# Calculation of the average rate of reaction!

1. Apr 3, 2013

### mcfaker

Hi,

Im stuck: I found this example in which they calculate the rate of reaction: the title is "calculating the average reaction rate"

Now how the hell is this average reaction rate if they didnt use stoichiometric coefficients!? (in this case it should be 2.

2. Apr 3, 2013

### Sunil Simha

Why and how are the stoichiometric coefficients to be used?

3. Apr 3, 2013

### mcfaker

because the average rate always is calculated using this formula:

4. Apr 3, 2013

### Sunil Simha

True that is the rate of the reaction.

The question asks, instead, for rate of decomposition of the reactant. Which is simply the rate of change of its concentration with time.

5. Apr 3, 2013

### mcfaker

Thanks, So u mean that it is not the rate of the reaction that is calculated in this example?

6. Apr 3, 2013

### Sunil Simha

Yes, that is correct. They have merely found the rate of change of concentration of the reactant (which in this case is because of decomposition. Note that if they had asked the rate of formation of one of the products, it would simply again be rate of change in its concentration with respect to time)

7. Apr 3, 2013

### mcfaker

And what if they would ask "the rate of disappearance"? Do u have any idea what that might mean? Thanks!

8. Apr 3, 2013

### Sunil Simha

Disappearance or appearance would simply mean change in concentration (in the first case, the conc. is decreasing and in the second it is increasing). So you just have to find rate of change of concentration.

Note: Some textbooks use the convention that rate of disappearance is $-\frac{dC}{dt}$
where C is the concentration. For appearance the expression is similar but with the absence of the minus sign.