If equal masses of powdered marble and marble chips are added separately to equal volume of 1. 0M hydrochloric acid at the same temperature, it is observed that the powdered marble takes less time to react with the acid while the marble chips take a longer time. This is because when the marble is powdered the total surface area of marble in contact with the acid increases and hence more effective collisions are made between the molecules of the acid and the particles of the marble. The overall effect is therefore an increase in reaction rate. On the other hand the reaction between the acid and marble chip is slow because a limited area of the chip is in contact with the dilute acid hence there are fewer effective collisions between the acid and the marble particles.

As a consequence, the reaction rate decreases, measuring the time required for each reaction to go to completion and plotting a graph of reaction rate against time for the powdered marble and marble chips will have a shape as shown below.

• Experimental investigation of particles size on the rate of reaction

Example

Reaction between marble (a form of calcium carbonate) with dilute hydrochloric acid.

Requirements

Lumps of marble, powdered marble, 2M hydrochloric acid a conical flask atop pan balance, a piece of wool.

• Place about 40cm3 of 2M hydrochloric acid (an excess) in a conical flask on a top pan balance and place a piece of cotton wool in the neck of the flask.

(The cotton wool prevents the acid from splashing on the pan). Add 6g lumps of marble. Note the total mass and immediately start the stop-watch. Note the mass every 30 seconds.

• Plot a graph of mass on the vertical scale against time on the horizontal scale.
• Repeat the experiment using 6g of powdered marble and take the same measurements. Draw the graph on the same axis. The following rate curves are obtained.

Observation

Effervescence is more vigorous and stops earlier in the case of the powdered marble than in that of lumps marble.

Conclusion

Increasing the surface area of solid reactants increases the rate of reaction.