CHEM 3490: Thermodynamics

Thermodynamics is the study of transformation of _________

energy

Examples of thermodynamics:

1. ___________ when a fuel burns

2. _________ when a chemical reaction pumps electrons through a circuit

1. Release of energy to heat surroundings

2. Electrical work

A/n ________ system can exchange both energy and matter between system and surroundings

open

A/n _________ system cannot exchange matter but can exchange energy between system and surroundings

closed

In a/n ________ system, neither matter nor energy can be exchanged between system and surroundings

isolated

A system (or surroundings) does work when it causes _________

motion against an opposing force

Energy is the capacity of the _________ to do work

system

When the energy of the system changes as a result of a temperature difference between the system and the surroundings, we say that energy has been transferred as _______.

heat (q)

Work and heat are both types of energy transfer that make use of the motion in the _________

surroundings

1. Work is energy transfer making use of the _________ in the surroundings

2. Heat is energy transfer making use of _________ in the surroundings.

1. organized motion of atoms

2. thermal motion

The ________ Law of Thermodynamics states that the internal energy (U) of a system is constant unless it is changed by doing work (w) or by heating (q)

First

(ΔU = q + w)

If a system has walls that do not allow for heat transfer, the walls are said to be ________.

adiabatic

Total energy of the system is referred to as _________.

internal energy (U)

The first law of thermodynamics is basically another law of _________

conservation of energy

1. For a ________ function the change in property is independent of the path taken to get there

2. It only depends on the _________

3. Different steps result in ________

1. state

2. initial and final point of the process

3. the same values

True or False?

For a path function, any number of steps results in the same value

False

Different steps result in different values at each step, which is why these are dependent on path taken

True or False?

For a state function, any number of steps results in the same value

True

No matter how many steps you take, or what you did at each step, the end result will be the same

Name the state functions we studied in thermodynamics:

Internal energy (U)

Enthalpy (H)

Entropy (S)

Gibbs free energy (G)

Temperature (T)

Pressure (P)

Volume (V)

1. For a _______ function, the change in property is dependent on the path taken to get there

2. Different steps result in _________

1. path

2. different values

Name the path functions we studied in thermodynamics:

Heat (q)

Work (w)

Heat capacity (C)

1. ________ work is the work associated with expansion and compression

2. It is also called _______ work

1. Mechanical

2. PV

The physical measurement of work is ________

force times the distance (F x d)

In thermodynamics, the physical measurement of work goes from force x distance (F x d) to:

pressure times volume (P x V)

where:

force (F) = pressure (P)

distance (d) = volume (V)

Reversible compression is the minimum work needed for the _________ to do work on the ________.

surroundings to do work on the system

1. In reversible ________, the pressure of the gas in the system (P) is just slightly less than the pressure of the gas in the surroundings (Pext)

2. This means the external pressure (Pext) is ________.

1. compression

2. not constant

Reversible expansion is the maximum work needed for the _________ to do work on the ________.

system to do work on the surroundings

1. In reversible ________, the pressure of the gas in the system (P) is just slightly greater than the pressure of the gas in the surroundings (Pext)

2. This means the external pressure (Pext) is ________.

1. expansion

2. not constant

The sign conventions and associated spontaneity of work (w):

1. work done on the system by the surroundings is ________, indicating that it is a _______ process.

2. work done by the system on the surroundings is _______, indicating that it is a _______ process.

1. positive; nonspontaneous

2. negative; spontaneous

We use the formula: w = −Pext∆V if the external pressure is _________

constant

(this is the formula for irreversible work)

We use the formula w = −nRTln(Vf/Vi) if the external pressure is _______

variable (changing at each step)

(this is the formula for isothermal reversible work)

What is the key condition for a reversible expansion of a gas?

The external pressure (Pext)​ must be infinitesimally less than the internal pressure of the gas (P).

This allows the gas to expand slowly

What is the key condition for a reversible compression of a gas?

The external pressure (Pext) must be infinitesimally greater than the internal pressure of the gas (P).

This forces the gas to compress very gradually

Work during reversible compression is the _______ work to compress a gas

minimum

Work during reversible expansion is the ________ work that can be obtained from the gas

maximum

Internal energy of an ideal gas is dependent only on __________

temperature

________ means that the temperature is kept constant

Isothermal

What is the expression that defines an isothermal process?

ΔU = 0

constant temperature = no change in internal energy

What does the First Law of Thermodynamics become in order to define an isothermal process?

q = -w

Derivation:

First Law: ΔU = q + w

Plug in zero for ΔU:

0 = q + w

Rearrange by moving work to other side:

-w = q

*This means that the heat added to the system equals the negative of the work done by the system.

How can we determine heat (q) in an isothermal process of an ideal gas?

By measuring the work done, since:

q = -w

*you can directly calculate the heat from the work.

If a substance is heated, it temperature typically rises but the change in temperature (ΔT) varies depending on the _________ of the substance.

"heat capacity" (C)

1. Heat capacity for a process held at constant ________ is termed Cv.

2. Heat capacity for a process held at constant ________ is termed Cp.

1. volume

2. pressure

What is the overall expression relating heat (q) to its specific heat capacity (C)?

C = q/ΔT

(from q = CΔT)

What is the specific expression for heat capacity if volume is constant?

Cv = ΔU/ΔT

(from C = q/ΔT)

*since changing volume is required for work, we can sub in 0 for w in the first law and get ΔU = q +0 or ΔU = q. Now we have a valid substitution if heat is not given in a problem with constant volume

At constant ________, no work is done

volume

*volume must be changing for work to have been done since work = -PΔ𝑉

If there is no heat exchange allowed, the system is said to be ________

adiabatic

Give the expression that defines the adiabatic system:

q = 0

What does the First Law of Thermodynamics become in order to define an adiabatic system?

ΔU = w

Derivation:

First Law: ΔU = q + w

Plug in zero for q:

ΔU = 0 + w

Kick off the zero:

ΔU = w

*This means that any change in the internal energy is a result of the work

Most processes take place at constant pressure (not volume), so what new property needs to be defined?

Enthalpy (H)

*so a condition of enthalpy is constant pressure

Enthalpy is given by the expression derived from The First Law:

H = U + PV

First Law: U = q + w

Derive: plug in H for q and -PV for w:

U = H - PV

rearrange:

H = U + PV

Enthalpy change at constant pressure is given by the expression derived from The First Law:

ΔH = ΔU + PΔV

First Law: ΔU = q + w

Derive: plug in ΔH for q and -PΔV for w:

ΔU = ΔH - PΔV

rearrange:

ΔH = ΔU + PΔV

True or False?

At constant pressure, the heat is nothing but the change in enthalpy

True

What is the specific expression for heat capacity if pressure is constant?

Cp = ΔH/ΔT

*since constant pressure means heat (q) is now defined as enthalpy (H) volume is required for work, we can sub in ΔH for q in the expression

For an ideal gas, H = U + PV:

What modification can you make to this expression in a case where you are not given pressure or volume, but given mol and temperature?

H = U + nRT

since PV = nRT

1. An _______ process is a process at constant pressure in which heat is released to the surroundings from the system

2. In this type of process, ΔH is ________.

1. exothermic

2. negative (ΔH < 0)

1. An _________ process is a process at constant pressure in which heat is transferred to the system from the surroundings

2. In this type of process, ΔH is ________

1. endothermic

2. positive (ΔH > 0)

For a chemical (or physical/phase change) reaction:

1. Change in enthalpy (Δ𝑟H) is given by the following expression:

2. This expression says that enthalpy changes for chemical and physical (phase transitions) reactions are _________

3. This expression is known as _________

1. Δ𝑟H = [sum of the enthalpy of products] − [sum of the enthalpy of reactants]

2. additive

3. Hess' Law

By Hess' Law, reversing the reaction means the __________ is also reversed.

sign of the enthalpy change

*so a reversed endothermic reaction (+ΔH), becomes (-ΔH), an exothermic reaction

1. ________ is a phase change from solid to gas

2. It's direction of heat flow is ________

1. Sublimation

2. endothermic

1. ________ is a phase change from solid to liquid

2. It's direction of heat flow is ________.

1. Fusion

2. endothermic

1. _________ is a phase change from liquid to gas

2. It's direction of heat flow is ________.

1. Vaporization

2. endothermic

1. ________ is a phase change from gas to liquid

2. It's direction of heat flow is _________.

1. Condensation

2. exothermic

1. _________ is a phase change from liquid to solid

2. It's direction of heat flow is _________.

1. Freezing

2. exothermic

Enthalpy changes depend on _________.

number of moles

The _________ of a chemical reaction is denoted by Δ𝑟H°and refers to the enthalpy change associated with one mole of a specified reagent when all reactants and products are in their standard states

standard reaction enthalpy

Give the chemical formula for combustion:

carbon based species of interest + O2 --> CO2 + H2O

What does standard temperature and pressure (STP) mean?

pressure of 1 bar

temperature of 298 K

The standard heat of formation (or standard enthalpy change of formation) of any element in its most stable state at STP is _____.

0

Method of using standard heats of formation to calculate the enthalpy change of a reaction:

Products minus reactants (Hess' Law)

aA + bB --> cC + dD

[cC + dD] - [aA + bB]

where:

A = chemical species

a = moles of A

_________ describes the temperature dependence of enthalpy change of a reaction

Kirchoff's law

Kirchoff's law requires only that we know:

1. the enthalpy change at temperature T1

2. the heat capacity of the system

True or False?

Kirchoff's law shows that heat capacities are dependent on temperature

False,

heat capacity is independent of temperature as shown in this derived expression for Kirchoff's law:

𝜟rH(T2) = 𝜟rH(T1) + ∆CP(T2 − T1)

Heat capacity at constant pressure (Cp) is the same as saying:

"Molar constant pressure heat capacity"

*so you may see her say this instead

Heat capacity at constant volume (Cv) is the same as saying:

"Molar constant volume heat capacity"

*so you may see her say this instead

The First Law of Thermo is concerned with energy changes that accompany a process, but does not say whether that process is __________ or not.

spontaneous

A spontaneous process is a change that has a tendency to occur without needing _______ done to bring it about

work

"natural direction of change"

True or false?

Nonspontaneous processes violate the First Law of Thermo

False

nonspontaneous just means that work has to be done to bring complete the process, and work is a part of the First Law

There are two types of spontaneous physical processes:

1. Matter dispersing into disorder

*ie: gas molecules expanding into a vacuum

2. Energy dispersing into disorder

*oscillating atoms in a hotter body spreading out their energy into a colder body

_________ is the measure of the disorderly dispersal of energy or matter in thermodynamics

Entropy (S)

The __________ Law of Thermodynamics states that entropy of an isolated system tends to increase during a spontaneous process.

Second

To determine the change in entropy you need to determine the heat from a ________ process that connects the same two end points

reversible

Entropy has units of ________

J/K (energy per temperature)

ΔS

Entropy change of a phase transition is related to the __________ change of the phase transition and is given by ________.

enthalpy;

ΔSpt = ΔHpt/Tpt

*where pt = phase transition

Remember at constant T and P, the heat of a process is equal to the __________

enthalpy change ΔH

*q = ΔH

The ________ Law of Thermodynamics states that entropy of all perfectly ordered crystalline materials is 0 at 0K.

Third

The _________ can be used to calculate the standard entropy change of a reaction

standard molar entropies

*like enthalpy

The second law of thermo specifically states for isolated systems. If a system isn't isolated, how is does the second law adjust?

it considers the system + surroundings to be isolated and the second law applies to the total entropy of the system + surroundings

True or False?

A process cannot be spontaneous if the change of the system entropy is negative.

False

It can still be spontaneous if the overall entropy (system + surroundings) is positive

If the process is at constant T and P, then the entropy change of the surroundings is due to the _________

enthalpy

(aka the heat in a constant T and P process)

The change in the Gibbs free energy of the system at constant _________ is

∆G = ∆H − T∆S

temperature

The expression for Gibbs at constant temperature is ∆G = ∆H − T∆S. What modification needs to be made for constant temperature and pressure?

since we are having to consider the system and surroundings, ∆S in the expression needs to be replaced by ∆S(total)

∆S(total) = ∆S − ∆H/T

so the new expression for constant T and P needs to be:

∆G = ∆H − T(∆S − ∆H/T)

Give the expressions for change in entropy with temperature (T) for constant volume and constant pressure

ΔS = nCvln(T𝑓/T𝑖)

ΔS = nCpln(T𝑓/T𝑖)

*use the heat capacity that supports the problem, as long as C is independent of T

Write the expression for Gibb's free energy of a spontaneous process at constant T and P:

∆G < 0

True or False?

∆G is the free energy change of the system only

true

maximum work is done by the system when the process is ________

reversible

Since ∆G deals exclusively with the energy of the system, how do relate ∆G to work?

∆G = maximum non expansion work of the system

If ∆G is negative (<0) then it is the ________ non expansion work that is obtained from the system during a ________ process.

maximum;

spontaneous

If ∆G is positive (>0) then it is the _________ non expansion work that is needed to make a _________ process occur.

minimum;

non-spontaneous