Answer to Question #205760 in General Chemistry for Fiona Yvonne Balia

Question

5 g of zinc pieces are added to 400 cm³ of 0.5 mol. dm-3 hydrochloric acid. To investigate the average rate of the reaction, the change in the mass of the flask containing the since and the acid is measured by placing the flask on a direct reading balance. The reading on the balance shows that there is a decrease in mass during the reaction. The reaction which takes place is given by the following equation: Zn(s)+2HCl(aq)→ZnCl2(aq)+H2(g)

Why is there a decrease in mass during the reaction?

The experiment is repeated, this time using 5 g of powdered zinc instead of pieces of zinc. How will this influence the average rate reaction?

The experiment bus repeated once more,this time using 5 g of the zinc pieces of 600 cm³ of 0.5 mol.dm-3 hydrochloric acid. How does the average rate of this reaction compare with the original reaction rate?

What effect would a catalyst have on the average rate of this reaction?

Answer 1

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why is there decrease in mass?

The decrease in mass could be most probably be because the experiment is performed in an open system. There is exchange in both mass and energy between the system and the surrounding. Notice that the experiment releases hydrogen gas which will most probably escape into the surrounding. This will cause reduction of mass for the contents in the flask.

The influence of Using powdered zinc of the same mass.

Using powdered zinc instead of metal pieces increases the rate of reaction. This is due to increase in surface area of the reacting species hence more exposure to the acid.

Effect of increasing volume of the acid.

increasing the volume of the acid from 400cm³ to 600cm³ has no effect on this reaction. Increasing volume has no effect since it only increases number of moles of the acid present and not the concentration of the acid. The moles of the acid are already in excess in the original reaction since 5g of zinc only requires 0.1538moles of hydrochloric acid which is equivalent to about 308ml of 0.5M hydrochloric acid. Increasing volume increases these moles instead of increasing concentration so there is no effect.

Effect of using a catalyst.

when a catalyst is used, the reaction takes a shorter time to reach completion. A catalyst increases the rate of reaction by lowering the activation energy. this is done occasionally by providing an alternative reaction mechanism that needs lower energy: an ideal transition state hence overcoming the energy barrier more easily.