Thermodynamics (Section C)

Question 1

What is the specific heat capacity?

Answer 1

The quantity of energy required to raise the temperature of a unit mass of a substance by 1K

Cs = q/m x deltaT
J g^-1 K^-1

Question 2

The specific heat capacity of liquid water is 4.18J/Kg. Calculate the energy required to heat 1.0mol of water from 25C to 90C?

Answer 2

4.9kJ

Question 3

What is an extensive property?

Answer 3

A property that does not depend on the quantity of the substance (e.g. specific and molar heat capacities)

Question 4

What is an intensive property?

Answer 4

A property that depends on the quantity of substance present (e.g. volume)

Question 5

What is a closed system?

Answer 5

There is exchange of energy between the system and surroundings but there is not exchange of matter between the system and surroundings

Question 6

What is an open system?

Answer 6

Energy and matter can be exchanged between the system and its surroundings

Question 7

What is an isolated system?

Answer 7

Neither energy or matter can be exchanged between the system and its surroundings

Question 8

What is heat?

Answer 8

The change in energy transferred as a result of temperature difference

Question 9

What is work?

Answer 9

Work involves the energy exchange as a result of motion against opposing force

Question 10

Define the standard enthalpy of fusion.

Answer 10

the energy required to melt on mole of a pure substance at its melting point at 1 bar pressure

Question 11

Define the standard enthalpy of vaporisation

Answer 11

The enthalpy required to vaporise one mole of a pure liquid, at its boiling point at 1 bar pressure

Question 12

Why is the standard enthalpy of vaporisation greater than the standard enthalpy of fusion for a specific substance?

Answer 12

More energy is needed to completely separate the molecules and overcome the intermolecular attractions holding the liquid together than in melting a solid to a liquid

Question 13

Why do liquids, such as water and ethanol have relatively high boiling points?

Answer 13

Liquids such as ethanol and water form hydrogen bonds. Vaporising the liquids, requires enough energy to overcome all the forces between the molecules and hydrogen bonds are the strongest intermolecular forces in liquids

Question 14

Calculate the enthalpy change when 1.00mol of ice at 0C is turned into steam at 100C.
Take the average molar heat capacity of liquid water to be 75.1J/Kmol
and then enthalpy of fusion of water to be +6.01kJ/mol
and then enthalpy of vaporisation of water to be +40.7kJ/mol

Answer 14

54.2kJ/mol

Question 15

What is enthalpy change?

Answer 15

It is the heat transferred by a chemical reaction or process at a constant pressure

Question 16

What is the enthalpy of sublimation

Answer 16

enthalpy of sublimation = enthalpy of fusion + enthalpy of vaporisation

Question 17

How do you convert degrees celsius to kelvin?

Answer 17

+273

Question 18

The heat capacity of air at room temperature (20C) is 21J/Kmol. How much heat is required to raise the temperature of a 5m x 5m x 3m room by 10C?

Answer 18

656kJ

Question 19

what volume of space does 1 mol of air occupy?

Answer 19

24dm^3

Question 20

What is power?

Answer 20

the rate at which energy if transferred

Question 21

What is a state function?

Answer 21

A property of a system that is path independent

Question 22

What is a path function?

Answer 22

A property of a system that depends on the path

Question 23

What is 1cal and 1Cal in Joules

Answer 23

1cal = 4.2J
1Cal = 4.2kJ

Question 24

What is the equation for the change in enthalpy?

Answer 24

deltaU + PdeltaV

Question 25

What are the corresponding wavelengths for ROYGBV?

Answer 25

R - 700nm
O - 620nm
Y - 550nm
G - 500nm
B/V - 450nm

Question 26

Why would a molecule give no UV-vis spectrum?

Answer 26

no multiple bonds
no aromatic rings
no O, N or S containing groups
- it is probably FULLY SATURATED CARBON

Question 27

Which hydrocarbons are colourless?

Answer 27

small unsaturated hydrocarbons and aromatics (e.g. ethylene, benzene) because their electronic absorptions are entirely in the UV range

Question 28

Strongest UV bands are given rise by

Answer 28

pi-pi* transitions of aromatics and unsaturated hydrocarbons (-c=c-)

UV - not coloured

Question 29

Molecules containing O, N, S, Cl etc contain non-bonded electrons on the hetero atom : these give rise to

Answer 29

Molecules containing O, N, S, Cl etc contain non-bonded electrons on the hetero atom : these give rise to n- π* transitions in the 200-400 nm range. Here the UV-vis absorption is usually ~10-100 times weaker than for π-π* transitions.

Question 30

How doe UV-vis spectrum arise

Answer 30

• Electrons orbit around nuclei, just like in atoms
• Electron energies are quantized : different types of molecular orbital - σ, σ, π, π : made by overlapping atomic
orbitals (also non-bonding, n)
• Electron jumps occur between molecular orbitals to give rise to UV-visible absorption bands
• The positions and relative intensity of the bands gives important information on the type of bonding; also the
nature of the atoms involved : C=O ; C=C ; aromatic rings ; presence of S, P ; transition metal ions , etc …..
• Light absorbing species are called chromophores

Question 31

What is red shift?

Answer 31

Red shift is when the pi-pi* transition shift to longer wavelengths as the chain of the polyunsaturated hydrocarbon increases

Question 32

What is the absorbance of carotene?

Answer 32

• polysunsaturated hydrocarbon
• 11 -C=C- bonds
• absorbance at 400-500nm
• the pi-p* orbitals interact along the -C=C- chain