4.19 understand the term activation energy and represent it on a reaction profile

The activation energy is just the minimum amount of energy the reactants needs for the reaction to start

4.21 explain that a catalyst speeds up a reaction by providing an alternative pathway with lower activation energy

Catalysts work by lowering the activation energy (the minimum energy required by reacting particles for the reaction to occur). This happens because a catalyst provides an alternative reaction pathway with a lower activation energy.

NOTE: with a catalyst, although the activation energy lowers, the enthalpy change (ΔH) is not affected.

4.18 describe the effects of changes in surface area of a solid, concentration of solutions, pressure of gases, temperature and the use of a catalyst on the rate of reaction

NOTE: this point is just about the changes that occur, if you want/need to understand why these changes occur, go to this point... 4.20


Changes in surface area
The bigger the surface area (to volume ratio) the faster the reaction. This is because there are more particles on the surface for the reactants to react with.  In a solid, to increase the surface area without decreasing the mass just break up the solid into smaller pieces.

Concentration of solutions
If a solution is very concentrated there are lots of particles for its volume (the particles are close together). Alternatively, if the concentration is very weak there are a very little amount of particles for its volume (the particles are very spaced out). 

Pressure of gases
This is very similar to concentration of solutions. If the gas is at high pressure there will be more particles squished into a certain space, more particles means a faster rate of reaction. Alternatively, low pressure results in little amount of particles meaning a slower rate of reaction.

Temperature
The hotter the reactants the faster the reaction. Alternatively, the colder the temperature of the reactants the slower the reaction rate. This is because the particles have very little energy. 

Catalyst
A catalyst works by giving the reactants a surface to 'stick' to. This will increase the rate of reaction.

4.20 explain the effects of changes in surface area of a solid, concentration of solutions, pressure of gases and temperature on the rate of reaction in terms of partial collision theory

Changes in surface area
The bigger the surface area (to volume ratio) the faster the reaction. This is because there are more particles on the surface for the reactants to react with, meaning more collisions with the particles meaning a faster reaction. In a solid, to increase the surface area without decreasing the mass just break up the solid into smaller pieces.

Concentration of solutions
If a solution is very concentrated there are lots of particles for its volume (the particles are close together). More particles means more collisions and therefore a faster reaction rate. Alternatively, if the concentration is very weak there are a very little amount of particles for its volume (the particles are very spaced out). This means collisions will be less frequent (are there are less particles to collide with) so the reaction is slower.

Pressure of gases
This is very similar to concentration of solutions. If the gas is at high pressure there will be more particles squished into a certain space, more particles mean more collisions which mean a faster rate of reaction. Alternatively, low pressure results in little amount of particles in a certain space meaning less collisions (as there are not as many particles to collide with) meaning a slower rate of reaction.

Temperature
The hotter the reactants the faster the reaction. This is because the particles have more energy due to the heat. Because they have more energy this means they will more faster and therefore collide more. Alternatively, the colder the temperature of the reactants the slower the reaction rate. This is because the particles have very little energy. 


NOTE: faster collisions are only increased by temperature, all other ways to increase reaction rate increase the amount of particles to collide with.

4.17 describe experiments to investigate the effects of changes in surface area of a solid, concentration of solutions, temperature and the use of a catalyst on the rate of reaction

Changes in surface area

A change in surface area will affect the rate of reaction as there will be more/less surface area, meaning more/less collisions with more/less particles.

Method
- Measure out 50g of large pieces of marble chips and put them in a conical flask
- Add 50ml of dilute HCl
- Straight after you add the acid, put a bung in the top and attach it to a delivery tube attached to a gas syringe and start a stopwatch
- For 5 minutes, and at 30 second intervals, record the volume of gas collected in the syringe.
- Plot your results on a graph with time (the independent variable) as x and volume of gas collected (the dependant variable) as y.
- Repeat this experiment with smaller marble chips (still the same amount of mass... 50g).
- Now repeat again with powdered marble chips
- Plot these results on the same graph for easy comparison

Conclusion
Should all go well, you should conclude that the experiment using powdered marble chips produced the most amount of CO2 in the same amount of time This is because an increased surface area causes more collisions, therefore the rate of reaction is faster, therefore more gas is produced.

NOTE: The gas given off is CO2, remember this is a small scale version of how to make CO2 in a laboratory, if you are unsure of how this is done, here it is... 2.20  :)



Changes in concentration

A change in concentration will affect the rate of reaction as there will be more particles to collide, so the reaction will be completed quicker

Method
- Measure 50g of magnesium metal and put it in a conical flask
- Put the flask on scales
- Add 50ml of least concentrated HCl (do not remove flask from scales)
- Record the mass of the flask with magnesium and HCl then immediately start a stopwatch
- Record the mass of the experiment for 5 minutes, at 30 second intervals
- Plot your results on a graph with time (the independent variable) as x and mass loss (the dependant variable) as y. NOTE: the mass will decrease as this reaction produces hydrogen gas, which 'floats' away.
- Repeat this experiment with different concentrations of HCl (ensure to keep the same mass of magnesium...50g and volume of acid...50ml) and plot your results on the same graph

Conclusion

You should find that the higher the concentration the steeper the graph (the quicker the reaction).

Changes in temperature

The higher the temperature the faster the reaction, this is because the particles have more energy from the heat.

Method

- Draw an X (or any clear shape) on a piece of paper and put a conical flask on top

- Add 50ml of sodium thiosulphate and 50ml of HCl and immediately start a stopwatch

- Time how long it takes for the X to 'disappear' (NOTE: it will become not visible as this reaction produced a yellow precipitate of sulfur which cloudy the flask so you can't see the X, just time how long it takes until you can't see the X).

- repeat the experiment but with the two solutions at a higher temperature (using a water bath to heat the two solutions before adding them together).

- Repeat 5 times, increasing the heat of the solutions each time.

- Plot your result in a table for easy comparison.

Conclusion

This reaction should show that the higher the temperature, the quicker the X disappears, therefore the quicker the reaction.

Using a catalyst (decomposition of hydrogen peroxide)

The use of a catalyst will increase the rate of reaction.

Method

- Add 50ml of hydrogen peroxide to a conical flask
- Put a bung on top and attach it to a delivery tube attached to a gas syringe
- Start a stopwatch
- Time how much gas is collected (in the gas syringe) in 10 minutes at 30 second intervals (probably VERY little as the reaction naturally is very slow)
- Plot your results on a graph with time (the independent variable) as x and volume of gas collected (the dependant variable) as y.
- Repeat experiment, but add in a bit of manganese (IV) oxide (this is a catalyst)
- Repeat this experiment with smaller marble chips (still the same amount of mass... 50g).
- Now repeat again with powdered marble chips
- Plot these results on the same graph for easy comparison

Incase you were wondering, the equation for this reaction is...

 2H₂O_2(aq) ---> 2H₂O_(l) + O_{2 (g)}

NOTE: The gas given off is oxygen, remember this is a small scale version of how to make oxygen in a laboratory, if you are unsure of how this is done, here it is... 2.18 :)