Chemistry Reviews (AS level)

Chapter 10: Rates of Reaction

 Wow, we've reached the 10th chapter, what an achievement. In actuality this isn't much of a anniversary, as this is like the 15th Chem review I've done. Also it's not even the 10th chronologically, as I guess chapter 1 doesn't exist or something. Regardless, this chapter is quite a big one. I'd count what chapter is the biggest, but that would probably be procrastination. The topic covers not only RoR (the official abbreviation of rates of reaction), but also equilibrium, a fan-favourite from GCSE which thankfully has returned this year. 

As it could be assumed by the name, RoR is the rate of a reaction, so how fast a reaction happens. If you wanted to say it another way you could say it is how fast a product is formed/ reactant is used up. This is often as seen of a graph, which with time on the x axis and the measured value on the Y, the graph looks like a log graph. How does one calculate the rate of reaction? This is done with the graph (we'll get to how the graph is made later). If you want the rate at a point, you need to find the tangent of the point, and work out the gradient of the tangent. If you need to find the mean over a time, then you draw a line from the 2 points, and find that gradient.

Before you draw a graph though, you need to actually do the experiment. There are a couple of different ways to do this. One way is with a solution that turns from colourless to not, by drawing an X (or any other shape or marker) underneath the flask, and timing the time for it to disappear. The only thing is that you can't really plot a graph for this one, as the rate of reaction will be just measured in time, like secs. There are 2 methods which involve another value to measure aside from time. The first is by using a gas syringe, and measuring the volume of gas released. The low-cost version of this would be with a measuring cylinder filled with water, and putting a tube connected to the experiment up the cylinder. The other method is by putting the experiment on a balance, and measuring the mass loss (from the gas releasing into the air). For this one the empty equipment will need to be measured. Also, for the graph of this, you could either measure the mass lost (graph will go down), or the gas mass over time. You will make recordings every set time interval, and when you get 2 results of the same. Then, plot the points, and draw a line of best fit (the line will be curved). 

What affects rate of reaction? There is a list of 4 things: pressure (if gases), temperature, concentration, catalysts and surface area of solid reactants (if applicable). Increasing any or all of these will increase the RoR. This is because of collision theory, which basically describes the reaction happening by the particles bashing into each other. For the reaction to happen in the collision, the particles have to be orientated the right way, and they have to have enough energy to be equal to or exceed the activation energy. If there are more collisions then the reaction happens faster. Increasing pressure increases collisions as the gas particles are contained in a smaller volume, so are closer together so more collisions occur. With concentration it is kind of the opposite initial change (for the same result), as you increase the amount of particles in the same volume, and therefore there are more collisions. Increasing temperature means the particles have more energy, which increases collisions (there's also a bit more about temp later on). With increased surface area, there is more chance of collision. For the opposite cause, there are less collisions, so there will be a lower RoR.