Gen 8 - Enthalpy & Thermochemistry

Question 1

What is thermochemistry ?

Answer 1

The study of energy absorbed or released in chemical reactions or any physical transformations (ie. phase change)

Question 2

What equation represents the enthalpy change of a reaction ?

Answer 2

Question 3

What does a positive enthalpy change (delta H⁺) represent ?

Answer 3

The flow of heat into a system as a reaction occurs - E.g. An Endothermic reaction

Question 4

What does a negative enthalpy change represent ?

Answer 4

Heat energy is given off to its surroundings - an exothermic reaction.

Question 5

What is the law of conservation of energy ?

Answer 5

The total energy of an isolated system remains constant over time (if there is no associated external force)

Question 6

What is the second law of thermodynamics ?

Answer 6

The entropy or randomness of the universe increases in any real system.

For a reaction to proceed, the entropy of the system must increase.

Question 7

What is the difference between enthalpy and entropy ?

Answer 7

Enthalpy is the heat transfer that takes place at constant pressure.

Entropy gives an idea of the randomness of a system.

Question 8

What is the first law of thermodynamics ?

Answer 8

Energy can neither be created nor destroyed.

Question 9

What is delta H_f ?

Answer 9

The standard enthalpy (or heat) of formation is the change of enthalpy that would occur when 1 mole of a substance is formed from its elemental constituents, in a standard state reaction.

Question 10

What is the standard enthalpy of formation of an element in its standard state ?

Answer 10

Zero, as no enthalpy change is involved in their formation.

Question 11

What are the SI units for enthalpy ?

Answer 11

KJ/mole

Question 12

What is delta H_rxn ?

Answer 12

The standard enthalpy change for a reaction - the change of enthalpy that would occur if one mole of matter is transformed by a chemical reaction with all reactants and products under standard state.

Question 13

What is Hess’s law ?

Answer 13

Question 14

Why is it important to define standard conditions in terms of Thermochemistry ?

Answer 14

Because enthalpy changes depend on the conditions under which the reactions are carried out.

Question 15

What is standard pressure ?

Answer 15

1 atmosphere, approx 1 barr

Question 16

What is standard temperature with regards to thermochemistry ?

Answer 16

Generally 25 ºC if unspecified

Question 17

What is the standard physical state of an element ?

Answer 17

The “natural” physical state of an element under standard temperature and pressure

Question 18

What is the standard temperature for an ideal gas ?

Answer 18

0ºC

Question 19

What is the bond dissociation energy ?

Answer 19

A measure of the bond strength within a particular molecule - the standard enthalpy change in the homolytic cleavage of a chemical bond.

Question 20

Why are bond energies always positive values ?

Answer 20

Because it always takes energy to break bonds apart.

Question 21

Describe bond dissociation energy and bond energy in relation to CH₄ - why are they not the same ?

Answer 21

The bond dissociation energies for each of the homolytic C-H bonds cleavage are all slightly different. The average of the four bond dissociation energies is known as the bond energy.

Question 22

When is bond dissociation energy and bond energy the same ?

Answer 22

In all diatomic molecules because there is cleavage of only one bond.

Question 23

Define bond energy

Answer 23

A measure of bond strength. The energy required to sever 1 mole of a chemical bond in a gas.

Question 24

How would you calculate Energy transferred (Q) in a thermochemical reaction ?

Answer 24

Question 25

When is heat absorbed or released but without a change in temperature of the surroundings ?

Answer 25

During a phase change

Question 26

What is a calorie ?

Answer 26

The amount of thermal energy required to raise the temperature of 1g of water by 1ºC

Question 27

What is a calorie in joules ?

Answer 27

1 cal = 4.184 Joules

Question 28

Describe the 3 enthalpy phase changes ?

Answer 28

1. Heat of fusion Solid --\> Liquid 2. Heat of vaporization Liquid --\> Gas 3. Heat of sublimation Solid --\> Gas

Question 29

How do we calculate the heat required for a phase change ?

Answer 29

Q = ML Where: Q = heat transferred to system M = mass of sample L = Heat of transformation

Question 30

What is Calorimetry ?

Answer 30

The science of measuring the heat evolved or exchanged due to a chemical reaction.

Question 31

What are the most spontaneous transformation with regards to the 2nd law of thermodynamics ?

Answer 31

Transformations which require the smallest amount of energy and lead to the largest disorder of the system are the most spontaneous.

Question 32

What is entropy (S) ?

Answer 32

A state function which measures the degree of disorder in a system. Generally speaking, it increases from a solid to a liquid to a gas.

Question 33

Describe entropy in terms of a rubber band

Answer 33

Entropy decreases when the elastic band is stretched. In the relaxed elastic band, the molecules of the rubber polymer are coiled up and form a disorganised structure. As the rubber is stretched, the molecules tend to line up with each other and adopt a more organised structure.

Question 34

What are the units of entropy ?

Answer 34

J/K or Cal/K

Question 35

What is the law of disorder ?

Answer 35

The natural tendancy is for systems to move to the direction of maximum disorder, not vice versa. E.g. Your room never cleans itself !

Question 36

What is the equation for entropy change ?

Answer 36

Question 37

What is Gibbs free energy ?

Answer 37

A state function of entropy that can be used as a criterion for spontaneity.

Question 38

How do you calculate Gibbs free energy ?

Answer 38

Question 39

What are the unit of Gibb's free energy ?

Answer 39

KJMol^-1

Question 40

How does the value of delta G carried out at constant pressure, determine spontaneity of a reaction ?

Answer 40

A reaction carried out at constant pressure is spontaneous is delta G \<0. It is not spontaneous if delta G \>0.

Question 41

How is a reaction proceeding if delta G = 0 ?

Answer 41

The reaction is at equilibrium.

Question 42

Describe the best time to add milk to a hot drink in order to maximally cool it ?

Answer 42

Heat moves to an environment fastest when there is the greatest temperature difference, therefore for maximal heat loss, milk should be added just before drinking.

Question 43

What is the difference between a burn from steam at 100ºC, and a burn from water at 100ºC ?

Answer 43

The steam burn would be more severe as it has more heat energy to be released onto the skin. Energy is lost to the skin by condensation.