Topic 9 - Rates of Reactions Flashcards
Describe the relationship between rate and time
Rate is inversely proportional to time
What can rate be measured in?
- change in mass
- change in volume
- change in pressure
- time taken to turn opaque
Why does surface area affect rate of reaction
- surface area:volume of reactant is increased
- more of the solid is accessible to the other reactant
- more collisions take place per unit time
Describe the experiment for investigating the relationship between surface area and rate
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)
Describe the graph produced when investigating the effect of surface area in time
Gradients differ because SA affects rate
All level off at same point because same mass was used
What is concentration?
Mass of a solute per unit volume
What is the relationship between rate and concentration and why?
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
Describe the experiment used to investigate the relationship between concentration and rate
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
What is the relationship between time and rate
Rate = 1/time
How does temperature affect rate?
- 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
Describe the practical used to investigate how temperature affects rate
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
How do catalysts affect rate of reaction
- 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
Why do reactions slow down?
- reactants are being used up
* concentrations are dropping
What is rate of reaction?
How fast reactants are turned into products
Graphs for rate
- 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
Describe collision theory
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
How to show the effect of a catalyst using a reaction profile
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
How to calculate rate
Amount of reactant used or amount of production/ time
How to do precipitation reactions to measure rate:
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
How to measure rate using an experiment which results in a change in mass
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
How to measure rate using an experiment where you measure the volume of gas given off
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
Describe how you can measure how the use of a catalyst to affect rate
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
