When calculating the heat of formation for MgO(s) + 2HCl(aq) -> MgCl2(aq) + H2O(

Question

When calculating the heat of formation for MgO(s) + 2HCl(aq) -> MgCl2(aq) + H2O(l) we use 30ml of 1M HCl and 0.5g of magnesium oxide. We must crush up the pieces of magnesium oxide because the lumps will slow down the reaction process.

If we do not grind up these lumps of magnesium oxide before performing the experiment, what will happen to the heat of formation for the reaction?

My guess is that if there are lumps, the reaction is slower and thus the final temperature is lower, which ultimately impacts the qsurr calculation ( qsurr = m x s x delta T ...lower total qsurr) and therefore less negative qsys?

Explanation / Answer

When we use lumps of magnesium oxide, only surface molecules of magnesium oxide will take part in the chemical reaction. The molecules of magnesium oxide that are present in the bulk will not take part in the chemical reaction.

Thus only a part of magnesium oxide is consumed during the chemical reaction. Hence, only a part of heat change is involved. Hence, the temperature difference is lower. Hence, the calculated value of heat of reaction will be lower than actual value.

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