The First Law Of Thermodynamics

Question 1

What’re the contributions to the energy of matter?

Answer 1

Kinetic and potential energy

Question 2

What is the difference between an intensive and an extensive property?

Answer 2

Extensive property = a property that depends on the amount of substance in a sample.

Intensive property = a property that does NOT depends on the amount of substance in a sample.

Question 3

What does reference state of an element mean?

Answer 3

It is an elements most stable form under the prevailing conditions

Question 4

What is standard state referring to?

Answer 4

Any specified state at 1 bar

Question 5

What is an open system?

Answer 5

Both matter and energy can be exchanged with surroundings

Question 6

What is a closed system?

Answer 6

Energy, but not matter, can be exchanged with its surroundings

Question 7

What is an isolated system?

Answer 7

Neither matter nor energy can be exchanged with its surroundings

Question 8

What is a state function?

Answer 8

A state function is a physical property that depends on the present state of the system and is independent of the path by which that state was reached.

Heat and work are NOT state functions

Question 9

What is the first law of thermodynamics?

Answer 9

That the internal energy of an isolated system is constant

Question 10

Is internal energy a state function?

Answer 10

Yes

Question 11

What equation relates to the first law?

Answer 11

∆U = w + q

Question 12

What equation gives the work done when a system expands against an external opposing pressure?

Answer 12

W = -Pex ∆V

∆V because it is ∆Area x ∆distance = ∆Volume

Question 13

What is free expansion?

Answer 13

It is expansion against zero external pressure,

Pex = 0

Question 14

What does reversible mean in thermodynamics?

Answer 14

It means the system can be reversed by an infinitesimal change to a variable

Question 15

What can be said about the expansion work done in a system which is in mechanical equilibrium with its surroundings?

Answer 15

It can be concluded that at all stages of the expansion, maximum expansion work is done

Question 16

How do you derive the equation for the work done for a reversible isothermal expansion of an ideal gas?

Answer 16

W = -nRT ln (Vf/Vi)

Question 17

What is ∆U for an ideal gas?

Answer 17

∆U = 0

Question 18

How do work done and heat relate to each other for the isothermal expansion of an ideal gas?

Answer 18

q = -w

Question 19

How do work done and heat relate to each other for the isothermal reversible expansion of an ideal gas?

Answer 19

Since q = -w

q = nRT ln (Vf/Vi)

Question 20

When Vf > Vi in the isothermal reversible expansion of an ideal gas, what can be said about q?

Answer 20

If Vf > Vi, then ln Vf/Vi is positive and hence q > 0 as expected; heat flows into the system to make up for the energy lost as work.

The greater the ratio of the final and initial volumes, the greater the influx of energy as heat.

Question 21

What is heat capacity by definition?

Answer 21

C = ∆q / ∆T

Or more generally, C = dq / dT

Question 22

If heat capacity, C, is constant, what does q = ?

Answer 22

q = C∆T

Question 23

What is Cs, Cm, Cp and Cv when talking about heat capacity?

Answer 23

Specific heat capacity, C / mass(g)

Molar heat capacity = C / n(mol)

Heat capacity at constant pressure

Heat capacity at constant volume

Question 24

Is heat capacity intensive or extensive property of a system?

Answer 24

It is an extensive property which means that it depends on the amount of substance in the sample.

Question 25

What is enthalpy by definition?

Answer 25

∆H = ∆U + ∆(p+v)

This applies to any system, not just ideal gases

Question 26

What can be said about the change in internal energy that is free to expand or contract and energy supplied as heat to a system?

Answer 26

The change in internal energy of a system that is free to expand or contract IS NOT EQUAL TO the energy supplied as heat.

Question 27

How do you find the enthalpy when considering an ideal gas?

Answer 27

∆Hm = ∆Um + R∆T

Sub pV = nRT into,
∆H = ∆U + ∆(pv)

Question 28

How do you find enthalpy at constant pressure?

Answer 28

∆H = ∆U + P∆V

Question 29

Is enthalpy a state function?

Answer 29

Yes

Question 30

At constant pressure, no nonexpansion work, what does ∆H = ? WHY?

Answer 30

∆H = q

1. ∆H = ∆U + p∆V = ∆U + Pex ∆V

∆U = w + q
W = -Pex ∆V
These expressions are subbed into eq. 1

To give:
∆H = (w + q) + Pex ∆V
∆H = (-Pex ∆V + q) + Pex ∆V = q

Question 31

At constant volume, no nonexpansion work, what does ∆U = ? WHY?

Answer 31

∆U = qv

∆U = w + q
∆U = -Pex ∆V + q

As there is no change in volume, ∆V = 0 so work done = 0.

Hence ∆U = qv

Question 32

What is a bomb calorimeter an example of?

Answer 32

A constant-volume calorimeter

Question 33

How can temperature dependent heat capacity be calculated with change in internal energy and change in temperature?

Answer 33

Well, C = dq / dT
And dU = qv at constant volume, no non-expansion work

Hence Cv = dU / dT
This is hence heat capacity is temperature dependent

Question 34

What equation gives heat capacity when it is independent of temperature?

Answer 34

Cv = ∆U / ∆T

Question 35

What can you find using a bomb calorimeter at constant pressure?

Answer 35

It tells you how the enthalpy of a system changes as its temperature is raised at constant pressure.

Question 36

How do you find temperature dependent heat capacity at constant pressure?

Answer 36

Well, C = dq / dT
And dH = qp at constant pressure

So Cp = dH / dT

Question 37

What is always greater, enthalpy or internal energy?

Answer 37

Enthalpy is always greater, this difference increases with increasing temperature

Question 38

What is heat capacity when independent of temperature at constant pressure?

Answer 38

Cp = ∆H / ∆T

Question 39

What is always greater, Cp or Cv?

Answer 39

Cp > Cv since H = U + pV and Cpm = R + Cvm

Cp is always greater than Cv, because work is done while volume changes, using up some of the energy added. Therefore, more energy must be added to increase temperature by the same amount to make up for the work done

Question 40

How does R relate to Cp and Cv for an ideal gas?

Answer 40