2A The First Law, Internal Energy

Question 1

For purposes of thermodynamics, the universe is divided into two parts…

Answer 1

1) The system

2) Its surroundings

Question 2

What is the key idea of the chapter?

Answer 2

The total energy of an ISOLATED system is constant.

The First Law

Question 3

What are the surroundings made of?

Answer 3

The surroundings comprise the region outside the system and are WHERE WE MAKE OUR MEASUREMENTS.

Question 4

What is an open system?

Answer 4

An OPEN system is a system that has a boundary that permits the transfer of matter between it and its surroundings.
An open system allows energy exchange.

Question 5

What is a closed system?

Answer 5

A CLOSED system is a system that has a boundary that DOES NOT permit the transfer of matter between it and its surroundings.
A closed system allows energy exchange.

Question 6

What is an isolated system?

Answer 6

An ISOLATED system is a CLOSED system that doesn’t have THERMAL nor MECHANICAL contect with its surroundings.
An isolated system DOES NOT allow energy exchange.

Question 7

How can a closed system exchange energy with its surroundings? (2 examples)

Answer 7

1. A closed system can expand and thereby raise a weight in the surroundings - do work.
2. A closed system can transfer energy in form of heat if the surroundings are at lower temperature.

Question 8

What is the definition of work?

Answer 8

Work is done to achieve motion against an opposing force.

Question 9

When does a process do work?

Answer 9

The process does work if in principle can be used to raise a weight somewhere IN THE SURROUNDINGS.
Example: expansion of a gas that pushes out a piston: this can be used to raise a weight.

Question 10

What is the energy of the system in terms of work?

Answer 10

The energy of a system is its capacity to do work.

1. When work is done on an isolated system, the capacity of the system to do work is increased - the energy of the system is increased.
2. When the system does work, the energy of the system is reduced and it can do less work than before.

Question 11

When can it be said that energy has been transfered as heat?

Answer 11

When the energy of the system changes as a result of a temperature difference between the system and its surroundings
Example: hot water can be used to do more work than cold water - its capacity to do work is increased.

Question 12

What are diathermic and adiabatic boundaries?

Answer 12

Diathermic boundary is a boundary that allows the transfer of energy as heat.
Adiabatic boundary DOES NOT permit the transfer of energy as heat.

Question 13

When an endothermic process occurs in an adiabatic system…

Answer 13

…it results in a lowering of temperature of the system.

Question 14

When an endothermic process occurs in a diathermic system…

Answer 14

…it results in energy flowing into the system as heat to restore the temperature to that of the surroundings.
- The system remains at the SAME temperature.
(isothermal process)

Question 15

When an exothermic process occurs in an adiabatic system…

Answer 15

…it results in a rise of temperature of the system.

Question 16

When an exothermic process occurs in a diathermic system…

Answer 16

…it results in energy flowing out of the system as heat into the surroundings.
- The system remains at the SAME temperature.
(isothermal process)

Question 17

What happens when a COMBUSTION reaction takes place in an ADIABATIC container?

Answer 17

The energy released as heat remains inside the container and results in a permanent rise in temperature.

Question 18

What happens when a COMBUSTION reaction takes place in a DIATHERMIC container?

Answer 18

Although the temperature typically rises in the course of the combustion, if we wait long enough, the system returns to the temperature of its surroundings.
So we can speak of a combustion at ‘25°C’.

Question 19

What is thermal motion?

Answer 19

Thermal motion is the disorderly motion of molecules.

Question 20

What is heating in molecular terms?

Answer 20

Heating is the transfer of energy that makes use of disorderly, apparently random, molecular motion IN THE SURROUNDINGS.

Question 21

How does thermal motion of the molecules in the HOT SURROUNDINGS increase the energy of the system?

Answer 21

It stimulates the molecules in a cooler system to move more vigorously and, as a result, the energy of the system is increased.

Question 22

How does thermal motion of the molecules in the HOT SYSTEM increase the energy of the surroundings?

Answer 22

When a system heats its surroundings, molecules of the system stimulate the thermal motion of the molecules of the surroundings - the energy of the surroundings is increased.

Question 23

What is work in molecular terms?

Answer 23

Work is the transfer of energy that makes use of the organized, orderly molecular motion IN THE SURROUNDINGS.

Question 24

When work is done on a system, molecules in the SURROUNDINGS are used to…

Answer 24

…transfer energy to it in an ORGANIZED way.

Question 25

Where the distinction between heat and work is made?

Answer 25

In the surroundings.
Work is identified as energy transfer making use of the organized motion of atoms in the surroundings.
Heat is identified as energy transfer making use of disorderly thermal motion in the surroundings.

Question 26

Can work stimulate thermal motion in the system?

Answer 26

Yes, it can, but it does NOT matter because it makes use of the orderly motion of atoms/molecules in the SURROUNDINGS.

Question 27

What is the internal energy?

Answer 27

The internal energy is the total energy of a system.
It is the sum of kinetic and potential energy of the constituents of the SYSTEM ONLY (atoms, ions, molecules, etc).
It is a STATE function, an EXTENSIVE property.
Measured in J.

Question 28

The change in internal energy between the initial energy and the final energy state:

Answer 28

ΔU=Uf - Ui

Question 29

Is internal energy a STATE function and EXTENSIVE property?

Answer 29

Yes, Internal energy is a STATE FUNCTION.
Changing any of the state variables (e.g. pressure), results in change in internal energy.
Yes, internal energy is an EXTENSIVE property - it depends of the amount of substance present.

Question 30

What is the molar internal energy?

Answer 30

Um=U/n
It is an INTENSIVE property.
Measured in kJ/mol

Question 31

What does the equipartition theorem say about raising the temperature of the system?

Answer 31

For translation and rotational modes of interaction between molecules, the contribution of a mode is proportional to the temperature, so the internal energy of a sample increases as the temperature is raised.

Question 32

Why the internal energy of a perfect gas is independent of the VOLUME it occupies?

Answer 32

Because there are no intermolecular intercations in a perfect gas, so the DISTANCE between the molecules has no effect on the energy.

Question 33

How can the internal energy of a system be changed?

Answer 33

By doing work on the system or by heating it - both ways are equivalent.

Question 34

The formulation of the First Law.

Answer 34

The internal energy of an isolated system is constant.

no change in the internal energy

Question 35

Mathematical statement of the First Law.

Answer 35

ΔU = q + w
q - heat transferred as heat to the system
w - work done on a system

Question 36

What does the mathematical statement of the First Law say about a closed system?

Answer 36

The change in internal energy of a CLOSED system is equal to the energy that passes through its boundary as heat or work.

Question 37

What is expansion work?

Answer 37

Expansion work - the work arising from change in volume.
This type of work includes the work done by a gas as it expands and drives back the atmosphere.
It also involves work associated with negative changes of volume - compression.

Question 38

How to calculate infinitesimal changes in the the internal energy?

Answer 38

dU = dq + dw

Question 39

The general equation for work.

Answer 39

dw = - |F| dz
F - opposing force of magnitude F
dz - the distance

Question 40

Expansion work (equation)

Answer 40

dw = -p(ex) dV

p(ex) - external pressure

Question 41

The external pressure determines the magnitude of work - why?

Answer 41

When a gas is compressed, the ability of the SURROUNDINGS to do work is diminished by the amount determined by the weight that is lowered, and it is the energy that is transferred to the system.

Question 42

An example of a reaction where the external pressure is constant throughout the expansion.

Answer 42

Expansion of a gas formed in a chemical reaction in a container that can expand.