6.2.4.- Predict and explain, using the collision theory, the qualitative effects of particle size, temperature, concentration and pressure on the rate of a reaction.

Question Answer
What was the independent variable The surface area of the Calcium Carbonate
What was the dependent variable Volume of gas produced
What variables were controlled The volume and concentration of acid used, the mass of Calcium Carbonate used. The same room temperature. Identical equipment used (same size: flask, delivery tube).
Using collision theory explain the following the shape of the graphs at the start of the reaction The smallest chip has the steepest slope suggesting it has the fastest rate of reaction. This is because as surface area increases, rate of reaction increases. - In a solid substance only the particles on the surface can come into contact with a surrounding reactant. The powder is supposed to be the fastest but in our reaction; the powder is blocking the gas from reaching the delivery tube so it doesn’t follow the theory. Our results for small chips are an anomaly as it doesn’t follow the trend. The big chip has the slowest rate of reaction as it takes the longest time until the reaction is over and the gradient is gentle.
What does the gradient of the graph at any one point represent The steeper the gradient, the faster the rate of reaction.
What are the units for the gradient of the graph Volume/unit time
Discuss the reasons for the differences in the shape of the graphs The steeper the gradient the faster the reaction. If we use the same amount of reactants assuming there is one that is in excess, we should get the same amount of volume produced. In this case, it is different so there must be something wrong.

Factors affecting the rate of reaction:
• Temperature- as the temperature increase (temperature is a measure of the average kinetic energy so if temp increases, the energy increases), the particles will move faster so there will be more chances of success collisions per time. However, the main reason why an increase in temperature increases the rate is that more of the colliding particles will possess the necessary activation energy (energies exceed Ea) resulting in more successful collisions. For many reactions, the rate approximately doubles for every 10K temp rises and there is an exponential relationship.
• Surface area (particle size) - In a solid substance only the particles on the surface can come into contact with a surrounding reactant. If the surface area increases/particle size decreases (e.g. in powder form), the rate of reaction should increase. Similarly, if the surface area decreases/ particle size increases (e.g. in big chips), the rate of reaction should decrease.
• Concentration- The more concentrated the reactants are, the more collisions there will be per second per unit volume. As the reactants got used up their concentration decreases. This explains why the rate of most reactions gets slower as the reaction proceeds. If there are particles in a volume then there are more chances of successful collisions.
• Catalyst- Catalysts increase the rate of a chemical reaction without themselves being chemically changed at the end of the reaction. They work essentially by bringing the reactive parts of the reactant particles into close contact with each other, this provides alternative pathway for the reaction with a lower Ea. If Ea is lowered, it means that the energy required for a reaction to take place is lowered.
• Pressure- For reactions involving gases, increasing pressure increases the rate of reaction. This is because the higher pressure compresses the gas, effectively increase its concentration. This will increase the frequency of collisions. The Haber process illustrates the effect of high pressure on reactions that involve gaseous reactants.

• light (EMR)- can promote some reactions e.g. dye fading, photosynthesis, skin tanning, methane/chlorine explosion, skin cancer, photography, vitamin D formation in skin and hydrogen peroxide photodecomposition (it is kept in brown bottles.