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Rates of Reaction 3
Reactions can only happen if the colliding particles have enough energy.

Rates of Reaction 3

Many factors affect rates of chemical reactions - pressure of gases, temperature, surface area of solids, concentration and if there is a catalyst. Anything that will change the probability of particles colliding or change the energy of the collisions will affect the rate of a reaction. This is the last of three GCSE Chemistry quizzes looking at these factors.

When investigating rates of reaction, it is necessary to make a series of measurements over a period of time, for example, how much hydrogen is produced during the reaction of an acid with zinc. The experiment should be repeated several times and, after discarding any anomalous results, the readings averaged and plotted on a scatter graph with time along the horizontal axis. The line of best fit will usually be a curve, with the steepest gradient at the start indicating the fastest rate of reaction. Where the curve is horizontal, is shows that the reaction had finished. The conditions of the experiment can then be changed and the whole process repeated. Plotting the results on the same graph, using different colours, gives a quick and easy visual interpretation, from which you can write your conclusion.

When working out rates of reaction mathematically, as higher tier candidates are more likely to do, dividing the amount of reactant used (or product formed) by the time taken gives a valid rate. This is effectively the same as working out the gradient of a graph. If 15 cm3 of carbon dioxide were released in the first 20 seconds of a reaction, the rate would be 0.75 cm3/s. Later in the reaction, it may take 45 seconds to produce 10 cm3 of carbon dioxide, in which case the rate would have dropped to 0.22 cm3/s. This slowing down of the rate occurs because the concentration of one or both of the reactants changes during the reaction. Professional scientists usually refer to the initial rate of reaction, in other words the rate at the very start.

How would a decrease in temperature affect the rate of a reaction? What about surface area? Have a go at this quiz and test your knowledge of the factors which affect rates of reaction, such as temperature, surface area or pressure.

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1.
What is activation energy?
The maximum energy that particles need to change into a new substance
The minimum energy required to break the bonds of the colliding chemicals
The minimum energy that particles need to change into a new particle
The maximum energy that particles need to change into a new substance when they collide with other particles
Reactions involve bonds being broken followed by new, different bonds being formed
2.
Calcium carbonate (marble) reacts with hydrochloric acid to produce water, carbon dioxide and calcium chloride. Which of the following combinations is likely to result in the fastest rate of reaction?
1g of marble chips with 50ml of hydrochloric acid at 20 °C
1g of marble powder with 50ml of hydrochloric acid at 20 °C
1g of marble chips with 50ml of hydrochloric acid at 30 °C
1g of marble powder with 50ml of hydrochloric acid at 30 °C
Increasing the surface area and the temperature will increase the rate of reaction greatly
3.
Which of the following statements about collisions is correct?
Only fast-moving particles collide with each other
All colliding particles have the same amount of energy
Reactions can only happen if the colliding particles have enough energy
All colliding particles are at the same temperature
If there is sufficient energy involved in a collision between particles, the bonds will break
4.
Which of the following is NOT a disadvantage of using a catalyst in a chemical reaction?
They can be reused multiple times
If a reaction is going too fast, simple mistakes can be made
They can be poisoned by impurities and stop working
They can be expensive
Catalysts are not used up in chemical reactions
5.
What apparatus is used to measure the volume of gas given off in a reaction?
Stop clock
Ruler
Gas syringe
Test tube
You can also use apparatus such as a measuring cylinder, a graduated test tube or even a burette
6.
Which of the following statements about the effect the size of the pieces of a solid on the rate of a chemical reaction is true?
The smaller the pieces, the bigger the surface area so there is more surface area to be attacked
The bigger the pieces, the smaller the surface area, so there is more surface area to be attacked
The bigger the pieces the bigger the surface area so there is more surface area to be attacked
The smaller the pieces, the smaller the surface area so there is more surface area to be attacked
The larger surface area means that there are more particles available to be involved in collisions. It is only ever the surface layer of particles of solids that are involved in reactions
7.
On an energy level diagram, what effect would adding a catalyst have to the reaction on the line showing the energy for a reaction?
It would go higher
It would not go as high
It would be the same
It would be a straight line not a curve
A catalyst lowers the activation energy or energy required for the reaction to take place. For the GCSE, make sure that you can label energy level diagrams
8.
Which of the following does NOT affect the rate of a chemical reaction between a solid and a liquid?
Concentration
Temperature
Altitude
Surface area
Carrying out the reaction at a different altitude would affect the pressure. Changes in pressure affect only reactions involving gases
9.
Platinum is used as a catalyst in the production of nitric acid. Which of the following statements is correct?
The catalysed reaction requires more energy than the uncatalysed reaction
The catalysed reaction will produce more nitric acid than the reaction without platinum
The mass of platinum will remain the same throughout the reaction
The particles in the catalysed reaction will have more energy than the particles in a reaction without the platinum
As the platinum does not take part in the reaction itself, it does not gain or lose mass
10.
What units do we use to measure concentration?
kmol/dm3
mol/dm3
mol/dm2
mol/dm
Moles per cubic decimetre
Author:  Kate Gardiner

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