profile in Figure 4. You should:

• label the activation energy

• label the overall energy change.

Draw a dot and cross diagram for a molecule of hydrogen chloride (HC l). Show the outer shell electrons only.

Figure 5 represents molecules of methane and of poly(ethene).

Methane is a gas at room temperature but poly(ethene) is a solid at room temperature.

Explain why methane and poly(ethene) exist in different states at room temperature.

Sodium carbonate reacts with hydrochloric acid in an exothermic reaction.

The equation for the reaction is:

A student investigated the effect of changing the mass of sodium carbonate powder on the highest temperature reached by the reaction mixture.

Plan a method to investigate the effect of changing the mass of sodium carbonate powder on the highest temperature reached.

Figure 17 shows a line of best fit drawn through the student’s results.

Determine the gradient of the line of best fit in Figure 17.

Use the equation:

Give the unit.

The initial temperature of the reaction mixture is where the line of best fit would meet the y-axis.

Determine the initial temperature of the reaction mixture.

Show your working on Figure 17.

Another student repeated the investigation but added sodium carbonate until the sodium carbonate was in excess.

Which sketch graph shows the results obtained when sodium carbonate was added until in excess?

Tick (✓) one box.

Figure 18 shows a reaction profile for the reaction of sodium carbonate with hydrochloric acid.

What do labels X and Y represent on Figure 18 ?

How does the reaction profile show that the reaction is exothermic?

Use Figure 18.

Sodium carbonate reacts with hydrochloric acid in an exothermic reaction. The equation for the reaction is:

A student investigated the effect of changing the mass of sodium carbonate powder on the highest temperature reached by the reaction mixture.

Plan a method to investigate the effect of changing the mass of sodium carbonate powder on the highest temperature reached.

Figure 3 shows a line of best fit drawn through the student’s results.

Determine the gradient of the line of best fit in Figure 3. Use the equation:

Give the unit.

The initial temperature of the reaction mixture is where the line of best fit would meet the y-axis. Determine the initial temperature of the reaction mixture. Show your working on Figure 3.

Another student repeated the investigation but added sodium carbonate until the sodium carbonate was in excess. Which sketch graph shows the results obtained when sodium carbonate was added until in excess? Tick (✓) one box.

Figure 4 shows a reaction profile for the reaction of sodium carbonate with hydrochloric acid.

What do labels X and Y represent on Figure 4 ?

How does the reaction profile show that the reaction is exothermic? Use Figure 4.

This question is about citric acid. Figure 7 represents one molecule of citric acid.

Complete the molecular formula of citric acid. Use Figure 7. C 6 H __ O __

What type of bonding is shown in Figure 7 ? Tick (✓) one box.

Figure 8 shows two representations of one molecule of citric acid, A and B.

Figure 8

Give two advantages of representation A compared with representation B. Advantages of A:

A student investigated the temperature change during the reaction between citric acid and sodium hydrogencarbonate solution. Citric acid is a solid. This is the method used.

1. Pour 25 cm³ of sodium hydrogencarbonate solution into a polystyrene cup.

2. Measure the temperature of the sodium hydrogencarbonate solution.

3. Add 0.25 g of citric acid to the cup.

4. Stir the solution.

5. Measure the temperature of the solution.

6. Repeat steps 3 to 5 until a total of 2.00 g of citric acid has been added.

Table 4 shows some of the student’s results.

How do the results in Table 4 show that the reaction is endothermic?

Three of the student’s results are plotted on Figure 9. A line of best fit for these points is drawn. Complete Figure 9. You should:

• plot the data from Table 4 on Figure 9

• draw a line of best fit through the points you have plotted

• extend your line of best fit to meet the line of best fit already drawn on Figure 9.

Determine the overall temperature change for the reaction. Use Figure 9.

What is the dependent variable in this investigation? Tick (✓) one box.

This question is about citric acid (C₆H₈O₇). Citric acid is a solid. A student investigated the temperature change during the reaction between citric acid and sodium hydrogencarbonate solution. This is the method used.

1. Pour 25 cm³ of sodium hydrogencarbonate solution into a polystyrene cup.

2. Measure the temperature of the sodium hydrogencarbonate solution.

3. Add 0.20 g of citric acid to the polystyrene cup.

4. Stir the solution.

5. Measure the temperature of the solution.

6. Repeat steps 3 to 5 until a total of 2.00 g of citric acid has been added.

The student plotted the results on a graph. Figure 6 shows the student’s graph.

Figure 6 shows an anomalous point when 0.60 g of citric acid was added. This was caused by the student making an error. The student correctly:

• measured the mass of the citric acid

• read the thermometer

• plotted the point.

Suggest one reason for the anomalous point.

Explain the shape of the graph in terms of the energy transfers taking place. You should use data from Figure 6 in your answer.

A second student repeated the investigation using a metal container instead of the polystyrene cup. The container and the cup were the same size and shape. Sketch a line on Figure 6 to show the second student’s results until 1.00 g of citric acid had been added. The starting temperature of the solution was the same. Explain your answer.

The student used a solution of citric acid to determine the concentration of a solution of sodium hydroxide by titration.

The student made 250 cm³ of a solution of citric acid of concentration 0.0500 mol/dm³ Calculate the mass of citric acid (C₆H₈O₇) required. Relative atomic masses (A_r): H = 1 C = 12 O = 16

This is part of the method the student used for the titration.

1. Measure 25.0 cm³ of the sodium hydroxide solution into a conical flask using a pipette.

2. Add a few drops of indicator to the flask.

3. Fill a burette with citric acid solution.

Describe how the student would complete the titration.

Give two reasons why a burette is used for the citric acid solution.

13.3 cm³ of 0.0500 mol/dm³ citric acid solution was needed to neutralise 25.0 cm³ of sodium hydroxide solution. The equation for the reaction is:

Calculate the concentration of the sodium hydroxide solution in mol/dm³

29

This question is about compounds of oxygen and hydrogen. Figure 2 represents the structure of hydrogen peroxide.

Figure 2

What is the correct formula of hydrogen peroxide? Tick (✓) one box.

Which type of bonding is shown in Figure 2 ? Tick (✓) one box.

Hydrogen peroxide decomposes in the presence of a catalyst. Which elements are often used as catalysts?

Tick (✓) one box.

Figure 3 shows the reaction profile for the decomposition of hydrogen peroxide. The word equation for this reaction is: hydrogen peroxide → water + oxygen

Labels A, B, C and D each represent a different part of the reaction profile. Use Figure 3 to answer Questions 02.4 and 02.5

Which label shows the activation energy? Tick (✓) one box.

Which label shows the energy of hydrogen peroxide? Tick (✓) one box.

The decomposition of hydrogen peroxide gives out energy to the surroundings. What type of reaction is this?

Tick (✓) one box.

Hydrogen and oxygen form water. A hydrogen atom contains one electron. An oxygen atom contains six electrons in the outer shell. Complete Figure 4 to show a dot and cross diagram for a water molecule. Show the outer electrons only.

This question is about energy changes in reactions.

Ammonium nitrate dissolves in water. The change is endothermic. Which piece of equipment uses this change?

Tick (✓) one box.

A student investigated the temperature change in the reaction between dilute sulfuric acid and potassium hydroxide solution. This is the method used.

1. Measure 25 cm³ of potassium hydroxide solution into a glass beaker.

2. Add 5 cm³ of dilute sulfuric acid.

3. Stir the solution.

4. Measure the temperature of the solution.

5. Repeat steps 2 to 4 until a total of 30 cm³ of dilute sulfuric acid has been added.

Figure 9 shows part of the scales of four thermometers, A, B, C and D.

The student wanted to measure the temperature to a resolution of 0.1 °C Which thermometer should the student use?

Tick (✓) one box.

Energy is lost to the surroundings during the reaction. What type of error does this cause in the results?

Tick (✓) one box.

The student used a glass beaker for the reaction. Name a container the student could use instead of the glass beaker to improve the accuracy of the results.

Table 3 shows the student’s results.

Plot the data from Table 3 on Figure 10. You should:

• draw a line of best fit

• extend your line of best fit to the y-axis.

The intercept on the y-axis of Figure 10 shows the starting temperature of the potassium hydroxide solution. Give the starting temperature of the potassium hydroxide solution.

Another student repeated the investigation and obtained an anomalous result. This result was lower than expected. What could have caused the anomalous result?

Tick (✓) two boxes.

This question is about ammonia, NH₃

Complete the dot and cross diagram for the ammonia molecule shown in Figure 6. Show only the electrons in the outer shell of each atom.

Give one limitation of using a dot and cross diagram to represent an ammonia molecule.

Explain why ammonia has a low boiling point. You should refer to structure and bonding in your answer.

Ammonia reacts with oxygen in the presence of a metal oxide catalyst to produce nitrogen and water.

Which metal oxide is most likely to be a catalyst for this reaction? Tick (✓) one box.

Figure 7 shows the displayed formula equation for the reaction.

Table 3 shows some bond energies.

Calculate the overall energy change for the reaction. Use Figure 7 and Table 3.

Explain why the reaction between ammonia and oxygen is exothermic. Use values from your calculation in Question 05.5

Figure 8 shows the reaction profile for the reaction between ammonia and oxygen. Complete Figure 8 by labelling the:

• activation energy

• overall energy change.

A student investigated the temperature change in the reaction between dilute sulfuric acid and potassium hydroxide solution. This is the method used.

1. Measure 25.0 cm³ potassium hydroxide solution into a polystyrene cup.

2. Record the temperature of the solution.

3. Add 2.0 cm³ dilute sulfuric acid.

4. Stir the solution.

5. Record the temperature of the solution.

6. Repeat steps 3 to 5 until a total of 20.0 cm³ dilute sulfuric acid has been added.

Suggest why the student used a polystyrene cup rather than a glass beaker for the reaction.

Table 6 shows some of the student’s results.

Figure 11 shows some of the data from the investigation.

Complete Figure 11:

• plot the data from Table 6

• draw a line of best fit through these points

• extend the lines of best fit until they cross.

Determine the volume of dilute sulfuric acid needed to react completely with 25.0 cm³ of the potassium hydroxide solution. Use Figure 11.

Determine the overall temperature change when the reaction is complete. Use Figure 11.

The student repeated the investigation. The student used solutions that had different concentrations from the first investigation. The student found that 15.5 cm³ of 0.500 mol/dm³ dilute sulfuric acid completely reacted with 25.0 cm³ of potassium hydroxide solution. The equation for the reaction is:

Calculate the concentration of the potassium hydroxide solution in mol/dm³ and in

Relative atomic masses (A_r): H = 1 O = 16 K = 39

A student investigated the temperature change in displacement reactions between metals and copper sulfate solution.

This is the method used.

1. Measure 50 cm³ of the copper sulfate solution into a polystyrene cup.

2. Record the starting temperature of the copper sulfate solution.

3. Add the metal and stir the solution.

4. Record the highest temperature the mixture reaches.

5. Calculate the temperature increase for the reaction.

6. Repeat steps 1-5 with different metals.

Draw one line from each type of variable to the name of the variable in the investigation.

The student used a polystyrene cup and not a glass beaker.

Why did this make the investigation more accurate?

Tick one box.

Table 5 shows the student’s results.

Complete Figure 9.

Use data from Table 5.

The student concluded that the reactions between the metals and copper sulfate solution are endothermic.

Give one reason why this conclusion is not correct.

The temperature increase depends on the reactivity of the metal.

Write the metals magnesium, nickel and zinc in order of reactivity.

Use Table 5.

Y is an unknown metal.

Describe a method to find the position of Y in the reactivity series in Question 06.5

Figure 10 shows the reaction profile for the reaction between zinc and copper sulfate solution.

Which letter represents the products of the reaction?

Tick one box.

Which letter represents the activation energy?

Tick one box.

A student investigated the temperature change in displacement reactions between metals and copper sulfate solution. Table 2 shows the student’s results.

Plot the data from Table 2 on Figure 4 as a bar chart.

The student concluded that the reactions between the metals and copper sulfate solution are endothermic. Give one reason why this conclusion is not correct.

The temperature change depends on the reactivity of the metal. The student’s results are used to place copper, iron, magnesium and zinc in order of their reactivity. Describe a method to find the position of an unknown metal in this reactivity series. Your method should give valid results.

Draw a fully labelled reaction profile for the reaction between zinc and copper sulfate solution on Figure 5.