Final Review Session

Give an example based on molecule–molecule interactions excluding chemical reactions, illustrating how the total pressure upon mixing two real gases could be different from the sum of the partial pressures.

Can temperature be measured directly? Discuss.

• Yes

• No

• Yes, but only in certain cases

• It depends on the definition of temperature

Answer explanation

No, at least not directly, we measure temperature by its resultant heat flow

The location of the boundary between the system and the surroundings is a choice that must be made by the thermodynamicist. Consider a beaker of boiling water in an airtight room. Is the system open or closed if you place the boundary just outside the liquid water? Is the system open or closed if you place the boundary just inside the walls of the room?

• Open/open

• Open/closed

• Closed/open

• Closed/closed

A process involving an ideal gas and in which the temperature changes at constant volume is carried out. For a fixed value of ΔT, the mass of the gas is doubled. The process is repeated with the same initial mass and ΔT is doubled. For which of these processes is ΔS greater? Why?

• Double mass

• Double ΔT

• ΔS is the same for both

• Not enough information is provided

Answer explanation

Two ideal gas systems undergo reversible expansion under different conditions starting from the same P and V. At the end of the expansion, the two systems have the same volume. The pressure in the system that has undergone adiabatic expansion is lower than in the system that has undergone isothermal expansion. Explain this result without using equations.

Answer explanation

All energy transfer in the adiabatic process comes from work (which causes pressure-decreasing volume change) while isothermal process has heat to balance against work.

Is the following statement correct? If not
rewrite it so that it is correct. Because the reaction
H2(g) + O2(g) ¡ H2O(l) is exothermic, the products are at a higher temperature than the reactants. Discuss.

• Correct

• Not correct

Answer explanation

The products release energy via enthalpy, but that is not necessarily converted to temperature. It is only true if the process is adiabatic, so the enthalpy can't escape

What is wrong with the following statement? Burns caused by steam at 100°C can be more severe than those caused by water at 100°C because steam contains more heat than water. Rewrite the sentence to convey the same information in a correct way.

Answer explanation

even at the same temperature, steam contains more energy (through enthalpy) than water

Why is q_v = ΔU only for a constant volume process? Is this formula valid if work other than P–V work is possible?

Answer explanation

w_v = 0

it would not be valid, as ΔU = q + w for all work

Why is it valid to add the enthalpies of any sequence of reactions to obtain the enthalpy of the reaction that is the sum of the individual reactions?

Answer explanation

Enthalpy being a state function, so you only need to look at the change between final and initial states

An ideal gas is expanded reversibly and isothermally. Decide which of q, w, ΔU, and ΔH are positive, negative, or zero.

Answer explanation

q: positive

w: negative

ΔU: 0

ΔH: 0