# Speed of reactions factors

1. Aug 26, 2013

### WavesOfPhysics

In Ox/Red reactions there 3 factors that can speed up a reaction between 2 compounds:

the first being a greater concentration, which is pretty easy to comprehend.

What I don't understand is how heat affects the speed of a reaction ( at a micro level)
and how is adding a third component to the reaction sometimes makes it faster.

2. Aug 27, 2013

### janhaa

It's related to collision theory, and the Maxwell-Boltzmann distribution.
This is shown mathematically in the Arrhenius equation

$$k=Aexp(-E_a/RT)$$
where k is rate constant
A: the collision factor (say A = 50000)
What happens if you increase the temperature from 293 K to 303 K?

i.e.
$$k(303) / k(293) \approx 2,0$$
You can see that the fraction of the molecules able to react has almost doubled by increasing the temperature by 10°C.

3. Aug 27, 2013

### Yanick

There are many more factors involved in rates of reaction other than simply concentration and temperature.

As above you can look at the Arrhenius equation to get some quantitative information. If you are interested in understanding what is happening (not the Maths) then you can check out "activation energy." You will undoubtedly see the Arrhenius equation on your travels through "activation energy land."

You need to be more specific about what you mean by "adding a third component." Are you referring to something like a catalyst? Catalysis is a huge field but it works by modifying the activation energy needed to overcome an energy barrier. Once again, google is your friend here.

4. Aug 27, 2013

### janhaa

A third factor are:

the speed of molecules also increasing with larger surface of the reactants

Last edited: Aug 27, 2013
5. Aug 27, 2013

### Staff: Mentor

Please elaborate, what you wrote doesn't make much sense to me.

6. Aug 27, 2013

### janhaa

Say we have CaCO3 into (large) pieces which react with 2 M HCl, versus crushed CaCO3 with the same HCl.
Latter reaction will most fizz, because the probability of favorable collisions increases with increasing surfaces

(hope you understand)

7. Aug 27, 2013

### WavesOfPhysics

thanks for these replies, most of my questions were very well answered but one which was complety forgotten, how does energy affects molecules ( that's what I meant by micro level )

8. Aug 27, 2013

### Staff: Mentor

Contrary to what you wrote earlier, it is not the speed of the molecules that changes the outcome in this case.