What does a rate of reaction graph show?

A graph of rate of reaction shows how the measured quantity of either product or reactant changes over time. The graph provides a visual representation of this.

Describe the overall shape of the graph. What does the shape of the graph tell us about the rate of reaction?

As the reaction proceeds the rate of reaction changes. Initially the rate is high. We can see this represented as a line with a steep gradient. Over time the gradient of the line decreases, reflecting the fact that the rate is decreasing.

How can we provide quantitative information about the progress of the reaction to aid our description of the change in rate of reaction? 

By using the method of drawing a tangent to the line and measuring the gradient we can give values for the relationship between the measured quantity (reactant or product) vs time. The value of this ratio changes throughout the experiment.

How valid are the values my students might have for the rate of reaction determined from the tangent method?

As with all practicals the data obtained will depend on a number of factors. It will depend on the ability of students to collect data such as gas volume either by using a gas syringe or by the displacement of water, or their skill in manipulating the colorimeter or recording mass loss from a top pan balance.

Assuming data is reliable there is the additional complication of accurately drawing tangents to the line and correctly performing algebraic functions on the data collected.

All of the above require practice so opportunities must be provided to develop these skills.