Rates of Reaction

Question 1

How could you measure the effect of changed on surface area on the rate of reaction change?

Answer 1

-Add calcium carbonate (marble chips) to HCL and measure the mass of carbon dioxide lost at intervals
1. Measure the volume of gas evolved with a gas syringe and take readings at regular intervals
2. Make a table of readings and plot them on a graph
-You chose regular time intervals, so time is the independent variable (x)
-Volume is the dependent variable (y)
(Different lines for different SA of marble chips)
3. Repeat the experiment with exactly the same volume of acid, and exactly the same mass of marble chips, but with the marble more crunched up
4. Then repeat the experiment with the same mass of powdered marble instead of marble chips
-At the end the reaction will finish when all the HCL has been used up, but there will still be some unreacted marble chips at the bottom.
CaCO3 (s) + 2HCL (aq) —> CaCl2 (aq) + H2O (l) + CO2 (g).

Question 2

Why would the graph be a curve for a mass change reaction?

Answer 2

1. The graph is very steep at the start a lot of carbon dioxide is being produced per minute (the reaction is fast)
2. The graph becomes less steep showing that the reaction is slowing down
3. The graph becomes horizontal showing that no more carbon dioxide is being produced (the reaction has stopped)
4. The steeper the slope of the line, the faster the reaction

Question 3

What factors affect the rate of reaction?

Answer 3

• Concentration (pressure for gases)
• Temperature
• Surface area
• Catalyst

Question 4

How does temperature affect the rate of reaction?

Answer 4

• The particles have more kinetic energy and so move around faster and collide more frequently.
• AND a greater proportion of particles will have enough energy to overcome the EA so more of the collisions will be successful.
• Faster collisions are only caused by increasing the temperature

Question 5

How does surface area affect the rate of reaction?

Answer 5

• The GREATER the Surface Area the FASTER the rate of reaction.
• When the surface area is larger there are more sites for particles to collide with the substrate then when there is a smaller surface area. This results in the reaction occurring quicker with a larger surface area.

Question 6

How does a catalyst affect the rate of reaction?

Answer 6

Catalysts speed up a reaction by provide an alternative reaction route with a lower activation energy. Increasing the number of successful collisions

Question 7

What is a catalyst? Give an example.

Answer 7

1. A catalyst is a substance that speeds up a reaction without being used up.
2. An enzyme is a biological catalyst, for example lipase.

Question 8

Describe collision theory and what is activation energy?

Answer 8

• Reactions take place when particles collide with a certain amount of energy.
• The minimum amount of energy needed for the particles to react/collide with is called the activation energy and is different for each reaction.
• Frequency of collisions between particles.
• Energy with which particles collide.

Question 9

How does concentration affect the rate of reaction?

Answer 9

• Concentration is the number of particles in a given volume.
• As the concentration increases there are more particles (of a substance) in that given volume. As the (other element) does not change in surface area or volume but there are more particles (of a substance) more frequent collisions will occur, causing more (of something) to be produced per (time value).
• More particles per unit volume, this results in more frequent and successful collisions.

Question 10

Why is a cotton wool used in a reaction with marble chips and HCL?

Answer 10

The cotton wool is to allow carbon dioxide to escape, but to stop any acid from spraying out.

Question 11

How could you change the marble chip and HCL experiment?

Answer 11

• Crush up the marble chips into powder to increase the surface area and then measure the time taken for the same volume of CO2 to be produced as when the marble chips are not crushed. (amount of CO2 produced is PROPORTIONAL to how much (mass) of the reactant there is).
• You can heat the acid, to increase the temperature and then measure the time taken for the same volume of CO2 to be produced as when the acid is not heated.
• You can increase the concentration of HCL by replacing with water (KEEP OVERALL VOLUME THE SAME!), and measure the time taken for a given volume of CO2 to be produced.

Question 12

How could you experimentally investigate the effect of concentration on the rate of a reaction?

Answer 12

1. Add sodium thiosulfate and HCL and mix.
2. Add increasing volumes of water, and decreasing volume of sodium thiosulfate to reduce it’s concentration. (KEEPING OVERALL VOLUME THE SAME).
3. Then stir the mixture, and measure the time taken for a cloudy precipitate to form and the cross is no longer visible.

Question 13

How does pressure in gases affect the rate of reaction?

Answer 13

When the pressure in gases is increased, the rate of reaction is faster as there are still the same number of particles but in a smaller volume causing collisions to become more frequent and so the rate is faster.

Question 14

How could you experimentally investigate the effect of a catalyst on the rate of a reaction?

Answer 14

Decomposition of hydrogen peroxide.
-Add manganese (IV) oxide as catalyst, other catalysts that work are found in potato peel and blood
-Oxygen gas is given off and can be measured gas syringe.
2H2O (aq) —-> O2 (g) + 2H2O (l).
-This experiment can also be used to investigate the rate of reaction if you, change mass of catalyst, change how ‘lumpy’ the catalyst is, different catalyst, concentration of hydrogen peroxide solution, change temperature of solution.
-Better catalysts give a quicker reaction, which is shown by a steeper graph which levels of quickly

Question 15

How could you investigate which catalyst was the best? Why would this be useful?

Answer 15

Add different catalysts to hydrogen peroxide and check at each 15 second interval how much oxygen is produced and whichever produces the largest volume of oxygen in the shortest amount of time is the most effective.
-Catalysts help substances to react quickly at lower temperatures and pressures that would other wise would be needed and this saves money.

Question 16

How is the rate and concentration related?

Answer 16

1. Rate is proportional to concentration. As the reactants are colliding with the substrate, the concentration of the reactants is decreasing. This therefore means that the rate is decreasing proportionally to the concentration.
   - Early on in the reaction there are lots of acid particles and lots of collisions.
   - Later on the reaction there are fewer particles left so less collisions.
   - As the reaction proceeds the collision rate of acid particles decreases.

Question 17

How do you calculate the rate of reaction?

Answer 17

• Amount of reaction used or amount of product formed / time
• There are different ways that the speed of a reaction can be measured (use stopwatch for all)
• The speed of a reaction can be observed either by how quickly the reactants are used up or how quickly the products are formed

Question 18

How can you use precipitation to measure the speed of a reaction? What can this reaction be used to measure?

Answer 18

• Sodium thiosulfate and HCL are both clear solutions and they react together to form a yellow precipitate of sulphur
  1. Observe a marker through the solution and measure how long it takes for it to disappear
  3. The quicker the marker disappears the quicker the reaction
  4. This only works for reactions where the initial solution is rather see-through
• The result is very subjective and different people might not agree over the exact point when the marker ‘disappears’
  5. The reaction can be repeated for solution at different temperature, although that is quite hard to do accurately and safely and it is not safe to heat an acid directly, the best way to do it is to use a water bath to heat both solutions to the right temperature before you mix them
• The depth of the liquid must be kept the same each time
• The results will show that the higher the temperature the quicker the reaction and therefore the less time it takes for the marker to disappear
• The reaction an also be used to test the effects of concentration

Question 19

How can you use change in mass (usually gas given off) to measure the speed of a reaction?

Answer 19

1. Measuring the speed of a reaction that produces a gas can be carried out on a mass balance
2. As the gas is released the mass disappearing is easily measured on the balance
3. The quicker the reading on the balance drops, the faster the reaction
4. When the mass stops changing, the reaction has finished
5. Rate of reaction graphs are particularly easy to plot using the results from this method
6. This is the most accurate of the three methods described on this page because the mass balance is very accurate
7. The disadvantage is that is releases gas straight into the room

Question 20

How can you use the volume of gas given off to measure the speed of a reaction?

Answer 20

1. This involves the use of a gas syringe to measure the volume of gas given off
2. The more gas given off during a given time interval, the faster the reaction
3. When gas stops being produced the reaction has finished
4. A graph of gas volume against time elapsed could be plotted to give a rate of reaction graph
5. Gas syringes usually give volumes accurate to the nearest millilitre so they are quite accurate
6. You have to be quite careful though, if the reaction is too vigorous you can easily blow the plunger out of the end of the syringe

Question 21

Describe the graph for the effect of using finer particles of solid

Answer 21

1. An increase in surface area cause more frequent collisions, so the rate of the reaction is faster
2. Another line could be plotted to show the reaction if a greater mass e.g. double the mass of small marble chips is added (steeper line at start and overall amount of gas evolved is double (higher line))
   - The extra surface area gives a quicker reaction and there is also more gas evolved overall (as long as the acid is in excess)
3. If larger SA steeper line at start (plateaus first) but all eventually plateau with same amount of gas evolved

Question 22

How could use experimentally measure the effect of increased concentration?

Answer 22

1. React magnesium metal with dilute HCL
2. This reaction gives off hydrogen gas, which we can measure with a mass balance
3. Take readings at regular time intervals
4. Put the results in a table and work out the loss in mass for each reading and plot a graph
5. Repeat with more concentrated acid solution, but always with the same amount of magnesium
6. The volume of acid must always be kept same too, only the concentration is increased
   - In this experiment time is again the independent variable and mass loss is the dependent variable (the marble chip reaction uses a gas syringe)

Question 23

Describe the graph to show the effect of using more concentrated acid solutions?

Answer 23

• Different lines for different concentrations
• A higher concentration gives a steeper graph with the reaction finishing (line plateaus) much quicker
• But all react same loss in mass (graph line plateau) just at different time

Question 24

How can you use a precipitation reaction to measure the rate of a reaction?

Answer 24

1. This is when the product of the reaction is a precipitate which clouds the solution
2. Observe a marker through the solution and measure how long it takes for it to disappear
3. The quicker the marker disappears, the quicker the reaction
4. This only works for reactions where the initial solution if rather see-through
5. The result is very subjective, different people might not agree over the exact point when the mark ‘disappears’