Section C- Thermodynamics

Question 1

What 3 types of chemical systems are there and what exchanges with the surroundings do each allow?

Answer 1

Isolated system- no exchange of matter or energy. Open system- exchange of both matter and energy. Closed system- exchange of energy but a fixed amount of matter.

Question 2

What is heat?

Answer 2

Energy transferred as a result of temperature difference. The transfer of heat energy is a random process.

Question 3

What is temperature?

Answer 3

The distribution of thermal energy and a measure of the average energy in a system.

Question 4

Define specific heat capacity.

Answer 4

The enthalpy change in Joules when 1kg of substance is heated by 1 degree Celsius. (or 1g by 1K)

Question 5

What is work (w)?

Answer 5

Energy exchange as a result of motion against an opposing force. It is a coordinated motion of molecules.

Question 6

What is a state function?

Answer 6

A function whose value only depends on the beginning and end points of a process and not on the path taken e.g. altitude but not distance.

Question 7

What is the molar heat capacity?

Answer 7

The energy required to raise the temperature of 1 mol of substance by 1K.

Question 8

What are extensive and intensive properties?

Answer 8

Extensive- depends on the quantity of the substance that is present. Intensive- do not depend on the quantity.

Question 9

Define enthalpy change (delta H). Is it a state function?

Answer 9

The heat transferred at constant pressure by a chemical reaction or process. Yes it is a state function as it only takes into account the start and end values and not the path taken (e.g. what method is used to transfer heat).

Question 10

Define standard enthalpy change of vapourization.

Answer 10

The enthalpy change when 1 mol of a liquid vapourizes at its boiling point (Tb), and 1 bar pressure.

Question 11

Define standard enthalpy change of fusion.

Answer 11

The enthalpy change when 1 mol of a substance melts at its melting point (Tm), and 1 bar pressure.

Question 12

What is the equation used to calculate the enthalpy change of sublimation?

Answer 12

Enthalpy change of fusion + enthalpy change of vapourization

Question 13

Define the standard enthalpy change of reaction.

Answer 13

The enthalpy change at 1 bar for a reaction with all the components in their standard states. The value obtained refers to the molar amounts given in the equation and are reported at 298K.

Question 14

Define the standard enthalpy change of formation.

Answer 14

The enthalpy change when 1 mol of substance is formed under standard conditions from its constituent elements in their standard states.

Question 15

What is the value of the standard enthalpy change of formation for elements in their standard states?

Answer 15

0

Question 16

What does Hess’s law state?

Answer 16

That the total enthalpy change for a chemical reaction is independent of the path by which the reaction occurs as long as the starting and finishing states are the same for each reaction path.

Question 17

How can the enthalpy change of the reaction be calculated?

Answer 17

(sum of enthalpies of formation of the products) - (sum of the enthalpies of formation of the reactants)

Question 18

Define the standard enthalpy change of combustion.

Answer 18

The enthalpy change when 1 mol of a substance reacts completely with excess oxygen gas at 1 bar pressure.

Question 19

What is the bond dissociation enthalpy? And the mean bond dissociation enthalpy?

Answer 19

The enthalpy change per mole when a particular chemical bond is broken under standard conditions in the gas phase. The mean is the average across a range of related compounds.

Question 20

What information does the sign of the energy change provide?

Answer 20

Positive- energy gained by a system (heat absorbed or work done on the system)

Negative- energy lost by a system (heat released or work done by the system)

Question 21

What is the zeroth law of thermodynamics?

Answer 21

Energy flows between objects such as to equalise their temperatures.

Question 22

What is the first law of thermodynamics?

Answer 22

Delta U= q + w

Where q is the energy transferred as heat, amd w is the energy transferred as work.

Also called the law of conservation of energy :

• Energy can be neither created nor destroyed, only interconverted between forms.
• The total quantity of energy in the universe is constant.
• The internal energy of an isolated system is constant (DeltaU=0)

Question 23

What is the internal energy (U)? Can it be measured?

Answer 23

The sum of the kinetic and potential energies of molecules in a sample. It cannot be measured, only the change in internal energy can.

Question 24

What is the equation used to calculate DeltaU for a process at a constant volume?

Answer 24

DeltaU = q_v

_{(only dependant on the heat transferred)}

Question 25

What equations show the relationship between the enthalpy change and change of internal energy?

Answer 25

DeltaH = DeltaU + pDeltaV

DeltaH = DeltaU + Deltan_gasRT

Question 26

What is the value of the gas constant (R)?

Answer 26

8.314 JK^-1

Question 27

How can DeltaU be measured?

Answer 27

Using a bomb calorimeter which is an insulated device and material is combusted within it, causing a temperature rise associated with a particular physical or chemical change.

Question 28

What is entropy?

Answer 28

Entropy is a measure of the randomness or disorder in a system.

It is related to the number of arrangements that a system can adopt by the Boltzmann formula:

S = k_BlnW

Where k_B is the Boltzmann constant (1.381x10^-23JK^-1), W is the number of ways of arranging the molecules and S is the entropy.

Question 29

What is the second law of thermodynamics?

Answer 29

Spontaneous processes increase the total entropy of the universe.

The total entropy is the entropy of the system plus the entropy of the surroundings.

Question 30

What is the third law of thermodynamics?

Answer 30

The entropy of a perfect crystal at zero kelvin is zero.

Question 31

What is the standard entropy of a substance?

Answer 31

The entropy of 1 mol at 298 K and 1 bar pressure. Determined by adding the entropy changes in heating 1 mol of the substance from 0K to 298K taking into account any phase changes that occur.

Question 32

For a chemical change to be spontanous, what must the entropy change of the Universe be?

Answer 32

Positive.

Question 33

What is an adiabatic system?

Answer 33

One in which energy is only transferred in the form of work.

Question 34

What does the Maxwell-Boltzmann distribution describe?

Answer 34

The speeds of molecules or the occupation of energy levels.

Question 35

What is entropy change?

Answer 35

The ratio of heat transferred to temperature. (q/T)

Question 36

What is the equation to calculate entropy (S)?

Answer 36

S = klnW

Where W is the number of microstates (degrees of freedom)

k is the Boltzmann constant (1.381x10^-23 JK^-1)

Therefore entropy is a measure of how dispersed the energy is.

It is a measure of the quality of the energy.

Question 37

What is G?

What is the equation used to calculate it under constant pressure?

Answer 37

Gibbs free energy- the energy available for useful work.

G = H -TS

Question 38

For a spotaneous process, what will the value of DeltaG be?

Answer 38

Negative.

Question 39

Name some entropy-driven processes.

Answer 39

Mixing and unmixing

Protein folding

Formation of membranes