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

# Surface Area and Reaction Rates

## Optional Experiment: Surface area and reaction rate

Marble $$(\text{CaCO}_{3})$$ reacts with hydrochloric acid $$(\text{HCl})$$ to form calcium chloride, water and carbon dioxide gas according to the following equation:

$$\text{CaCO}_{3}(\text{s}) + 2\text{HCl}(\text{l})$$ $$\to$$ $$\text{CaCl}_{2}(\text{s}) + \text{H}_{2}\text{O}(\text{l}) + \text{CO}_{2}(\text{g})$$

### Aim

To determine the effect of the surface area of reactants on the average rate of the reaction.

### Apparatus

• $$\text{2}$$ $$\text{g}$$ marble chips, $$\text{2}$$ $$\text{g}$$ powdered marble, concentrated hydrochloric acid ($$\text{HCl}$$)

• one beaker, two test tubes.

### Method

#### Warning:

Concentrated $$\text{HCl}$$ 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 handle with care.

1. Prepare a solution of hydrochloric acid in the beaker by adding $$\text{2}$$ $$\text{cm^{3}}$$ of the concentrated acid to $$\text{20}$$ $$\text{cm^{3}}$$ of water.

2. Place the marble chips into one test tube and the powdered marble into a separate test tube.

3. Add $$\text{10}$$ $$\text{cm^{3}}$$ of the dilute hydrochloric acid to each of the test tubes and observe the rate at which carbon dioxide gas ($$\text{CO}_{2}$$) is produced (you should see bubbles of $$\text{CO}_{2}$$).

### Results

Note (write down) what you observe.

### Questions and discussion

• Which reaction proceeds faster?

• Can you explain this?

### Conclusion

The reaction with powdered marble is faster. The smaller the pieces of marble are (in this case the powdered form is smallest), the greater the surface area for the reaction to take place.

Only the molecules at the surface of the solid can react with the hydrochloric acid. The next layer of molecules can only react once the surface molecules have reacted. That is, the next layer of molecules becomes the surface.

The chips of marble are relatively large, so only a small percentage of the molecules are at the surface and can react initially. The powdered marble has much smaller solid pieces, so there are many more surface molecules exposed to the hydrochloric acid. The more molecules exposed on the surface (the greater the surface area) the faster the reaction will be.

For the same amount of mass, smaller pieces of solid react faster as shown in Figure 7.2.

a) A large particle, b) small particles with the same volume as the large particle.

c) The surface area of large particles (shown in blue) is much smaller than that of small particles (shown in red).

### Optional Video: The Effect of Surface Area on Reaction Rate

Calcium carbonate reacts with hydrochloric acid according to the following reaction:

$$\text{CaCO}_{3}(\text{s}) + 2\text{HCl}(\text{aq})$$ $$\to$$ $$\text{CaCl}_{2}(\text{aq}) + \text{H}_{2}\text{O}(\text{l}) + \text{CO}_{2}(\text{g})$$

Consider the solid calcium carbonate.

If we react $$\text{1}$$ $$\text{g}$$ of $$\text{CaCO}_{3}$$ we find that the reaction is faster if the $$\text{CaCO}_{3}$$ is powdered when compared with the $$\text{CaCO}_{3}$$ being large lumps.

Explanation:

The large lump of $$\text{CaCO}_{3}$$ has a small surface area relative to the same mass of powdered $$\text{CaCO}_{3}$$. This means that more particles of $$\text{CaCO}_{3}$$ will be in contact with $$\text{HCl}$$ in the powdered $$\text{CaCO}_{3}$$ than in the lumps. As a result, there can be more successful collisions per unit time and the reaction of powdered $$\text{CaCO}_{3}$$ is faster.

$$\color{red}{\textbf{Increasing the surface area of the reactants increases the rate of the reaction.}}$$

The following video shows the effect of surface area on the time an effervescent tablet takes to fully dissolve. The tablet is fully dissolved once the bubbles ($$\text{CO}_{2}$$ gas) stop forming.