Multi-step processes
Many chemical products are produced by a series of reactions.
An example is the smelting of iron which involves the three reactions as shown below.
Reaction 1. C + O2 → CO2
Reaction 2. C + CO2 → 2 CO
Reaction 3. Fe2O3 + 3 CO → 2 Fe + 3 CO2
Combining equations
When there is a series of reactions it is sometimes useful, especially when solving calculation problems, to combine the reactions to produce an overall reaction.
Reactions can be added together, but this is best done by adding the equations one step at a time. In each addition, you need to ensure that the chemical that carries through to the next step cancels out when the equations are added. Click on the headings below to see how the three reactions required in the production of iron can be combined.
Add equation 2 to equation 1.
Now add equation A to equation 3.
(You have to multiply the equations to get the same number of CO molecules [six] on each side of the equations so they will cancel when the equations are combined.)
Now watch videos of two examples of combining multi-step processes.
Process 1
The two reactions are shown below.
Reaction A 2 NaOH + CO2 → Na2CO3 + H2O
Reaction B Na2CO3 + CO2 + H2O → 2 NaHCO3
When the reactions are added together, the sodium carbonate and the water molecules cancel out to give the final equation as shown below.
2 NaOH + 2 CO2 → 2 NaHCO3
You can also divide to remove the 2s as they appear in front of every species in the equation.
Click here to open the video in a new window.
Process 2
The two reactions are shown below.
Reaction A 4 NH3 + 5 O2 → 4 NO + 6 H2O
Reaction B 2 NO + O2 → 2 NO2
In this case we have to multiply the second equation by a factor of two because there only two moles of nitrogen monoxide used in this step, whereas there are four moles produced in step 1.
Then the reactions can be added together to give the final equation as shown below.
4 NH3 + 7 O2 → 4 NO2 + 6 H2O
Click here to open the video in a new window.
The yield of multi-step processes
If a process includes a series of reactions with a yield of less than 100%, you can calculate the overall yield of the reaction by multiplying the % yields of the various steps.
Click here to see an example
A two-stage process is used to remove of the impurity silica (SiO2) from the iron containing compounds in a blast furnace is shown below.
Reaction 1. CaCO3 
CaO + CO2 Yield = 60%
Reaction 2. CaO + SiO2 
CaSiO3 Yield = 75%
= 45%
Multi-step processes consolidation
Complete the problem sheet 
to apply your knowledge to a range of multi-step processes.
What problems may be encountered by chemical industries that rely on multi-step processes to manufacture products?
