the rate and extent of chemical changes qus

equilibrium

When a reversible reaction occurs in apparatus which prevents the escape of reactants and products, equilibrium is reached when the forward and reverse reactions occur at exactly the same rate.

Explain how a catalyst increases the rate of a reaction

• lowers activation energy

• provides different reaction pathway

A pressure of 100 atmospheres is used instead of atmospheric pressure. The higher pressure gives a greater yield of methanol and an increased rate of reaction. Explain why.

• equilibrium moves to the product side because fewer molecules

• more collisions per unit time because more molecules per unit volume

A pressure of 285 atmospheres is not used in the Haber process instead of 200 atmospheres. Give one reason why

• higher energy costs

Explain why a low pressure is used in stage 1. Give your answer in terms of equilibrium

• higher yield

• fewer molecules on the left hand side

The rate of reaction is different when manganese dioxide is used as a fine powder rather than coarse lumps. Explain why. You should answer in terms of collision theory.

• surface area greater

• more collisions per unit time

C2H4 (g) + H2O (g) → C2H5OH (g)

Explain how increasing the pressure of the reactants will affect the amount of ethanol produced at equilibrium.

• more ethanol produced

• system moves to fewer molecules

The forward reaction is exothermic. Use Le Chatelier’s Principle to predict the effect of increasing temperature on the amount of ethanol produced at equilibrium.

• amount decrease because equilibrium moves to the left

A few drops of a colourless solution containing a high concentration of thiocyanate ions (SCN–) are added to the orange equilibrium mixture.

Explain the colour change observed.

• becomes more red

• equilibrium moves to the right

• the concentration of thiocyanate is reduced

A water bath is set up at a temperature above room temperature.

When a test tube containing the orange equilibrium mixture is placed in the water bath, the mixture becomes more yellow.

Explain what this shows about the energy change for the forward reaction.

• the equilibrium moves to the left

• the temperature is reduced

• therefore the forward reaction is exothermic

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

1. measure volume of hcl with a measuring cylinder

2. pour hcl into a polystyrene cup

3. measure initial temperature with a thermometer

4. add a known mass of sodium carbonate measured with a balance and stri

5. measure the highest temperature reached

6. repeat with different masses of sodium carbonate

7. repeat the whole investigation

8. use the same starting temperature

9. use the same volume of hcl

10. use the same concentration of hcl

The activation energy for the reaction between zinc and sulfuric acid is lowered if a solution containing metal ions is added.

What is the most likely formula of the metal ions added?

Cu2+

The student concluded that the large calcium carbonate lumps reacted more slowly than the small calcium carbonate lumps.

How do the student’s results show that this conclusion is correct?

for large lumps a smaller number of moles of gas is collected in the same time

Give three conclusions that can be drawn about the rate of reaction between magnesium and dilute hydrochloric acid in this investigation.

Use data from the graph and the table above.

1. rate decreases

2. rate decreases more slowly as time increases

3. rate becomes 0 at 60s

Describe how the rate of reaction changes between 0 and 160 seconds.

• rate is greatest at the start

• then rate decreases

• then the reaction stops

The student measured the change in mass of the reactants.

Describe another method, other than measuring the change in mass of the reactions, that the student could have used to find the rate of the reaction between marble chips and hydrochloric acid.

• collect gas in a gas syringe

• measure the volume of gas produced

Explain, in terms of particles, how and why the rate of reaction changes during the reaction of calcium carbonate with hydrochloric acid.

• acid particles are used up so concentration decreases

• therefore there are less frequent collision so rate decreases

Give a possible reason for the anomalous result at 40 °C

* the temperature used was lower than 40C

What word describes an investigation by the same students which gives similar results each time?

repeatable

Describe how the students can use their results to improve the accuracy of the investigation.

• discard anomalies

• find a mean

Which two statements are correct when the reaction reaches equilibrium?

1. the mass of each substance does not change

2. the rates of the forward reaction and reverse reaction are equal

The student compares the rates of the reaction of zinc carbonate with:

• 0.01 mol/dm3 methanoic acid

• 0.01 mol/dm3 ethanoic acid.

The rate of the reaction with methanoic acid is greater than the rate of the reaction with ethanoic acid. Explain why.

You should refer to ions in your answer. Use the table above.

• methanoic acid has a lower pH

• methanoic acid has a higher concentration of hydrogen ions

• more collisions per unit time

Plan an investigation to show how the concentration of the sodium thiosulfate solution affects the rate of the reaction with dilute hydrochloric acid.

Your plan should give valid results

1. measure 10cm3 of sodium thiosulfate

2. place in conical flask

3. measure 10 cm3 of hcl

4. place conical flask on a cross

5. add hcl to flask

6. swirl

7. start stopclock

8. measure time for cross to become no longer visible

9. repeat and find mean

10. repeat for diff concentration of sodium thiosulfate

control

• conc of hcl

• volume of hcl and sodium thiosulfate