Thermodynamics: introduction

Question 1

Define a system

Answer 1

The system is what we are interested in, ie the reaction vessel

Question 2

Define the surroundings

Answer 2

The surroundings describes all space that is not the system in question

Question 3

Draw and describe an open system

Answer 3

Matter and energy can be exchanged with the surroundings. Eg an open beaker left to evaporate

Question 4

Draw and describe a closed system

Answer 4

Only energy can be exchanged with the surroundings, eg a reaction in a sealed tube

Question 5

Draw and describe an isolated system

Answer 5

Neither energy nor matter can be exchanged with the surroundings eg hot liquid in a thermos

Question 6

Describe Temperature, T

Answer 6

Measured in Kelvin. 0°C = 273.15K

Question 7

Describe Volume, V

Answer 7

The SI unit for volume is m³

Question 8

Describe pressure, P

Answer 8

Force, F (N) per area, A (m²)

The SI unit is Pa but there are many other units of pressure

Question 9

Define one bar

Answer 9

1 bar = 10^₅ Pa

Question 10

Define one atm

Answer 10

1 atm = 101.325 kPa

Question 11

Define the standard pressure in thermodynamics

Answer 11

1 bar (10⁵ Pa) is defined as the standard pressure, p⁰

Question 12

Give the equation linking amount of substance to mass

Answer 12

Question 13

Give the equation linking mole fraction and number of moles

Answer 13

Question 14

Describe Avogadro’s principle

Answer 14

For a system in which temperature and volume are fixed, pressure increases as gas is fed into the system.

Question 15

Describe Boyle’s law

Answer 15

For a system where n and T are fixed, volume and pressure are inversely proportional. ie pV = constant

Question 16

Describe Guy-Lussac’s law

Answer 16

For a system in which n and V are fixed, pressure increases as temperature increases

Question 17

Describe how to derive the ideal gas law.

Answer 17

By combining Avogadro’s principle (p ∝ n), Boyle’s law (p ∝ 1/v) and Guy-Lussacs law (p∝T) we can get the ideal gas law:

Question 18

Give the value for R, the gas constant

Answer 18

Question 19

Describe an ideal gas

Answer 19

1. The volume of the gas molecules = 0 (negligible)
2. There are no interactions between molecules
3. The gas does not become a liquid or solid upon cooling

Question 20

Define an isothermic process

Answer 20

One which takes place at constant temperature

Question 21

Define an isochoric process

Answer 21

One which takes place at constant volume

Question 22

Define an isobaric process

Answer 22

One which takes place at constant pressure

Question 23

Describe Dalton’s law of partial pressures

Answer 23

If we have a mixture of gases A and B, their partial pressures can be worked out.

total p = p_A + p_B

Mole fractions can be worked out using partial pressures:

n_A/(n_A + n_B) = p_A/p