# Homework Help: Why does a higher activation energy usually mean a slower reaction?

1. Nov 9, 2013

### needingtoknow

Why does a higher activation energy usually mean a slower reaction?

Doesn't activation energy just tell us whether or not a reaction will take place or not based on if the particles have enough energy to meet the activation barrier? How is it related to the rate of the reaction?

2. Nov 9, 2013

### Simon Bridge

It can also indicate the energy needed to maintain the reaction is also high.
You are right though - a high activation energy need not always indicate a slower reaction rate.
You need to look at why the activation energy is high.

3. Nov 9, 2013

### needingtoknow

But generally why do people say that reactions with high activation energies tend to be slow?

4. Nov 9, 2013

### Simon Bridge

<puzzled>
Because, generally, but not always, reactions with high activation energies tend to be slow, and "people" tend to be honest about such things.

For specific cases, it is probably possible to find a reaction with a higher activation energy that proceeds faster.
Or, for that matter, a reaction with a lower activation energy that does not proceed very fast at all.

In all cases, where does the energy for the reaction come from?
How is it supplied? Does it all come in one go?
How does the reaction rate depend on the available energy?

The activation energy and the reaction rate are not expected to be independent terms - they both depend on the chemicals involved. Therefore it is not surprising to find some correlation between them.

But we may be confusing terms - the activation energy is the minimum energy the reactant molecules need in order to form the products.

5. Nov 9, 2013

### Yanick

See the Arrhenius equation. Rate constants are directly proportional to temperature and inversely proportional to activation energy.

6. Nov 9, 2013

### Staff: Mentor

The temperature dependence of a reaction rate constant is usually expressed in terms of its activation energy as follows:

$$k=k_∞e^{-E/RT}$$

where E is the activation energy and T is the temperature. The rate constant at very high temperatures k is related to the collision frequency, which is on the same order of magnitude for most combinations of molecules in a gas. So, the higher the activation energy, the lower the rate constant is likely to be at a given temperature.

7. Nov 9, 2013

### nasu

The rate of reaction depends on how many molecules have energies above the activation rate.
I think that the missing part in your understanding is that the molecules have a distribution, a range of energies, at any given temperature. So is the activation is low (but within the range), the number of molecules with enough energy will be larger.

8. Nov 10, 2013

### needingtoknow

How does the rate of reaction depend on how many molecules have energies above the activation rate? If more molecules have energies above the activation energy doesn't that just mean that more of the substance will react? What does it have to do with rate is my question?

9. Nov 10, 2013

### nasu

The rate of reaction depends on the rate of molecules of the reactants encountering each other. Something like molecular collisions. Do you understand that the rate of molecule collision in a gas depend on the concentration of molecules?
Then assume than for one reaction 90% of these collisions satisfy the energy condition and for another reaction, with higher reaction energy, in only 1/10 of the collisions the energy will be high enough. Most of the encounters between the molecules of reactant will not result in a reaction.

This is a crude model but offers (i hope) some intuitive understanding of how the rate depends on reaction energy. Of course temperature is an important player too.

10. Nov 11, 2013

### Staff: Mentor

You seem to be confusing the activation energy with the heat of reaction. The heat of reaction determines the temperature dependence of the equilibrium constant, which determines how much of a substance reacts. The activation energy determines the temperature dependence of the reaction rate, by determining what fraction of collisions per unit time result in reaction.