The rate of chemical reactions

Rate of chemical reactions

Chemical reactions take place at different rates. Some reactions occur rapidly, while others are very slow. Here are some examples:

i. Fast reaction: addition of sodium metal to water

The reaction between sodium metal and water is:

$2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)$

This reaction occurs immediately and violently, making it a fast reaction.

ii. Slow reaction: rusting of iron

The rusting of iron in the presence of air and water produces hydrated iron(III) oxide:

$Fe(s) + O_2(g) + H_2O(l) \rightarrow Fe_2O_3 \cdot XH_2O(s)$

This is an extremely slow reaction.

Measuring the rate of a chemical reaction

The rate of a chemical reaction can be measured in various ways. For example, consider the reaction between zinc and sulfuric acid:

$Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)$

When zinc is added to dilute sulfuric acid in a flask, the zinc slowly disappears and hydrogen gas ($H_2$) bubbles off. Over time, the rate of bubble formation decreases as the reaction slows down, and eventually no more bubbles appear. At this point, the reaction is over because all the acid has been used up, and some zinc remains behind in the beaker.

The rate of this reaction could be determined by measuring:

• The amount of zinc used up per unit of time;
• The amount of sulfuric acid used up per unit of time;
• The amount of zinc sulfate produced per unit of time;
• The amount of hydrogen produced per unit of time.

The rate of a chemical reaction is determined by measuring either the amount of reactant used up per unit of time or the amount of product produced per unit of time.

Graphical representation of the rate of reaction

If a reaction proceeds over time, it can be graphed to determine the rate. The slope of the graph gives the rate of reaction:

• If the plot is of product concentration against time, the slope will be positive, indicating that the product is increasing.
• If the plot is of reactant concentration against time, the slope will be negative, indicating that the reactant is being used up.

The SI unit for the rate of reaction is $\text{mol dm}^{-3}\ \text{s}^{-1}$.