This section aims to cover the following topics:
- Define chemical reaction rate
- Define rate expression
- Relative rates of reaction and stoichiometry ratio
Rate – change in concentration over time
The rate of a chemical reaction can be thought of as the speed at which a chemical reaction happens and this can be expressed as the change in concentration of a substance over time:
Rate of reactant / product
For a reaction A + B → C, where A and B are the reactants and C is the product, you can measure how fast a reactant is disappearing (A or B), or how fast a product is appearing (C).
Rate of reaction graphs
The concentration vs time graph shows the amount of reactant being used up over time or the amount of product being produced over time. The graphs will have slopes going in opposite directions to each other as illustrated below.
Initial, instantaneous, & average rate
It’s important to know the difference between instantaneous and average rate of reaction. Click on the tabs below to learn more.
Given a reaction equation: aA + bB → cC + dD where A and B are the reactants and C and D are the products, and a,b,c and d are the coefficients, the rate of reaction between the substances are relative to each other and the relationship is linked via the stoichiometric ratio:
Change in concentration is calculated using [A]final – [A]initial and change in time is calculated using T(final) – T(initial)
The change in concentration for A and B is negative because reactants are being used up.
Speed cannot be negative so the reactant rates are given (-) sign in front in order to cancel out the negative values.
To learn about reaction rate, a simple experiment using magnesium and hydrochloric acid can be performed.
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
In this experiment a small piece of magnesium ribbon is placed in HCl solution and the reaction produces hydrogen gas. The gas is collected in a gas syringe and the volume produced is measured every 10 seconds. Using the data collected, a graph can be generated using an excel spreadsheet. From the graph, the initial, average, and instantaneous rate of reaction can be determined using the methods described above.
The change in volume must first be converted to change in concentration in order to plot the concentration vs time graph.
A graph of [H₂] vs time can be plotted. It will look like something below. The graph starts off very steep to denote the quick production of H₂. As time proceeds, the reaction slows down and eventually stops when all of the Mg is completely used up.