Chem 161-2009 Homework 7
Week and part of 8
Chapter 6: 21,23,25,27,33,37,39,41,43,45,49,51,55,59,63,65,67,69,71,73,75,79,88,90,97,98,101
First Law of Thermodynamics
How much heat, in joules, must be involved in a process in which a system does 123 cal of work if
the internal energy of the system is to remain unchanged?
1 cal = 4.184J
∆U = q + w
0 = q - 123cal
q = +123cal
+123 cal x (4.184J/cal) = +515J
Can a system do work and absorb heat at the same time?
Can it do so while maintaining constant
Yes, a system can do work and absorb heat at the same time.
For example, if a system forms gas then,
since its gas is expanding, the surrounding’s gas is being compressed, and therefore work is being done on
If this reaction is endothermic, then the system is absorbing heat, while doing work on
It can do so while maintaining a constant internal energy.
If the amount of joules
absorbed in the form of heat is exactly equal to the amount of joules lost in the form of work on the
surroundings, then the internal energy will not have changed.
Enthalpy and Enthalpy Changes
At high temperatures, water is decomposed to hydrogen and oxygen.
O(g) → H
(g) + ½ O
Decomposition of 67.3 g H
O at constant pressure requires that 902 kJ of heat be absorbed by the system.
Is the reaction endothermic or exothermic?
What is the value of q for the reaction, per mole of water?
the value of q equal to ∆U or ∆H?
mol = g/MW
) = 3.735 mol
If heat is absorbed by the system the reaction is endothermic.
902 kJ/3.735 mol = 242 kJ/mol of water.
At constant volume, ∆U = q
At constant pressure, ∆H = q
Since this reaction is being done at constant pressure, then q = ∆H.
When 0.1500 mol of solid calcium oxide is mixed with 0.1500 mol of liquid water, solid calcium
hydroxide is formed and 9.78 kJ of heat is released.
Write a chemical equation for the reaction producing
1 mol of calcium hydroxide, including the physical states of all the substances and the value of ∆H.
∆H = -9.78J
∆H = -65.2J
Heats of Reaction and Reaction Stoichiometry