Chem 1B Final Concepts

When is q positive?

Endothermic, absorbing heat (bond breaking, vaporization, melting, sublimation)

When is q negative?

Exothermic, releasing heat (making bonds, condensation, freezing, deposition)

When an ideal gas is cooled @ constant pressure, how does delta H relate to delta E?

delta H < delta E, because temperature decreases, so does volume

Which has the highest entropy?

g > l > s, no bonds = =low entropy, single bonds = highest entropy, double bonds > triple bonds

When a gas is expanded isothermally and reversibly, the gas performs 300J of work. If the same process had happened isothermally and irreversibly…?

The reversible work is the maximum possible amount of work; if done irreversibly, it’ll perform less than 300J of work.

When does delta S have the largest possible value?

largest possible amount of moles of gas = largest possible value of delta s

If something is added to a reaction, what happens to delta G?

If the reaction shifts, the delta G value will decrease (become more negative) because it is becoming more spontaneous.

Predict the sign of delta S surrounding for the vaporization of H2O at 100 degrees C

Since the system gains heat, the surroundings must lose heat, delta S surroundings < 0

For a given process, the sign of delta S surrounding is always…

The opposite of the sign of the delta S system

An equal number of moles of helium gas and fluorine gas are subjected to the same amount of heat at a constant volume. Which gas changes in temperature by the largest amount?

Helium has a smaller heat capacity because fluorine is a polyatomic gas

How do you write a formation reaction?

Only make 1 mole of substance, reactants are in standard element form (Na, not Na^+), pure elements on the reactant side (Na not NaCl)

For the vaporization of a liquid, delta G will _______ as temperature is increased.

decrease

Which gas will have a higher entropy? A gas at 1 atm or 0.1 atm?

The gas at 0.1 if temperature and moles of gas are constant. Decreasing pressure increases volume, which increases entropy.

If a reaction is isothermal:

delta H = 0, temperature is constant, delta E = 0

Under what conditions will the following reaction be spontaneous? N2O4(g) → 2NO2(g)

moles of gas increase, delta H > 0 (must add heat to break bond), for this process to be spontaneous, T must be at high temperatures

a spontaneous process ________

will continue on its own once begun

Consider the following: N2 (g) + 3 H2 (g) 2 NH3 (g) ∆Go = –33 kJ at 298 K, predict the sign of ∆Ho

∆Ho < 0

If a sample of He gas is cooled at a constant pressure, it will release ______ it would if the sample were cooled at a constant volume.

more heat than

When volume decreases, pressure…

increases

When temperature rises, volume …

increases

When temperature rises, pressure ….

increases

When delta G is negative, the reaction will shift …

right

When delta G is positive, the reaction will shift

left

What will make the best buffer?

[HA] = [A-], and larger concentrations of HA and A-

When volume increases, the reaction will shift

to the side with more moles of gas

An oxidizing agent:

gets reduced (gains electrons)

A reducing agent:

gets oxidized: loses electrons

What rxn happens in the anode?

oxidation half reaction, loses electrons

What rxn happens in the cathode?

reduction half rxn, gains electrons

When volume decreases, the reaction will shift

to the side with less moles of gas

When ammonium chloride is dissolved in water (both initially at the same temperature), the resulting solution decreases in temperature. What is the sign of delta H for the dissolution reaction?

positive