Thermodynamics lecture 3

Question 1

In general, what is the change in the total internal energy of a system?

Answer 1

U = dq + dwadd + dwexp

where dwadd is the work done in addition to the work down by expansion

Question 2

when the total volume of a system is kept constant what happens to the overall internal energy

Answer 2

when total volume remains constant no work can be down for expansion therefore dw = 0 so internal energy is then defined as

dU = dq

which can also be represented as dU = dqv, where v represents a measure change between the state of i and f at a constant volume. Therefore change in U = qv

Question 3

what is the heat capacity? How does it relate to the change in internal energy to the change of temperature at a constant volume? And how does it relate to the heat supplied?

Answer 3

notesssssss

Question 4

What is the molar heat capacity? What is teh specific heat capacity?

Answer 4

molar heat capacity at a constant volume is the heat capacity per mole of a substance defined as:
Cv,m = Cv/n

the specific heat capacity is the heat capacity of the sample divided
by its mass defined as:

Cv,s = Cv/m

Question 5

What is enthalpy

Answer 5

H = U + pV

where H is enthalpy, U is internal energy, p is pressure, and V is volume.

note: enthalpy is a state function

Question 6

what is the change in enthalpy and derive its equation

Answer 6

It is teh amount of energy supplied as heat under the condition of constant pressure

the derivation is in the notessss

Question 7

what is the relationship between enthalpy and temperature?

Answer 7

as the ethalpy of a substance increases, the temperature increases as when molecules are excited to higher energies their total energy increase

Question 8

What is the heat capacity at constant pressure? How does it relate enthalpy to the temperature? And how does it relate the heat supplied to the change in temperature?

Answer 8

notesssssssss

Question 9

what is the molar heat capacity and constant pressure and how is it used to get an empirical approximation for temp?

Answer 9

molar heat capacity is the heat capacity at constant pressure divided by the number of moles and it can be used to get an empirical approximation for temperature when a nonideal gas is used as seen below:

Cp,m = a + bT + c/T^2
where a,b, and c are independent of temperature

Question 10

What is the relation between the two heat capacity constants?

Answer 10

When heat is applied to a system under constant pressure the volume might expand, this expansion of volume causes heat to be lost as work is done by the system to the surroundings. therefore the temperature increase is less, and the heat capacity increases with the smaller increase of temperature for the same heat therefore the Cp is larger than Cv and is represented by the following relation:

Cp -Cv =nR

Question 11

what are exenthalpic and endenthalpic reactions?

Answer 11

exenthalpic - an exothermic reaction where the enthalpy of the system is negative

endenthalpic - an endothermic reaction where the enthalpy of a system is positive

Question 12

what is the enthalpy change of transition?

Answer 12

it is the molar ethalpy change that accompanies a change of physical state represented by ∆trsH.

an example of an enthalpy change of transition is the enthalpy change of vaporization

Question 13

how do you calculate the enthalpy change of the reaction?

Answer 13

∆rH = the enthalpy change of the products - the enthalpy change of the reactants

Question 14

what is Hess’s law?

Answer 14

The standard reaction enthalpy is the sum of the values for the individual reactions into which the overall reaction may be divided.

Question 15

What is the problem with the standard enthalpy change of formation for ions? How can we solve it?

Answer 15

The problem: we can’t make a solution using cations and anions alone.

solution: set the standard enthalpy change of formation for H^+ ion to be zero at all temperatures.

Question 16

what happens when work is done on a system when it expands adiabatically?

Answer 16

when a gas expands in an adiabatic system work is done by the gas itself, but no heat enters the system so the internal energy falls, the temperature falls, the kinetic energy of molecules falls, and their speed also falls

Question 17

How does the change in temperature and volume affect the change of internal energy in an adiabatic system? How is this change related to the work done?

Answer 17

The change of the internal energy from the change in volume and temperature can be expressed by the sum of two steps, the first step is when the volume changes but the temp remains the same and that doesn’t have any effect on the internal energy, the second step is the increase of temp for a constant volume which causes an increase in internal energy, which then is defined as ∆U = Cv∆T. (note look at the graph on page 99 for reference)

Furthermore, the ∆U is defined as q + w whereas in an adiabatic system the q = 0 so ∆U = Wad

So work done in an adiabatic system (Wad) = Cv∆T

Question 18

What is the derivation for expression of the change of temperature in a reversible adiabatic expansion? What is the derivation of the expression that relates pressure and volume in a reversible adiabatic expansion?

Answer 18

Notessssss

Question 19

what is adiabat and how is it compared with an isotherm?

Answer 19

adiabat is a graph of pressure against volume in an adiabatic system, and compared with the isotherm the drop in pressure as volume increases is larger since y >1 and in the isotherm, the temp is maintained as heat is applied but that doesn’t happen in an adiabatic system