HOMEWORK WEEK 8 CHAPTER 6
An exothermic reaction is a chemical reaction or a physical change in which heat is
evolved (ΔH is negative). For example, burning one mol of methane, CH
carbon dioxide, water, and 890.3 kJ of heat. An endothermic reaction is a chemical
reaction or physical change in which heat is absorbed (ΔH is positive). For example, the
reaction of one mol of barium hydroxide with ammonium nitrate absorbs 170.4 kJ of heat
in order to form ammonia, water, and barium nitrate.
The heat capacity (C) of a substance is the quantity of heat needed to raise the
temperature of the sample of substance one degree Celsius (or one Kelvin). The specific
heat of a substance is the quantity of heat required to raise the temperature of one gram
of a substance by one degree Celsius (or one Kelvin) at constant pressure.
The equation for the formation of H
(I would accept S(s) instead of 1/8 S
i.e. forming one
mole of H
S from its elements
in their normal states
[Note that the “enthalpy of formation” of H
S(g) is just the ΔH of the formation
Using the fact that ΔH>0, the answer is c, endothermic, and the temperature of the
resulting solution falls.
Let’s assume that ΔH of reaction equals -1000kJ, i.e.
O(g), ΔH = -1000kJ
This gives us 4 separate pieces of information:
When 1 mole of C
reacts completely, 1000kJ of heat are released.
When 13/2 mole of O
react completely, 1000kJ of heat are released.
When 4 mole of CO
are formed by this reaction, 1000kJ of heat are released.
When 5 mole of H
O are formed by this reaction, 1000kJ of heat are released.
The question concerns one mole
of each species and we know that the ΔH or heat
released is directly proportional
to the moles involved. For instance for O
Heat released by reaction of 1mol = 1 mol O
13/2 mol O
Similarly when 1 mole of CO
is formed, we get 1000kJ/4mol = 250kJ
Similarly when 1 mole of H
O is formed, we get 1000kJ/5mol = 200kJ
Thus (b), 1 mol O
, yields the smallest amount of heat, because O