First Law

Question 1

What is pressure-volume work?

Answer 1

• Mechanical work that results when a system is compressed or expanded as a result of an outside pressure
• Force exerted by the gas inside = PA, because P = F/A
• If the piston is not moving, there is no acceleration, so no work is being done on it and outside force exactly counterbalances inside force
• If outside pressure < inside pressure, the gas will expand, and the outside pressure will do negative work: -F_ext(Hf-Hi) = PA ΔH = P ΔV
• In an expansion, the system does positive work; in a compression, the surroundings do net work

Question 2

What is internal energy?

Answer 2

• Total energy of a system arising from potential energy between molecules, kinetic energy of molecular motions, and chemical bonds
• When a system does work, its internal energy decreases; when work is done on it, its internal energy increases

Question 3

Relationship between energy and temperature

Answer 3

q = mcΔT

• Q is the amount of energy put into the system – but doing work on the system or heating it, and ΔT is the change in temperature
• Double the mass or the specific heat and you double the amount of energy needed to achieve a given temperature increase
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Question 4

Mathematical expression of the first law

Answer 4

If both heat and work can cross the boundaries of a system, then the change in internal energy, U = q + w

• q and w depend on the path followed (they are not state functions) but their sum is
• q = thermal energy added to system and w = work done on the system
• Work is only done on the system when there is an unbalanced external force
• work done on system = -work done on surroundings; any time there is net work done on the system, there is an exchange of energy between the system and its surroundings

Question 5

Is the heat capacity at constant pressure or at constant volume greater?

Answer 5

• Heat capacity is a measure of the amount of energy needed to cause a certain increase in temperature
• C_P > C_v because some of the energy is spent on doing work to maintain a constant pressure; in a constant volume container, all of the energy goes into increasing the internal energy

Question 6

How does bomb calorimetry work?

Answer 6

• Reacting species in small closed container surrounded by water in calorimeter can
• Because volume is fixed, all the heat released or absorbed by reaction is translated into changing the internal energy and the temperature; can calculate amount of energy transferred

Question 7

What is enthalpy in terms of pressure, volume, and internal energy?

Answer 7

• At constant pressure, change in internal energy equals the heat put into the system minus the work done by the system, ΔPV
• So, H = U + PV
• Enthalpy is a “corrected” internal energy accounting for the work done by the system when heat is added to it