5 Steps to a 5 AP Chemistry

ΔH°=ΣΔHf°products −ΣΔHf°reactants

ΔG°=ΣΔGf°products −ΣΔGf°reactants

ΔG° = ΔH°−TΔS°

=−RTInK=−2.303RTlogK

=−n F E°

ΔG=ΔG°+RTlnQ=ΔG°+2.303RTlogQ

q=mcΔT

Gas constant, R= 8.31 J mol–1K–1

Calorimetry

Calorimetryis the laboratory technique used to measure the heat released or absorbed

during a chemical or physical change. The quantity of heat absorbed or released during a

chemical or physical change is represented as q and is proportional to the change in tem-

perature of the system being studied. This system has what is called a heat capacity(Cp),

which is the quantity of heat needed to change the temperature 1 K. It has the form:

Cp=heat capacity =q/ΔT

Heat capacity most commonly has units of J/K. The specific heat capacity(or specific

heat) (c) is the quantity of heat needed to raise the temperature of 1 g of a substance 1 K:

c=q/(m×ΔT) or q=cmΔT,

where mis the mass of the substance.

The specific heat capacity commonly has units of J/g.K. Because of the original defini-

tion of the calorie, the specific heat capacity of water is 4.184 J/g.K. If the specific heat

capacity, the mass, and the change of temperature are all known, the amount of energy

absorbed can easily be calculated.

Another related quantity is the molar heat capacity(C), the amount of heat needed

to change the temperature of 1 mol of a substance by 1 K.

Calorimetry involves the use of a laboratory instrument called a calorimeter. Two types

of calorimeter, a simple coffee-cup calorimeter and a more sophisticated bomb calorimeter,

are shown in Figure 9.1. In both, a process is carried out with known amounts of substances

and the change in temperature is measured.

The coffee-cup calorimeter can be used to measure the heat changes in reactions or

processes that are open to the atmosphere: qp, constant-pressure reactions. These might be

reactions that occur in open beakers and the like. This type of calorimeter is also commonly

used to measure the specific heats of solids. A known mass of solid is heated to a certain

temperature and then is added to the calorimeter containing a known mass of water at a

known temperature. The final temperarure is then measured allowing us to calculate the ΔT.

We know that the heat lost by the solid (the system) is equal to the heat gained by the sur-

roundings (the water and calorimeter, although for simple coffee-cup calorimetry the heat

gained by the calorimeter is small and is ignored):

−qsolid=qwater

C

H

p T

=

Δ

Δ

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