Chemistry » Energy and Chemical Reactions » Rates Of Reaction And Factors Affecting Rate

What is a reaction rate?

What is a reaction rate?

In a chemical reaction, the substances that are undergoing the reaction are called the reactants, while the substances that form as a result of the reaction are called the products. The reaction rate describes how quickly or slowly the reaction takes place. So how do we know whether a reaction is slow or fast? One way of knowing is to look either at how quickly the reactants are used during the reaction or at how quickly the products form. For example, iron and sulfur react according to the following equation:

$$\text{Fe}(\text{s}) + \text{S}(\text{s})$$ $$\rightarrow$$ $$\text{FeS}(\text{s})$$

In this reaction, we can observe the speed of the reaction by measuring how long it takes before there is no iron or sulfur left in the reaction vessel. In other words, the reactants have been used. Alternatively, one could see how quickly the iron sulfide (the product) forms. Since iron sulfide looks very different from either of its reactants, this is easy to do.

In another example:

$$2\text{Mg}(\text{s}) + \text{O}_2(\text{g})$$ $$\rightarrow$$ $$2\text{MgO}(\text{s})$$

In this case, the reaction rate depends on the speed at which the reactants (oxygen gas and solid magnesium) are used, or the speed at which the product (magnesium oxide) is formed.

Definition: Reaction rate

The average rate of a reaction describes how quickly reactants are used or how quickly products are formed during a chemical reaction.

The average rate of a reaction is expressed as the number of moles of reactant used, divided by the total reaction time, or as the number of moles of product formed, divided by the total reaction time.

Average reaction rate for:

• the use of a reactant = $$\dfrac{\text{moles reactant used}}{\text{reaction time (s)}}$$

• the formation of a product = $$\dfrac{\text{moles product formed}}{\text{reaction time (s)}}$$

Using the magnesium reaction shown earlier:

• Average reaction rate of Mg being used = $$\dfrac{\text{moles Mg used}}{\text{reaction time (s)}}$$

• Average reaction rate of $$\text{O}_2$$ being used = $$\dfrac{\text{moles O}_2\text{ used}}{\text{reaction time (s)}}$$

• Average reaction rate of MgO being formed = $$\dfrac{\text{moles MgO formed}}{\text{reaction time (s)}}$$

Example: Reaction Rates

Question

The following reaction takes place:

$$4\text{Li}(\text{s}) + \text{O}_{2}(\text{g})$$ $$\to$$ $$2\text{Li}_{2}\text{O}(\text{s})$$

After two minutes, $$\text{4}$$ $$\text{g}$$ of lithium has been used. Calculate the rate of the reaction.

Step 1: Calculate the number of moles of lithium that are used in the reaction

$$\text{n} = \dfrac{\text{m}}{\text{M}}$$

$$\phantom{\rule{6.pt}{0ex}}=\dfrac{\text{4}\text{ g}}{\text{6.94}\text{ g.mol^{-1}}}$$

$$\phantom{\rule{6.pt}{0ex}}=$$ $$\text{0.58}$$ $$\text{mol}$$

Step 2: Calculate the time (in seconds) for the reaction

t = $$\text{2}$$ $$\text{minutes}$$ = $$\text{2}$$ $$\times$$ $$\text{60}$$ $$\text{s}$$ = $$\text{120}$$ $$\text{seconds}$$

Step 3: Calculate the rate of the reaction

Rate of reaction of $$\text{Li}$$ used $$= \dfrac{\text{moles of lithium used}}{\text{time}}$$

$$\phantom{\rule{73.pt}{0ex}} = \dfrac{\text{0.58}\text{ mol}}{\text{120}\text{ s}}$$

$$\phantom{\rule{73.pt}{0ex}} =$$ $$\text{0.005}$$ $$\text{mol.s^{-1}}$$

The rate of the reaction is $$\text{0.005}$$ $$\text{mol.s^{-1}}$$