Chemical Thermodynamics Practice Questions with complete Solutions Graded A+
Reactions for this lab - Answers H2 (g) + 1/2 O2 → H2O (l) (heat of formation of water) (a)
Mg (s) + 2 HCl(aq) →MgCl2 (aq) + H2 (g) (b)
Ha= -68.3 kcal/ mol
MgO(s) + 2HCl (aq) -> MgCl2 (aq) + H2O (l) (c)
Mg(s) + 1/2 O2 (g) --> MgO(s) (d)
Hd= Hf (MgO) = Hb - Hc + Ha = Hb - Hc + (68.3 kcal/mol)
Why is it very difficult to measure the heat of formation of MgO? - Answers Because without a pure
oxygen environment, it is not possible to prevent the formation of Mg3N2 when Mg(s) burns in air (79%
of air is N2)
Enthalpy of formation (Hf) - Answers the heat given off when one mole of a substance is prepared from
its constituent elements in their standard states
Why do you clean the Mg ribbon with the steel wool? - Answers To prevent the formation of MgO that
could screw up results
Heat capacity - Answers ability to absorb heat
How will you find the heat capacity of the calorimeter? - Answers since almost all the heat given off in
the reaction is absorbed by the water in the calorimeter, you will use the heat capacity of water.
How do you determine the amount of heat given off? - Answers heat= heat capacity x mass x
temperature change
What are the units of heat= heat capacity x mass x temperature change? - Answers cal = cal/ g °C x g x °C
What is the heat capacity of water? - Answers 1.0 cal/ g °C
-water has one of the highest heat capacities known (and specific heats)
What does a heat capacity of 1.0 cal/ g °C mean? - Answers It takes 1.0 cal of heat to raise the
temperature of 1.0 g of water by 1.0 °C
How do you determine the heat given off in the experiment? - Answers heat = (1.0 cal/ g C)(100g)
(temperature change)
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