Earth Chemical Processes



Earth Chemical Processes

Question 1

The equation for the reaction is:magnesium + hydrochloric acid ? magnesium chloride + hydrogen

Mg(s) + 2HCl(aq) ? MgCl2(aq) + H2(g)

3 cm of magnesium ribbon typically has a mass of 0.04 g and yields 40 cm3 of hydrogen when reacted with excess acid. 50 cm3 of 1M hydrochloric acid is a six-fold excess of acid.

In this reaction, the magnesium and acid are gradually used up. However the acid is in excess, so it is mainly the loss of magnesium (surface area becomes smaller) that causes the change in the rate.

If a graph of volume (y-axis) against time (x-axis) is drawn, the slope of the graph is steepest at the beginning. This shows that the reaction is fastest at the start. As the magnesium is used up, the rate falls. This can be seen on the graph, as the slope becomes less steep and then levels out when the reaction has stopped (when no more gas is produced). (Aristides 2006)

The reaction is exothermic, but the dilute acid is in excess and the rise in temperature is only of the order of 3.5°C. There is some acceleration of the reaction rate due to the rise in temperature. Some students might notice the flask becoming slightly warm and they could be asked how this would affect the rate of reaction, and how they might adapt the experiment to make it a 'fair test'.

Question 2

Time of dissolution is inversely proportional to the dissolution rate, i.e. if dissolution rate increases; the time of dissolution is decreased.

Question 3

Acidity increases the dissolution rate. Increased acidity generally results in increased dissolution rate.

Question 4

No

Question 5

Typical data obtained using a 5.0-cm length of Mg ribbon, 20.0 mL HCl in a small beaker, measuring the total time for all the magnesium to be consumed:

Concentration of HCl ...