Concentration of Acid
• Concentration is one factor which has an effect o the rate of a chemical reaction. In chemistry we describe concentration in terms of the number of moles of a substance there are in every cubic decimetre (litre) of solution, written as moles.dm-3 of M for short.
• In this experiment you will find out how increasing the concentration of the acid solution changes the rate of the reaction between dilute hydrochloric acid and magnesium metal.
• As the metal reacts with the acid, hydrogen gas is produced and the metal dissolves. To get an estimate of the rate of reaction, the time taken for a small piece of magnesium ribbon to dissolve (disappear) can be measured. This is a “clock reaction” and the reaction rate is proportional to 1/time taken
Dilute hydrochloric acid is harmful. Wear safety spectacles. Rinse any spillages with water.
1. Collect a strip of Mg ribbon about 10cm long. Clean the surface with emery paper, then cut it into strips of 1cm each as accurately as you can.
2. Set up a 250cm3 beaker half-full of tap water and clamp a boiling tube in the water, as shown in the diagram opposite. (You may find it easier to see the reaction if you stand the beaker on a tripod and gauze.
3. Use a small measuring cylinder to measure out 10cm3 of 1.5M hydrochloric acid and place this in the boiling tube. Put the thermometer into the acid in the tube.
4. Note down the temperature of the acid in the results table, then drop in one piece of Mg ribbon and start the stop-clock immediately.
5. Stir the acid carefully with the thermometer and stop the clock when the Mg ribbon just disappears (dissolves completely). Note down the final temperature of the acid and the time taken for the magnesium to dissolve in the results table.
6. Rinse out the boiling tube, measure out 10cm3 of 1.2M acid and repeat steps 4 and 5 for the other concentrations of acid.