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Chemistry Honors/AP

Chapter 5: 4-5

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WebLecture: Heat Capacity

Kotz and Triechel, Chemistry and Chemical Reactivity Chapter 5: 4-5.

5.4 Changes in energy content take two forms: heat flow and work. We measure work by movement, remember. Work done on by a system on its environment means that the system must expand into the environment; work done on the system by the environment means the environment pushes on the system. We measure this as a change in volume. If volume doesn't change (usually the case in most solution reactions), then all energy changes are heat flows into or out of the system.

5.5 Be sure that you understand what enthalpy is: the amount of heat energy stored in a system. Like potential energy in gravity systems, we always measure this as a change from some base position and express it as a function of the amount of matter involved. Heat LOSS is expressed as a negative amount. ΔH = -243kJ means that heat was released by the reaction.

Important Equations and Conversion Factors

ΔU = q + w
where U is the change in energy of the system as a result of
heat flow q into or out of the system and
work w done on (positive) or by the system (negative)

w = -P ΔV
where w = work done at constant pressure P
while volume changes ΔV = Vfinal - Vinitial

H = U + PV
H is the instantaneous total energy or enthalpy of the system

ΔH = ΔU + PΔV [where P is held constant]
In solutions, ΔV = 0, so ΔH = ΔU: it is equal to the heat flow into or out of the chemical system.

Videos for Chapter 5: Energy and Chemical Equations

Review the Videos at Thinkwell Video Lessons under THERMOCHEMISTRY.


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LAB #3 GUIDED INQUIRY: Measuring the enthalpy of reaction-- Phase II

Use your calorimeter to determine the specific heat of a metal.