The First Law of Thermodynamics

Question 1

What is Thermodynamics

Answer 1

Is the study of the way different types of energy interconver

Question 2

Thermodynamics divides the world into two parts: they System and the Surroundings
What is the difference between the two

Answer 2

System: the part of the world we want to study
Surroundings: the rest of the world outside the system

Question 3

We can only measure energy as

Answer 3

A change in the energy of a system
This can be achieved by observing its transfer to and from the surroundings

Question 4

What is an open system

Answer 4

A system in which both matter and energy can be exchanged with the surroundings
For example: a reaction taking place in a beaker is an open system

Question 5

What is a closed system?

Answer 5

A system that contains fixed amount of matter but allows the exchange of energy
For example: a sealed reaction vessels holds all the content in but heat can be conducted

Question 6

What is an isolated system?

Answer 6

A system where there can be no exchange of energy or matter with the surroundings
For example: a sealed, well-insulated vaccum flask

Question 7

What is energy

Answer 7

Energy can be thought of as the capacity to do work

Question 8

What is work

Answer 8

Work is done when moving against a force

Question 9

What is the equation for Work Done

Answer 9

w = Fs

w = work done (J)
F = Force (N)
s = distance (m)

Question 10

What is Internal Energy (U)

Answer 10

The total capacity of a system to do work

Question 11

What is an ideal gas

Answer 11

There are no forces between the molecules

Question 12

How do internal energy and ideal gases related

Answer 12

• Ideal gases have no intermolecular potential energy
• The internal energy of an ideal gas is purely kinetic and therefore depend only on temperature
• The internal energy of an ideal gas is independent of volume

Question 13

What is the First Law of thermodynamics

Answer 13

Energy can neither be created nor destroyed BUT is just transferred from one form into another

Question 14

What happens to the internal energy of a system if work is doen

Answer 14

Internal energy decreases

Question 15

What happens to the internal energy of a system, if work is done to it

Answer 15

Its internal energy increases

Question 16

How does Work and Heat related to an isolated system

Answer 16

No work can be done on or by an isolated system
No heat flows to or from and isolated system

Question 17

How can the internal energy of a system be changed

Answer 17

By transferring energy to or from the system as heat

Question 18

Describe the movement of heat

Answer 18

Heat is energy which flows from high-temperature objects to low-temperature ones

Question 19

What is thermal equilibrium

Answer 19

Objects at the same temperature are in thermal equilibrium, meaning there is no net heat flow between them
(Zeroth law of thermodynamics)

Question 20

What is Molar Heat Capacity (C) of a substance
What is its units

Answer 20

• is a measure of how much heat must be supplied to raise its temperature by 1K
• Units = J mol⁻¹ K⁻¹

Question 21

What is the equation used to work out the amount of heat supplied (J) relating to the Molar Heat Capacity (J mol⁻¹ K⁻¹)

Answer 21

q = nCΔT

q = amount of heat supplied (J)
n = amount of substance (mol)
C = molar heat capacity ( J mol⁻¹ K⁻¹)
ΔT = change in temp (K)

Question 22

For gases, the value of heat capacity depend on wheather the measurements are carried out under which two conditions

Answer 22

• Constant pressure (Cp)
• Constant Volume (Cv)

Question 23

The heat capacity of a particular substance is related to the number of degrees of freedom possessed by its molecules
How?

Answer 23

As the molecules become more complex there are more degrees of freedom which can absorb the heat
Hence have a higher Heat Capacity (C)

Question 24

A system can do two kinds of work: expansion and non-expansion
What is the difference

Answer 24

Expansion: work involved a change in the volume of the system, such as the work done by a reaction with gaseous products
Non-expansion: work does not involved a change in volume, such as the electrical work done by the reaction in a battery

Question 25

Imagine a gas confined to a cylinder by a disc, where the surroundings exert a constant pressure
If the pressure inside the cylinder (pi) builds up to become greater than that of the surrondings (pex), what will happen

Answer 25

• The disc will be push forward
• Work is done moving the disc through a displacement z

Question 26

What is the equation for Force (F)

Answer 26

Force = pressure x area

Question 27

If the equation for work done it W= Fs
Using the imaginary cylinder from before, how would you work out work done, from this displacement Z
(Hint: Pi = pressure initial, Vi = volume inital, Pex = external pressure)

Answer 27

w = Fz
= Pex x Area x z
= Pex (Vf - Vi)

Question 28

Using the equation from before for working out the Work Done by the imaginary cylinder, how would the work done on the gas by the surrounding be work out?

Answer 28

It has the same value but opposite sign

w = -Pex (Vf - Vi)

Question 29

What is Reversible Isothermal Expansion (An expanded gas becomes a compressed gas)
What is the Work Done equal to

Answer 29

• For an ideal gas expanding reversibly at constant temperature (isothermally)
• The Work done corresponds to the area under a Boyle’s law curve
• This is larger than irreversible

Question 30

How does Reversible Isothermal Expansion link to the second law of Thermodynamics

Answer 30

Second Law: A process must occur reversibly to do the masimum possible work
The Work done for a Reversible Isothermal Expansion is great than irreversible, shown by the blue space under these graphs

Question 31

Reversible Isothermal Expansion can only occur…

Answer 31

For an ideal gas

Question 32

Taking both Work and Heat into account gives a mathematical statement for the first law being:

Answer 32

ΔU = q + w

ΔU = internal energy
q = Heat (J)
w = Work done

Question 33

For ΔU = q + w
w is postive when … and negative when…

Answer 33

w is positive if work is done on the system
and negative if work is done by the system

Question 34

The internal energy of an ideal gas is independent of volume, since there are no forces between molecules, and therefore only depends on temperature
If temperature is kep constant (isothermal process) then the internal energy

Answer 34

Is constant

IF ΔT = 0 THEN ΔU = 0

Question 35

For a system at constant volume, there is no expansion work. Therefore heat released/absorbed by the system contributes only to the internal energy change
This makes internal energy a

Answer 35

state function

Question 36

What is a state function

Answer 36

Are quantities such as pressure, volume and temperature, which depend only on the state of the system
If the system is changed from one state to another, the change in a state function is independent of how the change was brought about

Question 37

What is a path function

Answer 37

Are quantities like work and heat, which depend on the path between states
Path functions depend on how the change was brought about