Chemical Thermodynamics

Question 1

What is the first law of thermodynamics?

Answer 1

The amount of energy in the universe is constant. Energy can only be converted from one form to another; it cannot be created or destroyed

Question 2

What is the unit of energy?

Answer 2

The joule (J)

Question 3

What is 1J also equivalent to?

Answer 3

1 kg m^2 s^-2

Question 4

What is a calorie?

Answer 4

A unit of energy equivalent to the heat energy required to raise the temperature of 1g of water by 1 degree C

Question 5

What is universe generally equal to in thermodynamics?

Answer 5

System + Surroundings = Universe

Question 6

What is system in thermodynamics?

Answer 6

System = the molecules that are to take part in the reaction, and the products formed from them

Question 7

What is surroundings in the thermodynamics?

Answer 7

The rest of the universe

Question 8

What is change in enthalpy equal to?

Answer 8

Change in enthalpy = Enthalpy of products - Enthalpy of reactants

Question 9

If change in enthalpy is positive, the reaction is…

Answer 9

Endothermic

Question 10

If change in enthalpy is negative, the reaction is…

Answer 10

Exothermic

Question 11

What is the definition of enthalpy of combustion?

Answer 11

Enthalpy change when 1 mole of a compound is burned completely in oxygen

Question 12

What is the definition of standard enthalpy of formation?

Answer 12

Enthalpy change when 1 mole of a substance is formed from its constituent elements

Question 13

What does Hess’s Law state?

Answer 13

That enthalpy change involved in going from reactants to products in a given state is the same regardless of the path taken

Question 14

What is entropy a measure of?

Answer 14

Entropy is a measure of disorder

Question 15

How does entropy change through the states?

Answer 15

Most ordered = Solid < Liquid < Solution < Gas = Least ordered

Question 16

What is C.I entropy of universe equal to?

Answer 16

C.I entropy of universe = C.I entropy of system + C.I entropy of surroundings

Question 17

How is Gibbs free energy related to entropy and enthalpy?

Answer 17

Gibbs = C.I enthalpy - (T x C.I entropy)

Question 18

At what values of Gibbs energy are reactions thermodynamically feasible?

Answer 18

A reaction will proceed if the system can lose free energy, I.e if C.I Gibbs free energy is negative

Question 19

What does endergonic mean in terms of Gibbs?

Answer 19

Positive Gibbs energy = unfavourable

Question 20

What does exergonic mean in terms of Gibbs?

Answer 20

Negative Gibbs energy = favourable

Question 21

How can you work out the temperature at which a reaction is thermodynamically feasible using Gibbs energy?

Answer 21

If Gibbs energy < 0, the reaction is thermodynamically feasible. Therefore you can rearrange the Gibbs energy equation to make temperature the subject.
Temperature = C.I Enthalpy / C.I Entropy

Question 22

What is reaction quotient?

Answer 22

The reaction quotient (Q) measures the relative amounts of products and reactants present during a reaction at a particular point in time

Question 23

What is the formula for reaction quotient (Q)?

Answer 23

Reaction quotient = [Products]^(no. moles of each) / [Reactants]^(no. moles of each)

Question 24

How are gases represented in the reaction quotient?

Answer 24

p[X]

Question 25

How are solutions represented in the reaction quotient?

Answer 25

[X]

Question 26

How are solids represented in the reaction quotient?

Answer 26

For solids, their concentration is 1, so they are not included in the reaction

Question 27

What is the reaction relating Gibbs and equilibrium constant, and what does each symbol represent and what are the units?

Answer 27

Gibbs = -RTln(K)
R = gas constant = 8.314 (J mol^-1 K^-1)
T = temp (K)
K = equilibrium constant (J mol^-1)

Question 28

How is the Gibbs-equilibrium constant rearranged to make equilibrium constant the subject?

Answer 28

K = e^(Gibbs/RT)

Question 29

What is k equal to?

Answer 29

[Products]^(no. moles) / [Reactants]^(no. moles)
(at equilibrium)

Question 30

What does a large equilibrium constant indicate?

Answer 30

The reaction will favour the forward direction

Question 31

What does a small equilibrium constant indicate?

Answer 31

The reaction will favour the backwards direction