Introduction  (Page 23/3)

Introduction

Before we begin this section, it might be useful to think about some different types of reactions and how quickly or slowly they occur.

Thinking about reaction rates

Think about each of the following reactions:

• rusting of metals
• photosynthesis
• weathering of rocks (e.g. limestone rocks being weathered by water)
• combustion

1. For each of the reactions above, write a chemical equation for the reaction that takes place.
2. How fast is each of these reactions? Rank them in order from the fastest to the slowest.
3. How did you decide which reaction was the fastest and which was the slowest?
4. Try to think of some other examples of chemical reactions. How fast or slow is each of these reactions, compared with those listed earlier?

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 up during the reaction or at how quickly the product forms . For example, iron and sulfur react according to the following equation:

$Fe+S\to FeS$

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 up. Alternatively, one could see how quickly the iron sulfide product forms. Since iron sulfide looks very different from either of its reactants, this is easy to do.

In another example:

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

Reaction rate

The rate of a reaction describes how quickly reactants are used up or how quickly products are formed during a chemical reaction. The units used are: moles per second (mols/second or mol.s ${}^{-1}$ ).

The average rate of a reaction is expressed as the number of moles of reactant used up, divided by the total reaction time, or as the number of moles of product formed, divided by the reaction time. Using the magnesium reaction shown earlier:

or

or

Reaction rates

1. A number of different reactions take place. The table below shows the number of moles of reactant that are used up in a particular time for each reaction.

   Reaction	Mols used up	Time	Reaction rate
   1	2	30 secs
   2	5	2 mins
   3	1	1.5 mins
   4	3.2	1.5 mins
   5	5.9	30 secs

   1. Complete the table by calculating the rate of each reaction.
   2. Which is the fastest reaction?
   3. Which is the slowest reaction?
2. Two reactions occur simultaneously in separate reaction vessels. The reactions are as follows: $Mg+C{l}_2\to MgC{l}_2$ $2Na+C{l}_2\to 2NaCl$ After 1 minute, 2 g of MgCl ${}_2$ have been produced in the first reaction.
   1. How many moles of MgCl ${}_2$ are produced after 1 minute?
   2. Calculate the rate of the reaction, using the amount of product that is produced.
   3. Assuming that the second reaction also proceeds at the same rate, calculate...
      1. the number of moles of NaCl produced after 1 minute.
      2. the mass (in g) of sodium that is needed for this reaction to take place.