5 Steps to a 5 AP Chemistry 2019

130 ❯ STEP 4. Review the Knowledge You Need to Score High

Note: ° = 298 K

Answer:

∆°=−

=

−

×

=− ×

GRTKln

(8.314J)

(molK)

(298 K)ln4.17 10

8.34 10 J/mol

14

4

Experiments

The most common thermodynamic experiment is a calorimetry experiment. In this experi-

ment, the heat of transition or heat of reaction is determined.

The experiment will require a balance to determine the mass of a sample and possibly a

pipet to measure a volume, from which a mass may be calculated using the density. A calo-

rimeter, usually a polystyrene (Styrofoam) cup, is needed to contain the reaction. Finally, a

thermometer is required. Tables of heat capacities or specific heats may be provided.

Mass and possible volume measurements, along with the initial and final temperatures,

are needed. Remember: you measure the initial and final temperature so you can calculate

the change in temperature.

After the temperature change is calculated, there are several ways to proceed. If the

calorimeter contains water, the heat may be calculated by multiplying the specific heat of

water by the mass of water by the temperature change. The heat capacity of the calorimeter

may be calculated by dividing the heat by the temperature change. If a reaction is carried

out in the same calorimeter, the heat from that reaction is the difference between the heat

with and without a reaction.

Do not forget, if the temperature increases, the process is exothermic and the heat has

a negative sign. The opposite is true if the temperature drops.

Common Mistakes to Avoid

1. Be sure your units cancel giving you the unit desired in the final answer.


2. Check your significant figures.


3. Don’t mix energy units, joules, and calories.


4. Watch your signs in all the thermodynamic calculations. They are extremely important.


5. Don’t confuse enthalpy, DH, and entropy, DS.


6. Pay close attention to the state of matter for your reactants and products, and choose
   the corresponding value for use in your calculated entropies and enthalpies.


7. Remember: products minus reactants.


8. DHf and DGf are for 1 mol of substance. Use appropriate multipliers if needed.


9. DGf and DHf for an element in its standard state are zero.


10. All temperatures are in kelvin.


11. When using DG° = DH°rxn - T DS°rxn, pay particular attention to your enthalpy and
    entropy units. Commonly, enthalpies will use kJ and entropies J.

STRATEGY

KEY IDEA

TIP