At the start of a reaction, a 1.00 dm solution contains 0.300 mol of ethanol.
after 100 seconds the concentration of the ethanol has decreased to 0.296 mol/dmº
what is the rate of reaction over the first 100 seconds?
a 2.96 x 10-3 mol/dm/s
b 3.00 x 10 mol/dm/s
c 4.00 x 10 mol/dm®/s
d 8.00 x 10 mol/dm/s

Explanation:

The rate of a chemical reaction is defined as the rate of change of molarity with respect to time. That is, finding the change in molarity and dividing by the change in time will yield the average rate of a reaction.

In this problem, we firstly need to identify the initial molarity, the final molarity, find their difference and divide by the time passed.

Firstly, find the initial molarity dividing the number of moles by the given volume:

Secondly, define the equation based on our definition to calculate the reaction rate:

Given the final molarity of:

We obtain the average rate of change as:

This is, however, the rate of disappearance of ethanol. The reaction rate with respect to the reactants is just the negative value of the rate of change of the reactants: