4. Thermodynamics

Question 1

What is the zeroth law of thermodynamics?

Answer 1

Heat flows from areas of increased temperature to areas of decreased temperature, until thermodynamic equilibrium is achieved.

Question 2

What is the first law of thermodynamics?

Answer 2

Total energy is the universe is constant. This law also states that energy cannot be created or destroyed, only transferred.

In an isolated system, energy must be constant because it cannot be transferred.

Question 3

t or f, temperature is a measure of average kinetic energy.

Answer 3

True, this is exactly temperature.

Question 4

What is ΔH?

Answer 4

ΔH = H (products) - H (reactants)

A measure of the heat of a reaction. Note: bond formation is ΔH < 0, destruction ΔH > 0.

Question 5

t or f, a catalyst does not affect Keq, ΔH, ΔS, or ΔG in any-way.

Answer 5

true, catalysts only decrease activation energy.

Question 6

What are standard conditions? What is STP (standard pressure and temperature?

Answer 6

standard conditions: T = 298K (25C°), P = 1atm, [] = 1M. liquids and solids are assumed to be pure. Values under SC have the ° symbol.

STP is 273K (0C°) and 1atm

Question 7

What is standard heat of formation (ΔH°f) ?

Answer 7

Standard heat of formation is the amount of heat needed to form 1 mole of a compound from its constituent elements in their natural state !!

Question 8

What is the equation for standard heat of formation of a given reaction (ΔH°f) ?

Answer 8

ΔH°f = (sum(n) x ΔH°f, products) - (sum(n) x ΔH°f, reactants)

n = stoichiometric coefficients

Question 9

what is the ΔH°f of O2?

Answer 9

0 because O2 exists in its natural state (diatomic)

Question 10

how does each halogen exist in their natural state?

Answer 10

All diatomic,
F and Cl = gases
Br = liquids
I = solid

Question 11

What is Hess’s Law?

Answer 11

If a reaction occurs in multiple steps, we can sum all of the ΔH°f reactions together to get the overall ΔH°f of the entire process.

Question 12

Hess’s Law, what if

1. a reaction is reversed
2. an equation is multiplied by some coefficient

Answer 12

1. if the rxn reverses, the sign of ΔH°f flips

2. if the rxn is multiplied by n, the ΔH°f is multiplied by n too.

Question 13

fill in the blank: Energy is needed to ___ a bond. Energy is released to ____ a bond.

Answer 13

Energy is needed to break a bond. Energy is released to form a bond.

Question 14

How do we use bond dissociation energy (BDE) to calculate ΔH of a reaction?

Answer 14

ΔHrxn = Σ(BDE bonds broken) - Σ(BDE bonds formed)

note: we account for stoichiometry still. Thus, we multiple by the actual number of bonds

Question 15

What is the second law of thermodynamics?

Answer 15

Nature has a tendency to become increasingly disorganized (random).

The second law states that all processes increase disorder (entropy).

Question 16

Explain the entropy of gases, liquids, solids, and aqueous solutes.

Answer 16

g > aq > liq > solids

Question 17

What is the third law of thermodynamics?

Answer 17

The third law of thermodynamics explains the least thermally energetic state. This is a state of zero-entropy which occurs at absolute zero (K = 0)

0 kelvin = the most highly ordered state possible

Question 18

What is the formula for Gibbs free energy ΔG?

Answer 18

ΔG = ΔH - TΔS

thus negative ΔH and positive ΔS define a spontaneous reaction

Question 19

Explain the role of Temperature in Gibbs free energy when ΔH and ΔS have the same sign.

Answer 19

ΔG = ΔH - TΔS

If ΔH is + and ΔS is +
At high temperatures, the rxn will occur

If ΔH is - and ΔS is -
At low temperatures, the rxn will occur

Question 20

t or f, enthalpy is often given in KJ anc entropy is often given in J.

Answer 20

True - always convert units before doing calculations

Question 21

t or f, activation energy and gibbs free energy change in magnitude for the forward and reverse reaction.

Answer 21

False, gibbs free energy only changes its sign. In contrast, the activation energy will be different depending which direction the reaction goes (unless ΔG = 0)

Question 22

t or f, the internal energy of an object is proportional to its temperature.

Answer 22

true

Question 23

If ΔG = 0, what is true?

Answer 23

the reaction is at equilibrium