1.2 Reaction Rates

Reaction rate is a measure of how fast a reaction occurs, or how something changes during a given time period.

Before continuing this section you may want to do this simple lab activity.

Consider the oxidation of glucose, \(\ce{C_6H_1_2O_6(s)}\):

\(\ce{C_6H_1_2O_6(s) + 6O_2(g)\rightarrow6CO_2(g) + 6H_2O(g)}\)

One of the things that happens during this reaction is simply that glucose gets used up as it reacts with oxygen in the air, and carbon dioxide and water start to form.

How could we measure how fast this occurs? In this example, we might want to measure how quickly the mass of solid glucose decreases, or how quickly the gases (carbon dioxide and water) form. We might want to measure the volume of these gases.

Other reactions suggest other ways to measure their rates. Here are some other possible things that could be measured as they change:

• change in conductivity
• change in pH
• colour change
• change in pressure as gases are formed or used up

A common measure of reaction rate is to express how the concentration of a reaction participant changes over time. It could be how the concentration of a reactant decreases, or how the concentration of a product increases. This is the standard method we will be using.

Now that we have something that changes to measure, we must consider the second key aspect of determining rate - time. Rate is a measure of how something changes over time. You measure your rate of speed when driving by determining how many kilometres you travel in an hour; you could measure your rate of speed during a race by dividing the distance of the course by the number of minutes it took to run the rate.

We often define the rate of a chemical reaction as:

change in concentration change in time

Chemistry Notation

In chemistry, we typically represent concentration by using square brackets around the chemical formula of the substance. For example to indicate the concentration of \(\ce{SO_2(g)}\) in the following reaction we would write it as \(\ce{[SO2]}\).

Also, the delta symbol, Δ is used to indicate a change. ΔT, for example, means "the change in temperature."

Therefore, if we wanted to express the rate of the following reaction:

\(\ce{SO_2(g) + NO_2(g) \rightarrow SO_3(g) + NO(g)}\)

we could either measure the change in concentration of a reactant or product and might use either of the following expressions to calculate our rate:

\(\mathrm{\dfrac{\Delta[SO_2]}{\Delta{min}}}\)

\(\mathrm{\dfrac{\Delta[NO]}{\Delta{sec}}}\)

EQUATION

A graph illustrating reaction rate is often useful. What would it look like and why? If we were measuring the change in concentration of \(\ce{SO_2(g)}\) this is what we would see:

When the reaction first starts, the reactants ( \(\ce{SO_2(g)}\) and \(\ce{NO_2(g)}\)) are plentiful so things move along pretty fast - there is a relatively rapid decline in concentration of \(\ce{SO_2(g)}\)as it gets used up.

As the reaction continues, there will be less and less \(\ce{SO_2(g)}\) available so the reaction will slow down.

Eventually the reaction stops once all of the \(\ce{SO_2(g)}\)is gone. We notice this as the graph begins to level off - time continues to march along but there is no more any change in the concentration of \(\ce{SO_2(g)}\).

Moral of the story - if we measured the rate at the start of the reaction, it would be different than if we measured the rate near the end of the reaction. Rate is not a constant - it changes during the course of the reaction.

For this reason, we will generally work with the average rate of reaction, and measure rate over a longer period of time.

(If you measured rate at a specific point in time, you would be measuring the instantaneous rate. Instantaneous rate will differ depending on when the rate is actually calculated.)

For example, if you were taking a long car trip, there might be times when you are driving fast (on the highway) and also times when you drive more slowly (through the cities). If you calculated your average rate for the entire trip, you would average out these fast and slow periods.