Rates of Chemical Reactions

Ways of measuring the rate of a reaction:
To measure how fast a reaction is happening, you either need to be able to measure how much is being used up per unit of time or how much of a product is being made per unit of time. The best involves the production of a gas as this can be collected in a gas syringe and the volume recorded every 5s or 10s then plotted.
Another method will give the time for a reaction to run to completion, this can't be plotted during the course of a reaction, but it can be used to compare several where we have changed one variable. The best example of this reaction is the "disappearing cross" reaction with sodium thiosulfate. These are both required practicals.

Factors affecting Rate & Collision Theory:
Collision theory is all about how often particles (of reactants) collide and whether or not they have sufficient energy (activation energy)to make the collision a successful one. There are 4 ways of speeding up a chemical reaction.
The first way to increase the rate of a reaction is to increase the temperature. When the temperature increases, the particles move faster and so collide more often. As these particles have more energy, they are more likely to have the activation energy which will result in more successful collisions per second
The second way to increase the rate is to increase the concentration of a solution, by doing this, there will be more particles in the same unit of volume, this means that there will be more successful collisions per second. This is the same as increasing the pressure in a gas, the particles are pushed closer together which is the same as increasing the concentration.
The third way is to increase the surface area of the solid reactants, by doing this, there will be more reactants in contact with other reactants, this will result in more successful collisions per second
Finally, you can add a catalyst which will lower the activation energy. This will mean that more of the particles will have activation energy so without increasing the temperature, there will be more successful collisions per second

Practical - measuring how concentration or temperature affects rate:
Here, you need to complete a required practical. For this one, you will either add different concentrations of hydrochloric acid to a sodium thiosulfate solution in a conical flask that is sitting on a card that has a cross drawn on it or you may use thiosulfate solution of differing temperatures. In this method, you will start your timer when you add the HCl and stop it when you can no longer see the cross through the top of the conical flask. You will then be able to plot time taken for the cross to disappear (dependent variable on the Y-axis) against either the temperature or concentration (independent variable on the X-axis).

Practical - measuring how surface area affects rate:
For this second one, you need to complete a different required practical. Here you will add a specific mass of limestone chips (small surface area) to a conical flask containing hydrochloric acid that has a delivery arm attached by a pipe to a gas syringe. Once you add the limestone, add the bung to the top and start the timer. Record the volume of CO₂ released every 5 seconds. You repeat this for smaller pieces of limestone (larger surface area) and finally for limestone powder (largest surface area). You can then plot time vs volume and you will see the rate as the steepness of the three graphs. Steeper = faster. You can draw tangents and calculate the rate in cm³/s.

Catalysis and energy profiles:
A catalyst will speed up a reaction but the catalyst never gets used up in the process. A catalyst is specific to a reaction so it will only speed up the specific reaction and will have no effect at all in others. Enzymes are biological catalysts, without them, reactions in nature would not be able to happen in their natural conditions.
In the presence of a catalyst, the reaction is provided with an alternative pathway, this new pathway will still produce the safe final product, however, it will not need as much energy to get it started (activation energy). In energy profiles that we saw last module, we have seen the "hump" that shows the activation energy. When you add a catalyst to the reaction, the "hump" reduces in height.

Reversible reactions and Le Chatelier's principle:
In a reversible reaction, reactants can become products and products can become reactants. When the reaction is a equilibrium, it has not stopped. We call this dynamic equilibrium and the rate of the forward reaction is equal to the rate of the reverse reaction. Because energy cannot be created or destroyed, one direction will be endothermic and the other will be exothermic and their values will be equal and opposite.
In a closed system (nothing enters or leaves), the equilibrium will shift in order to try and compensate for any external changes. This is called Le Chatelier's principle. These are the ways in which it will move to try and compensate. If you heat the mixture, the equilibrium will move in the endothermic direction (to absorb some of the heat). If you remove a product, it will move to the right to replace some of the product that you took. In a gas, if you increase the pressure, the equilibrium will move to the side with the fewest number of moles of gas.

The Haber Process:
Fritz Haber, along with engineer Carl Bosch, came up with the Haber-Bosch process (commonly known and the Haber process). This process uses the reversible reaction:
N₂ + 3H₂ ⇋ 2NH₃
This reaction uses both a reversible reaction and a catalyst (iron). This is a fascinating reaction and it is the best way of making synthetic fertilisers. Without this reaction, there is no way that we could grow enough food to feed billions of the people alive on our planet.