Reaction Rates

In my last post I said that I was going to talk about reaction rates next. Well, here's a post about reaction rates for you!

When we talk reaction rates, we're talking how fast a reaction occurs. There are different ways to measure reaction rates, for example, measuring how fast the products are used up (by measuring mass or volume) or measuring how fast the products are being produced (by measuring mass or volume).

The main concept to know here is the collision theory. The collision theory states that there are 3 conditions that have to take place for a reaction to occur.

1. The reacting particles must collide.
2. The colliding particles must have enough energy to react. (They must have more energy than the activation energy required- see Energy Effects.
3. The particles must collide with a favourable orientation.

We can then extrapolate on this to explain why certain factors affect the rate of reaction.

Concentration- If you have more particles in the same amount of space, then there's a higher chance that the particles will collide. More collisions in the same amount of time = faster reactions. This is a bit of a silly example, but if you squish more people onto a 6-car train, there's a higher chance that people are going to be squashed into one another. More squashiness in the same amount of time = faster spread of annoyance among passengers, particularly when the idiots at the next station try and squish onto an obviously already overcrowded train.

Temperature- Increasing temperature increases the average energy of the particles. Because of this, more particles will have sufficient energy to react. The increased energy also makes the particles move faster, making collisions slightly more likely.

State of sub-division and surface area- Dividing up substances increases the overall surface area. This means that more particles are exposed to the reaction, making the number of collisions greater.

Catalysts- Catalysts affect reaction rates through a range of different means. They might make the particles more likely to collide with a favourable orientation, for instance. The main effect of catalysts is a reduction in the activation energy required for a reaction to occur. Here's the same diagram that I used in the last post, but with an extra line showing a possible catalysed pathway.

As you can see, if a catalyst is present, the activation energy is not as high, but the overall enthalpy change remains the same.

There's an extra dot point here about explaining the relationship between collision theory, kinetic energy distribution graphs and the rate of a reaction. I feel that I've alluded to it enough when I talked about temperature, but if you want me to elaborate some more, just ask!