Factors Affecting Reaction Rates

Factors affecting reaction rates

Several factors affect the average rate of a reaction. It is important to know these factors so that reaction rates can be controlled. This is particularly important when it comes to industrial reactions, where greater productivity leads to greater profits for companies. Let's talk about some of the factors that affect the average rate of a reaction.

Nature of reactants

Substances have different chemical properties and therefore react differently, and at different rates (e.g. the rusting of iron vs. the tarnishing of silver).

Optional Video: Reaction Rate - Nature of Reactants

Fact:

Oxalic acid is abundant in many plants. The leaves of the tea plant (Camellia sinensis) contain very high concentrations of oxalic acid relative to other plants. Oxalic acid also occurs in small amounts in foods such as parsley, chocolate, nuts and berries. Oxalic acid irritates the lining of the gut when it is eaten, and can be fatal in very large doses.

In the nature of reactants, surface area and concentration experiments learners are required to work with concentrated, strong acids. These acids can cause serious burns. Please remind the learners to be careful and wear the appropriate safety equipment when handling all chemicals, especially concentrated acids. The safety equipment includes gloves, safety glasses and protective clothing.

Optional Experiment: The nature of reactants

Aim

To determine the effect of the nature of reactants on the average rate of a reaction.

Apparatus

You will need the following items for this experiment:

Oxalic acid $((\text{COOH})_{2})$, iron(II) sulfate $(\text{FeSO}_{4})$, potassium permanganate $(\text{KMnO}_{4})$ and concentrated sulfuric acid $(\text{H}_{2}\text{SO}_{4})$
a spatula, two test tubes, a medicine dropper, a glass beaker and a glass rod.

Method

Warning:

Concentrated $\text{H}_{2}\text{SO}_{4}$ can cause serious burns. We suggest using gloves and safety glasses whenever you work with an acid. Remember to add the acid to the water and to avoid sniffing the acid. Handle all chemicals with care.

Label one test tube $\text{1}$. Prepare an iron(II) sulfate solution in test tube $\text{1}$ by dissolving two spatula tips of iron(II) sulfate in $\text{10}$ $\text{cm$^{3}$}$ of water.
Label the other test tube $\text{2}$. Prepare a solution of oxalic acid in test tube $\text{2}$ in the same way.
Prepare a separate solution of sulfuric acid by adding $\text{2}$ $\text{cm$^{3}$}$ of the concentrated acid to $\text{10}$ $\text{cm$^{3}$}$ of water. Remember always to add the acid to the water, and never the other way around.
Add $\text{2}$ $\text{cm$^{3}$}$ of the sulfuric acid solution to the iron(II) sulfate and oxalic acid solutions respectively.
Using the medicine dropper, add a few drops of potassium permanganate to the two test tubes. Observe how quickly the potassium permanganate solution discolours in each solution.

Results

You should have seen that the the potassium permanganate discolours in the oxalic acid solution much more slowly than in the iron(II) sulfate solution.

These reactions can be seen in the following videos:
It is the oxalate ions $(\text{C}_{2}\text{O}_{4}^{2-})$ and the $\text{Fe}^{2+}$ ions that cause the discolouration. It is clear that the $\text{Fe}^{2+}$ions react much more quickly with the permanganate than the $(\text{C}_{2}\text{O}_{4}^{2-})$ ions. The reason for this is that there are no covalent bonds to be broken in the iron ions before the reaction can take place. In the case of the oxalate ions, covalent bonds between carbon and oxygen atoms must be broken first.

Conclusions

Despite the fact that both these reactants (oxalic acid and iron(II) sulfate) are in aqueous solutions, with similar concentrations and at the same temperature, the reaction rates are very different. This is because the nature of the reactants can affect the average rate of a reaction.

The nature of the iron(II) sulfate in solution (iron ions, ready to react) is very different to the nature of oxalic acid in solution (oxalate ions with covalent bonds that must be broken). This results in significantly different reaction rates.

Note:

The $\text{KMnO}_{4}$ with oxalic acid and iron(II) sulfate pictures are screenshots from videos by katalofuromai and Aaron Huggard on Youtube.

Factors Affecting Reaction Rates