The Rate & Extent of Chemical Change (AQA GCSE Combined Science: Synergy: Physical Sciences): Exam Questions

How must the apparatus for the reaction be designed so that dynamic equilibrium can be reached?

Explain what happens to the relative amounts of nitrogen, hydrogen and ammonia after dynamic equilibrium has been reached.

How does the equation show that the reaction is reversible?

When does a reversible reaction reach dynamic equilibrium?

How must the apparatus for the reaction be designed so that dynamic equilibrium can be reached?

Hydrogen bromide decomposes to form hydrogen and bromine.

The equation for the reaction is:

$2\text{HBr}\to {\text{H}}_2+{\text{Br}}_2$

The energy needed to break the existing bonds is 103 kJ/mol greater than the energy released in forming new bonds.

TABLE 3 shows some bond energy values.

TABLE 3

Y is the bond energy of a H–Br bond.

Calculate Y. [5 marks]

Y = _________________ kJ/mol

FIGURE 12 shows part of the reaction profile when hydrogen bromide decomposes to form hydrogen and bromine.

Complete FIGURE 12.

You should:

• label the axes

• draw the reaction profile

• label the reactant and the products

• label the activation energy.

Calcium carbonate reacts with hydrochloric acid.

The equation for the reaction is:

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

A student investigated the effect of changing the surface area of the calcium carbonate on the rate of this reaction.

The student used calcium carbonate in the form of lumps and of powder.

Figure 4 shows the apparatus.

The rate of reaction is determined by measuring the decrease in mass of the conical flask and contents at regular time intervals.

Give the reason why the mass of the conical flask and contents decreases.

The mean rate of reaction using the calcium carbonate lumps was 0.0012 g/s.

Calculate the time taken for the mass of the conical flask and contents to decrease by 0.36 g.

The results showed that calcium carbonate powder reacted faster than calcium carbonate lumps.

Explain why calcium carbonate powder reacted faster than calcium carbonate lumps.

The surface area of a calcium carbonate lump can be estimated by comparing the lump with a cube.

Figure 5 shows a cube and a similar-sized calcium carbonate lump.

Calculate the total surface area of the cube in Figure 5.

The total surface area of the calcium carbonate lump in Figure 5 is estimated rather than measured.

Suggest one reason why.

A student investigated the reaction between calcium carbonate and hydrochloric acid.

The equation for the reaction is:

${\text{CaCO}}_3+2\text{HCl}\to {\text{CaCl}}_2+{\text{H}}_2\text{O}+{\text{CO}}_2$

The student investigated the effect of changing the concentration of hydrochloric acid on the rate of the reaction.

FIGURE 16 shows the apparatus.

This is the method used.

1. Add 50 cm³ of hydrochloric acid to the conical flask.

2. Add 2.0 g of calcium carbonate to the conical flask.

3. Immediately insert the stopper into the conical flask and start the timer.

4. Record the volume of gas collected in the gas syringe every 30 seconds.

5. Stop recording when the volume of gas in the gas syringe does not change.

6. Repeat steps 1 to 5 using different concentrations of hydrochloric acid.

Name the independent variable in this investigation.

FIGURE 17 shows the results for one concentration of hydrochloric acid.

Determine the rate of reaction at 60 seconds.

Show your working on FIGURE 17.

Give your answer to 2 significant figures.

How does FIGURE 17 show that the rate of reaction decreases with time?

Explain why the rate of reaction between calcium carbonate and hydrochloric acid decreases with time.

Complete the sentence.

The minimum amount of energy that particles must have to react is called the ________ .

This question is about a reversible reaction.

Ethene reacts with steam to produce ethanol.

The equation for the reaction is:

${\text{C}}_2{\text{H}}_4\left(g\right)+{\text{H}}_2\text{O}\left(g\right)\rightleftharpoons {\text{C}}_2{\text{H}}_5\text{OH}\left(g\right)$

FIGURE 14 shows the percentage yield of ethanol using different reaction conditions.

Predict the percentage yield of ethanol at a pressure of 100 atmospheres and a temperature of 350 ºC.

Use FIGURE 14.

Percentage yield =___________________ %

Explain why changing the temperature affects the percentage yield of ethanol.

Use FIGURE 14.

How must the apparatus for the reaction be designed so that equilibrium can be reached?

A catalyst is used to increase the rate of the reaction.

Define the term 'catalyst'.

What is the effect of the catalyst on the percentage yield of ethanol at equilibrium?

Give ONE reason for your answer.

Calcium carbonate reacts with hydrochloric acid.

The word equation for the reaction is:

calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide

A student investigated the effect of changing the surface area of the calcium carbonate on the rate of this reaction.

The student changed the surface area of the calcium carbonate by using different-sized lumps.

Figure 6 shows the apparatus.

The rate of reaction is determined by measuring the decrease in mass of the conical flask and contents at regular time intervals.

This is the method used.

1. Place a conical flask on a balance.

2. Add 50 cm³ of hydrochloric acid to the conical flask.

3. Add 2 g of small lumps of calcium carbonate to the hydrochloric acid.

4. Put cotton wool in the top of the conical flask.

5. Record the mass every 60 seconds until the mass remains constant.

6. Repeat steps 1 to 5 with 2 g of large lumps of calcium carbonate.

Why was cotton wool put in the top of the conical flask?

What was the independent variable in this investigation?

Give one control variable used in this investigation.

Table 3 shows some of the results.

Table 3

Calculate the mean rate of reaction from 0 to 60 seconds for the small lumps.

Use the equation:

$\text{mean rate of reaction}=\frac{\text{decrease in mass}}{\text{time taken}}$

Use Table 3.

Complete Figure 7.

You should:

• label the y-axis

• plot the data from Table 3 as a bar chart

• label each bar.

Why are the results plotted as a bar chart and not as a line graph?

Table 3 is repeated below.

Table 3

What effect does the size of the calcium carbonate lumps have on the rate of reaction?

Use Table 3 .

The surface area of a calcium carbonate lump can be estimated by comparing the lump with a cube.

Figure 8 shows a cube and a similar-sized calcium carbonate lump.

Calculate the total surface area of the cube in Figure 8.

Use the equation:

$\text{total surface area of cube}=6\times \text{length of one side}\times \text{length of one side}$

Suggest one reason why the total surface area of the lump in Figure 8 is estimated rather than measured.

Magnesium reacts with hydrochloric acid.

A student investigated the effect of changing the hydrochloric acid concentration on the rate of this reaction.

Figure 22 shows the apparatus.

This is the method used.

1. Add 50 cm³ of hydrochloric acid to the conical flask.

2. Add a 3 cm strip of magnesium to the hydrochloric acid in the conical flask.

3. Fit the stopper and delivery tube to the top of the conical flask and start timing.

4. Record the volume of hydrogen gas collected in the measuring cylinder every 20 seconds for a total of 100 seconds.

5. Repeat steps 1 to 4 with a different concentration of hydrochloric acid.

What volume of hydrogen gas has been collected in the measuring cylinder in Figure 22?

The stopper and delivery tube were fitted to the conical flask in step 3.

Explain why the time taken to fit the stopper and delivery tube may cause an error in this investigation.

Figure 23 shows the results for one concentration of hydrochloric acid.

Determine the time taken for the reaction to be complete.

Use Figure 23.

The student repeated the method using a higher concentration of hydrochloric acid.

How would the line of best fit for a higher concentration of hydrochloric acid compare with the line of best fit on Figure 23?

A student investigated the rate of the reaction between calcium carbonate and hydrochloric acid.

The word equation for the reaction is:

calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide

Figure 6 shows the apparatus.

This is the method used:

1. Add 50 cm³ of hydrochloric acid to the conical flask

2. Add 2.0 g of calcium carbonate to the conical flask

3. Immediately insert the stopper into the conical flask and start the timer

4. Record the volume of gas collected in the gas syringe every 30 seconds

5. Stop recording when the volume of gas in the gas syringe does not change

Which piece of equipment is the most suitable to measure the volume of hydrochloric acid?

Tick (✓) one box.

Name one piece of equipment that can be used to measure the mass of calcium carbonate.

Why must the stopper be inserted immediately after the reactants are added to the flask?

Figure 7 shows the results.

Describe the trend for the volume of gas collected.

Use Figure 7.

Determine the mean rate of reaction from 0 to 30 seconds.

Use Figure 7.

Use the equation:

$\text{mean rate of reaction}=\frac{\text{volume of gas collected}}{\text{time taken}}$

Choose the unit from the box.

Mean rate of reaction =_____________ Unit _____________

The student repeated the investigation using hydrochloric acid of higher concentration.

Complete the sentences.

Choose answers from the box.

When the concentration of hydrochloric acid increases, the rate of reaction increases.

This is because the mean distance between particles ________________.

Therefore the frequency of collisions between particles______________.

What is the minimum amount of energy that particles must have to react?

Tick (✓) one box.

Give two measurements the student should make to determine the rate of reaction.

Ammonia reacts with hydrogen chloride to produce ammonium chloride.

The reaction is reversible.

Write the word equation for the reaction.

Include the symbol for a reversible reaction.

________________+__________________ ________________

Which condition needs to be changed so that the direction of the reaction is reversed?

Enzymes are large molecules and biological catalysts.

What type of molecule is an enzyme?

A catalyst changes the rate of a chemical reaction.

What happens to the mass of a catalyst during a chemical reaction?

Tick (✓) one box.

Figure 13 shows a model of enzyme action.

Name X and Y in Figure 13.

Name the model shown in Figure 13.

Amylase is an enzyme that breaks down starch.

A student investigated how changing the pH affected the activity of amylase.

This is the method used.

1. Heat a test tube containing 2 cm³ of amylase solution at pH 5.0 in a water bath at 35 °C.

2. Heat a test tube containing 2 cm³ of starch solution in the same water bath.

3. Transfer the amylase solution into the test tube of starch solution and mix.

4. After 30 seconds remove a drop of the amylase–starch mixture and test the drop for starch.

5. Repeat step 4 until no starch is detected.

6. Record the total time taken for no starch to be detected.

7. Repeat steps 1 to 6 using amylase solution at different pH values.

Explain why the solutions are all placed in the same water bath.

Describe the test for starch.

Give the result of the test if starch is present.

Test:______________________________________________________

Result:____________________________________________________

Table 4 shows the results.

Table 4

Which range of pH values had the fastest rate of reaction?

Tick (✓) one box.

How could the method be improved to obtain a more accurate pH value for the fastest rate of reaction?

The rate of chemical reactions can be changed by changing the conditions.

Methane burns in oxygen to produce carbon dioxide and water.

The activation energy for the reaction is 2648 kJ/mol.

The reaction gives out 818 kJ/mol of energy.

Figure 1 shows the reaction profile for this reaction.

Figure 1

Complete the reaction profile.

Draw arrows to represent:

• the activation energy

• the energy given out.

What percentage of the activation energy is the energy given out?

Calcium carbonate decomposes when it is heated.

The decomposition of calcium carbonate is an endothermic reaction.

How would the reaction profile for decomposition of calcium carbonate be different from the reaction profile of methane burning in oxygen?

Catalysts are used in chemical reactions in industry.

Give two properties of catalysts.

For each property, explain why it makes the catalyst useful in industry.

Enzymes are biological catalysts.

What type of molecule is an enzyme?

Tick (✓) one box.

If enzymes are denatured they stop working.

Give two ways an enzyme can be denatured.

An enzyme called lactase catalyses the reaction that breaks down lactose to smaller molecules.

One model used to explain how enzymes affect reactions is called the lock and key model.

Use the lock and key model to explain why lactase cannot be used to speed up all chemical reactions.

Farmers use ammonium nitrate as a fertiliser.

Farmers want to slow down the rate at which ammonium nitrate fertiliser dissolves in the water in the soil.

Suggest why they spread the fertiliser in the form of small beads instead of a fine powder.

Ammonia is needed to make ammonium nitrate.

The reaction used to make ammonia is:

${\text{N}}_2\left(g\right)+3{\text{H}}_2\left(g\right)\rightleftharpoons 2{\text{NH}}_3\left(g\right)$

The forward reaction is exothermic.

At equilibrium, about 35% of the nitrogen and hydrogen are converted to ammonia at 450 °C and 200 atmospheres pressure.

Explain the effects of increasing the temperature, or increasing the pressure, on the amount of ammonia produced at equilibrium.

A student investigates how the concentration of an acid affects the rate of a reaction.

This is the method used.

1. Put a 3 cm piece of magnesium ribbon into a conical flask.

2. Add 50 cm³ of 0.5 mol/dm³ hydrochloric acid to the flask.

3. Collect and measure the volume of gas produced at 10 second intervals.

4. Repeat with different concentrations of hydrochloric acid using the same length of magnesium ribbon and volume of acid.

The student's results are shown in Figure 3.

Figure 3

How do the results show that increasing the concentration of acid increases the rate of reaction?

Explain why the rate of reaction changes as the concentration of the acid increases.

You should answer in terms of particles.

Student A said that the final volume of gas collected was lower for a concentration of 0.5 mol/dm³ because the reaction had not finished.

Student B said it was because all the acid had reacted.

Describe further experimental work the students could do to find out which student was correct.

Amylase catalyses the breakdown of starch into sugars.

A student investigated the effect of amylase on the reaction at different temperatures.

Figure 14 shows the apparatus the student used.

Figure 14

Method:

1. Put starch suspension into a test tube.

2. Add amylase solution.

3. Put the test tube in a beaker of water at 15 °C.

4. Remove a small sample of the mixture every 30 seconds and put in a spotting tile.

5. Test the sample for starch.

6. Time how long it takes to break down all of the starch in the mixture.

7. Repeat steps 1–5 at 20 °C, 25 °C and 30 °C.

8. Repeat for each temperature twice more.

Table 8 shows the student's results.

Table 8

One of the results in Table 8 is anomalous.

Draw a ring around the anomalous result.

Calculate the mean for 30 °C.

Figure 15 shows a graph of the student's results.

Use the graph to predict how long it would take to break down all of the starch at 10 °C.

Time = __________________ minutes

The student tested samples of the mixture for starch every 30 seconds.

In each test she added one drop of iodine to the sample in the spotting tile.

Predict the colour of the samples from the 20 °C test at 4.0 minutes and 7.0 minutes.

Colour at 4.0 minutes: _________________________

Colour at 7.0 minutes: _________________________

The student did a fourth test at 30 °C.

In this test the starch did not break down, even after 45 minutes.

Why did the amylase not break down the starch in this test?

Tick (✓) one box.

The student made the following conclusion about the optimum temperature for amylase to work at.

'Amylase works fastest at 40 °C'

Her teacher said that this is not a valid conclusion from her results.

Describe how the student could change her method to give results that would improve the validity of her conclusion.