Topic 9 - Rates of Reactions Flashcards Preview

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Flashcards in Topic 9 - Rates of Reactions Deck (22)
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1
Q

Describe the relationship between rate and time

A

Rate is inversely proportional to time

2
Q

What can rate be measured in?

A
  • change in mass
  • change in volume
  • change in pressure
  • time taken to turn opaque
3
Q

Why does surface area affect rate of reaction

A
  • surface area:volume of reactant is increased
  • more of the solid is accessible to the other reactant
  • more collisions take place per unit time
4
Q

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

A

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)

5
Q

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

A

Gradients differ because SA affects rate

All level off at same point because same mass was used

6
Q

What is concentration?

A

Mass of a solute per unit volume

7
Q

What is the relationship between rate and concentration and why?

A

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

8
Q

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

A

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

9
Q

What is the relationship between time and rate

A

Rate = 1/time

10
Q

How does temperature affect rate?

A
  • 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
11
Q

Describe the practical used to investigate how temperature affects rate

A

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

12
Q

How do catalysts affect rate of reaction

A
  • 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
13
Q

Why do reactions slow down?

A
  • reactants are being used up

* concentrations are dropping

14
Q

What is rate of reaction?

A

How fast reactants are turned into products

15
Q

Graphs for rate

A
  • 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
16
Q

Describe collision theory

A

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

17
Q

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

A

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

18
Q

How to calculate rate

A

Amount of reactant used or amount of production/ time

19
Q

How to do precipitation reactions to measure rate:

A

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

20
Q

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

A

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

21
Q

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

A

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

22
Q

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

A

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