6.1c Calculating rate

There are two types of rate of the reaction that you can calculate: Average rate and instantaneous rate

Instantaneous rate is more accurate but harder to find.

Example classic experiment we use to understand how to find the rate is Calcium carbonate (solid) and Hydrochloric Acid (aqueous), measured using inverted cylinder. See video below.

1. Average rate

The average rate of the reaction in the first 20s is the change in volume over 20s                      

The average rate of the reaction in the first 10s is the change in volume over 10s

It's easy to see that the average rate depends on the time span that you are looking at. The longer/ later the time span, the slower the rate is because there are less available reactants to collide and react.

2. Instantaneous rate: is the more accurate way, measuring the rate of the reaction at a specific time point. Mathematically, it's the slope of the curve at that time point.

To find instantaneous rate, you draw a gradient line at time t (a straight line that intersects the curve only at that point). Then from the gradient line, find the change in y-axis divided by the change in x-axis. See picture below. Yes, there are certain level of eye-balling.

Initial rate: the instantaneous rate at t = 0, usually the fastest rate.

Same curve as above. The initial rate is (draw the tangent line) = 60 mL / 5 s = 12 mL/s