S. Work is done in this reaction when the H2 gas expands out of the test tabe an

Question

S. Work is done in this reaction when the H2 gas expands out of the test tabe and pushes the atmosphere asik. W Pav where W wok, P pressure, AV Vnal-Vinitiol 12 Vi CI Volume of the initial system includes the Mg metal and the acid Volume of the final system includes the solution cond the H2 gas How much work is done when one mole of Mg reacts with excess acid at STP? z19 a) ) H-2 029 W: Pay b) Estra Credit: Why is it approxinately comect to assue o change in the volume of the Mg and acid solution (i.e. the solid& liquid phases) during the reaction?

Explanation / Answer

(a)

let us assume

Intial volume = volume of Mg and HCl = volume of MgCl2 solution = V

Mg (s) + 2 HCl (aq.) -----------> MgCl2 (aq.) + H2 (g)

From the balanced equation,

1 mol of Mg produces 1 mol of H2 gas

At STP 1 mol of any ideal gas occupies 22.4 L of volume

Volume of of H2 gas released = 22.4 L

So, final volume = Volume of H2 gas + volume of solution

Final volume = 22.4 + V

Change in volume = Volume of H2 gas + volume of solution - volume of Mg and acid

deltaV = 22.4 + V - V = 22.4 L

At STP, pressure = 1 atm

Work done = P * deltaV = 1 * 22.4 = 22.4 atm.L

But, 1 atm .L = 101.325 J

So, work done = 22.4 * 101.325 = 2269.7 J = 2.27 kJ

(b)

Because solids and liquids do not expand much and hence it is assumed that no volume change is observed for solids and liquids compared to gases.

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