Topic 9 - Rates of Reactions

Question 1

Describe the relationship between rate and time

Answer 1

Rate is inversely proportional to time

Question 2

What can rate be measured in?

Answer 2

• change in mass
• change in volume
• change in pressure
• time taken to turn opaque

Question 3

Why does surface area affect rate of reaction

Answer 3

• surface area:volume of reactant is increased
• more of the solid is accessible to the other reactant
• more collisions take place per unit time

Question 4

Describe the experiment for investigating the relationship between surface area and rate

Answer 4

Calcium carbonate + (dilute) hydrochloric acid -> calcium chloride + water + carbon dioxide

CaCO3 (s) + 2HCl (aq) -> CaCl2 + H2O + CO2

Independent variable: surface area of CaCO3 ; whole, smashed up, powdered
Dependant variable: volume of CO2 produced measured using a gas syringe (making sure its airtight)
Take readings at regular intervals and record the results in a table
Plot a graph of time (x) and volume (y)

Question 5

Describe the graph produced when investigating the effect of surface area in time

Answer 5

Gradients differ because SA affects rate

All level off at same point because same mass was used

Question 6

What is concentration?

Answer 6

Mass of a solute per unit volume

Question 7

What is the relationship between rate and concentration and why?

Answer 7

Rate is proportional to concentration

When concentration increases, there are more particles per unit volume, so there are more collisions per unit time

In a gas, increasing the pressure means that the particles are closer together, increasing the frequency of collisions

Question 8

Describe the experiment used to investigate the relationship between concentration and rate

Answer 8

Magnesium ribbon + dilute hydrochloric acid in a conical flask bubbles through a delivery tube into an upside down test tube in a trough of water, producing magnesium chloride + hydrogen

Mg (s) + 2HCl (aq) -> MgCl2 (aq) + H2 (g)

Independent variable: concentration of HCl
Dependent variable: time for all H2 to be produced

Question 9

What is the relationship between time and rate

Answer 9

Rate = 1/time

Question 10

How does temperature affect rate?

Answer 10

• particles move faster- more collisions per unit time
• particles have more energy, collide with more energy
• a higher percentage of collisions have energy greater than the activation energy

Question 11

Describe the practical used to investigate how temperature affects rate

Answer 11

Na2S2O3 (aq) + 2HCl (aq) in a beaker with an X underneath. Produces 2NaCl (aq) + S (s) + SO2 (g) + H2O (l)

Independent variable: temperature of solution - 40°C-> 80°C using a waterbath
Dependent variable: time taken for X to become invisible
Control: volume

Works because they are both clear solutions which produce a yellow precipitate of sulphur

Question 12

How do catalysts affect rate of reaction

Answer 12

• they provide an alternative reaction pathway with a lower activation energy
• therefore a higher percentage of collisions have energy higher than the activation energy and are therefore successful

Question 13

Why do reactions slow down?

Answer 13

• reactants are being used up

* concentrations are dropping

Question 14

What is rate of reaction?

Answer 14

How fast reactants are turned into products

Question 15

Graphs for rate

Answer 15

• graphs can be made with time on the x axis and either amount of product formed or amount of reactant used up on the y axis
• over time, the line becomes less steep as the reactants are used up
• quickest reaction have steepest gradient and flatten out in the shortest time

Question 16

Describe collision theory

Answer 16

The rate of a chemical reaction depends on:

1) the collision frequency of recaring particles -> the more collisions there are, the faster the reaction is
2) the energy transferred during a collision -> particles have to collide with enough energy for a collision to be successful

Question 17

How to show the effect of a catalyst using a reaction profile

Answer 17

1) activation energy is shown as the offered e between the nervy of reactants and the highest point on the curve
2) reaction profiles can be used to compare the reaction with and without a catalyst
3) the highest point on the curve for a reaction with a catalyst will be lower than without a catalyst

Question 18

How to calculate rate

Answer 18

Amount of reactant used or amount of production/ time

Question 19

How to do precipitation reactions to measure rate:

Answer 19

1) works for when two clear solutions produce a precipitate, which clouds
2) mix the two reactant solutions and out the comical flask in a piece of paper marked with a large X
3) observe the mark through the mixture from above and measure how long it takes until you can no longer see the mark-> the faster it disappears, the faster the reaction
4) it is subjective

Question 20

How to measure rate using an experiment which results in a change in mass

Answer 20

1) a production that produces a gas can be used to calculate rate using a mass balance
2) as the gas is released, the lost mass is measured on the balance- the quicker the reader drops, the faster the reaction
3) you know the reaction has finished when the reading stops changing
4) plot a graph of change in mass against time
5) do the experiment in a fine cupboard, stopper the conical flask with cotton wool to let the gases through but stop ant solid, liquid or aqueous reactants flying out

Question 21

How to measure rate using an experiment where you measure the volume of gas given off

Answer 21

1) use a gas syringe to measure the volume of gas given off
2) the more has given off during a set time interval, the faster the reaction
3) you can tell the reaction has finished when no more gas is produced
4) plot a graph of gas volume against time elapsed
5) make sure you use the right size gas syringe- if the reaction is too vigourous, you can blow the plunger out of the end of the syringe

Question 22

Describe how you can measure how the use of a catalyst to affect rate

Answer 22

1) decomposition of hydrogen peroxide: 2H2O2(aq) -> 2H2O(l) + O2 (g)
2) use of manganese (IV) oxide (MnO2) or copper (II) oxide (CuO) or zinc oxide (ZnO)
3) measure oxygen given off using a gas syringe at regular time intervals
4) change catalyst, control mass of catalyst used