CHEM 112 Thermochemistry Memorize

Sign Conventions for Heat (q) and Work (w)

what does heat capacity of a system depend on

Mass and type of material

Specific heat capacity

Units J/(g·°C)

Measure of a substance’s intrinsic ability to absorb heat (heat required to raise temp by one gram of substance by 1°C)

q=mc_^pΔT

Molar Heat Capacity

Measure of a substance’s intrinsic ability to absorb heat (heat required to raise the temperature of one mole of a substance by 1 °C.

Units: J/(mol · °C)

q=C_p,mΔT

What is pressure-volume (PV) work?

Work that results from a volume change against an external pressure.

What happens to work when a gas expands?

• ΔV is positive.

• When a gas expands, it pushes against the external pressure and transfers energy to the surroundings. Since the gas loses energy by doing work, (w) is negative.

formula for PV work at constant external pressure

- w = −PₑₓₜΔV

how do you convert pressure × volume into Joules?

• 1 kPa·L = 1 J.

• 1 atm·L = 101.325 J, so multiply atm·L by 101.325 to convert to Joules.

What is the First Law of Thermodynamics?

• Energy cannot be created or destroyed during chemical or physical processes.

• The change in internal energy of a system equals the heat gained or lost by the system plus the work done on or by the system: ΔU = q + w.

internal energy (U)

total amount of kinetic and potential energy a system possesses

contributions to the internal energy

• Translational kinetic energy

• Rotation

• Intermolecular attractions

• Nuclear energy

• Bond vibration

• Electron motions

• Chemical bonds

TRINBEC

Ahat is a state function?

A property that depends only on the initial and final states, not on the pathway taken.

Are ΔU q and w state functions?

ΔU is a State Function, but q and w are not State Functions!

Why does heat transferred at constant volume equal the change in internal energy?

• At constant volume, ΔV = 0, so PV work w = 0.

• From ΔU = q + w, therefore ΔU = qᵥ. Since ΔU is a state function, heat transferred at constant volume equals the change in internal energy.

what is enthalpy defined as

H = U + PV

What is the relationship between enthalpy change and heat at constant pressure?

At constant pressure, the heat transferred equals the change in enthalpy: ΔH = qₚ.

How are q_p and q_v related?

qₚ = qᵥ + RTΔn₍gas₎.

This accounts for the change in the number of gas moles.

What does Δn₍gas₎ mean?

Δn₍gas₎ = gaseous product moles − gaseous reactant moles.

When are qₚ and qᵥ equal?

when Δn₍gas₎ = 0

What are the standard states for gases, pure solids, pure liquids, and solutions?

• Gas: pure gas at exactly 100 kPa.

• Solid or liquid: pure substance in its most stable form at 100 kPa and the temperature of interest.

• Solution: concentration of 1 M.

• The temperature is usually 25°C, unless otherwise specified.

What is standard reaction enthalpy (ΔᵣH°)?

The enthalpy change when a reaction occurs with all reactants and products in their standard states.

The symbol ° indicates standard-state conditions.

What are the two main types of energy?

• Kinetic energy: energy of motion.

• Potential energy: stored energy due to position or interactions.

What is the difference between heat and work?

• Heat is energy transferred because of a temperature difference.

• Work is energy transferred when a force acts through a distance or another process occurs against a force.

What is the standard enthalpy of formation of an element in its standard state?

Zero