Enthalpy/Entropy/Gibbs Free Energy

Question 1

Is bond “breaking” (or dissociation) an endothermic or exothermic process?

Answer 1

Endothermic

(requires energy)

Question 2

Is bond “making” (or formation) an endothermic or exothermic process?

Answer 2

Exothermic

(produces energy)

Question 3

What is the one letter abbreviation for enthalpy?

Answer 3

Enthalpy = (H)

Question 4

What is the one letter abbreviation for Gibbs Free Energy?

Answer 4

Gibbs Free Energy = (G)

Question 5

What is entropy (S)?

Answer 5

Measure of the dispersal/disorder of matter and energy.

(Entropy NEVER decreases.)

Question 6

Rank solids, liquids and gases from lowest to highest entropy.

Answer 6

Solids < liquids < aq < gases

Gases have the highest entropy because gases are more random, have more arrangements and thus more disorder.

Question 7

What does an +ΔS indicate?

(Remember S is entropy.)

Answer 7

An increase in entropy (more disordered).

Example: liquid → gas

Question 8

What does a -ΔS indicate?

(Remember S is entropy.)

Answer 8

A decrease in entropy (less disordered).

Example: liquid → solid

Question 9

What is the one letter abbreviation for entropy?

Answer 9

Entropy = (S)

Question 10

What is the equation for Gibbs Free Energy (ΔG)?

Answer 10

ΔG = ΔH – TΔS

Question 11

What is Gibbs Free Energy?

Answer 11

A measure of the change in enthalpy and entropy of a reaction which determines spontaneity.

Question 12

[Mnemonic]

ΔG = ΔH – TΔS

Goldfish are (equals sign) Horrible without (minus sign) Tartar Sauce.

(how to remember the Gibbs Free Energy equation)

Answer 12

ΔG = ΔH – TΔS

Goldfish are (equals sign) Horrible without (minus sign) Tartar Sauce.

Question 13

For Gibbs Free Energy, if ΔG is negative (ΔG < 0), is the reaction spontaneous or nonspontaneous?

Answer 13

Spontaneous

Question 14

For Gibbs Free Energy, if ΔG is positive (ΔG > 0), is the reaction spontaneous or nonspontaneous?

Answer 14

Nonspontaneous

Question 15

For Gibbs Free Energy, if ΔG is zero what does that mean?

Answer 15

The system is in a state of equilibrium (ΔH = TΔS).

Question 16

[Mnemonic]

My negative ex-boyfriend was spontaneous.

(how to distinguish if ΔG is spontaneous or nonspontaneous)

Answer 16

(My negative ex-boyfriend was spontaneous.)

If ΔG is negative, the reaction is spontaneous and exergonic.

Question 17

When determining the molecule with the most exothermic standard heat of combustion, what is a factor?

Answer 17

The larger the alkane, the more exothermic (release of heat).

Example: N-butane has a higher standard heat of combustion than ethane.

Question 18

Can the spontaneity of a reaction be determined from enthalpy?

Answer 18

NO!

The spontaneity of a reaction can only be determined from Gibbs Free Energy!

Enthalpy (H) ≠ Spontaneity

Question 19

What is the value for ΔH°_f (heat of formation) of an element in its standard state?

Answer 19

ΔH°_f of an element in its standard state = 0 (arbitrary).

Example: ΔH°_f of H₂ = 0