Thermodynamics

Question 1

What is bond dissociation energy (ΔEd)?

Answer 1

The energy required to break a bond

Question 2

What is bond enthalpy (ΔHB)?

Answer 2

The change in heat accompanying the dissociation of a bond (measured at constant P)

Question 3

Equation for bond enthalpy

Answer 3

Question 4

Difference in ΔHB and ΔE for gases and liquids/solids

Answer 4

• For gases, ΔHB and ΔE differ by 1-2 %.
• For liquids and solids, negligible difference.

Question 5

ΔHB is always ___

Answer 5

Positive

Question 6

Why is ΔHB always positive?

Answer 6

Because it always takes energy (heat) to break a bond

Question 7

Bond breaking is always ___

Answer 7

Endothermic

heat is added, ΔHB > 0

Question 8

For an endothermic reaction, heat is added, ΔHB __ 0

Answer 8

ΔHB > 0

Question 9

Bond formation is always ___

Answer 9

Exothermic

heat is released, ΔHB < 0

Question 10

For an exothermic reaction, heat is added, ΔHB __ 0

Answer 10

ΔHB < 0

Question 11

Symbol for standard bond enthalpy

Answer 11

ΔHB°

Question 12

What does standard bond enthalpy indicate?

Answer 12

Indicates that reactants are products are in the standard state (pure form) at 1 atm, 25°C

Question 13

What is the importance of bond enthalpies?

Answer 13

The difference between bond enthalpies in products and reactants gives an estimate of the enthalpy of reaction (∆Hr°)

Question 14

Enthalpies of reaction

Answer 14

Question 15

The difference between the bond enthalpies in products and reactants gives an estimate of the ___

Answer 15

enthalpy of reaction (∆Hr°)

Question 16

Calculation of reaction enthalpies (ΔHr°)

Answer 16

Question 17

If bonds are stronger in the products than in the reactants, ΔH is

a) negative (exothermic)
b) positive (exothermic)
c) negative (endothermic)
d) positive (endothermic)

Answer 17

a) negative (exothermic)

Question 18

If bonds are stronger in the reactants, ΔHr° is ___

Answer 18

Positive (endothermic)

Question 19

Calculating bond enthalpies (ΔHB°)

Answer 19

Question 20

Calculating ΔHr° from Standard Heat of Formation (ΔHf°)

Answer 20

ΔHr° = ΔHf°

For a reaction forming 1 mol of compound from pure elements in their most stable form at standard state (P = 1 bar and T = 298K)

Question 21

Hess’s Law

Answer 21

• Enthalpy is a ‘state function’, ∆H is independent of path
• If two or more chemical reactions are added to give another chemical reaction, corresponding enthalpies ∆Hr° can be added

Question 22

Hess’s Law example

Answer 22

Solution

Question 23

Rules for Hess’s Law

Answer 23

1) When you multiply a reaction by a coefficient, the ∆H˚also multiplies by that coefficient
2) When you flip the direction of a reaction, the sign of the ∆H˚flips too
3) Cancel out the elements (or compounds) that are not in the final balanced equation (these are called intermediates)

Question 24

What is entropy (S)

Answer 24

• The measure of the disorder of a system
• There is a tendency in nature to proceed in a direction that increases the randomness of a system

Question 25

ΔS =

Answer 25

Change in entropy

Question 26

What does S stand for?

Answer 26

Entropy

Question 27

If ΔS is positive, this means there is a ___ in disorder

Answer 27

Increase

Question 28

If ΔS is negative, this means there is a ___ in disorder

Answer 28

Decrease

Question 29

S (entropy) in solid, liquid, & gas)

Answer 29

Question 30

Entropy for reactions, ∆Sr°

Answer 30

- Calculated from standard entropies (S°) - ∆Sr° = ΣS°(products) - ΣS°(reactants) - Where S° (absolute standard entropy) represents the increase in entropy that occurs when a substance is heated from 0K to 298K at 1atm For most cases, S > 0 (with disorder) S has an absolute zero – the perfect crystal at T = 0 K (no disorder)

Question 31

Generally, the more complex the molecule, the ___ the standard entropy (ΔS) value

Answer 31

Higher

Question 32

What characterizes a spontaneous reaction?

Answer 32

It proceeds in the forward direction wihtout the need for outside intervention

Question 33

Spontaneity depends on ___

Answer 33

Gibbs free energy ΔG

Question 34

Gibbs Free Energy

Answer 34

Question 35

Free energy of formation, ∆Gf°

Answer 35

- Analogous to the standard heat of formation (ΔHf°) - ∆Gf° ≡ standard Gibbs free energy of formation ≡ ∆Gf° for formation of 1 mol of compound from its elements in their most stable form in the standard states, 1atm and 25°C

Question 36

Equation for ∆Gf°

Answer 36

Question 37

What is ∆Gf° a measure of?

Answer 37

A compound's stability relative to its elements

Question 38

If ∆Gf° < 0, a compound is thermodynamically ___ relative to its elements

Answer 38

Stable

Question 39

If ∆Gf° > 0, a compound is thermodynamically ___ relative to its elements

Answer 39

Unstable

Question 40

What two equaitons can you use to calculate free energy for reactions, ∆Gf°?

Answer 40

Question 41

Summary of effect of temperature on spontaneity

Answer 41

Question 42

What is the enthalpy of formation?

Answer 42

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.

Question 43

A reaction is spontaneous when ΔG is ___

Answer 43

Less than 0