Thermodynamics: Common Mistakes to Avoid for AP Chemistry
The most common thermodynamic experiment is a calorimetry experiment such as experiment 13. In this experiment 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 calorimeter, 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
- Be sure your units cancel giving you the unit desired in the final answer.
- Check your significant figures.
- Don't mix energy units, joules, and calories.
- Watch your signs in all the thermodynamic calculations. They are extremely important.
- Don't confuse enthalpy, ΔH, and entropy, ΔS.
- 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.
- Remember: products minus reactants.
- ΔHf and ΔGf are for 1 mol of substance. Use appropriate multipliers if needed.
- ΔGf and ΔHf for an element in its standard state are zero.
- All temperatures are in kelvin.
- When using ΔG° = ΔH°rxn – T Δ S°rxn, pay particular attention to your enthalpy and entropy units. Commonly, enthalpies will use kJ and entropies J.
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