1. Enthalpy, entropy and free energy

Question 1

What are the two ways in which energy is released from stores in molecules?

Answer 1

heat or work

Question 2

What is internal energy?

Answer 2

It is the total energy of all particles (atoms and molecules) in a sample and is the letter U.

Equal to the sum of potential and kinetic energies of the particles of the system

Question 3

What is the difference between potential and kinetic energy?

Answer 3

PE is energy that is stored or ready to be released

KE moving objects have energy of motion

Question 4

How can energy be defined?

Answer 4

As the capacity to do work or supply heat

Question 5

What is enthalpy? H

Answer 5

Function related to the heat absorbed or evolved by a chemical system

Question 6

What is entropy? S

Answer 6

A measure of the number of ways energy is distributed throughout a chemical system

Question 7

What is Gibbs energy? G

Answer 7

The energy that is available to do work

G= H - TS

Question 8

What is the first law of thermodynamics?

Answer 8

delta U = q + w

Where q is the transfer of heat and w is the work done on a system or the system doing work

Question 9

For the first law of thermodynamics, if heat enters/exits the system then what is the sign of q?

Answer 9

If heat enters the system then q is positive and if heat exits the system then q is negative

Question 10

For the first law of thermodynamics, if work enters/exits the system then what is the sign of w?

Answer 10

If work is into the system then w s positive and if work is out of the system then w is negativ

Question 11

What is the SI unit of energy?

Answer 11

1 joule = 1 kg m2 s-2

Question 12

What happens in a closed system bs and isolated system for the first law of thermodynamics?

Answer 12

W=0 in a closed system and q=0 in an isolated system

Question 13

What is the equation for work?

Answer 13

W = -P x delta V

Question 14

What happens to W if v increases or decreases?

Answer 14

If V increases then the system is doing work (w is negative)

If V decreases then work is being done on system (w is negative)

Question 15

What can enthalpy be defined as?

Answer 15

H = U + PV

Question 16

What happens to enthalpy at constant pressure?

Answer 16

At constant pressure (where only PV work is allowed), change in enthalpy (delta H) equals the energy flow as heat

Question 17

What happens to enthalpy at constant volume?

Answer 17

At constant volume no work is done (p delta V = 0)

Question 18

What is the difference between heat and temperature?

Answer 18

Heat is not temperature
During a phase change such as evaporation, the substance undergoes a change in its enthalpy without experiencing a change in temperature
Heat is the actual energy measured in J
Heat is a measure of some of the energy in a substance

Question 19

Explain the enthalpy of the physical changes of water

Answer 19

When liquid water is cooled, its temp falls steadily until it drops just below the freezing point at 0 degrees. The temp then remains constant at the freezing point while the water crystallises. Once the water is completely frozen its temperature continues to fall

Question 20

What is the standard state of a substance?

Answer 20

Standard state of a substance is in its pure form at 1 atm pressure, where temp is usually 25 degrees

Question 21

What is a standard state reaction?

Answer 21

A standard state reaction enthalpy, delta H, is measured when reactants in their standard states change to products in their standard states

Question 22

What is delta H of formation if the element is in ts most stable form?

Answer 22

Zero

Question 23

What is entropy?

Answer 23

Measure of the randomness or disorder of a system, the larger the entropy the greater the disorder

Question 24

Which is more disordered out of a glass of ice chips or a glass of water?

Answer 24

The water is made of little molecules while the ice cubes are made of chunks of molecules, within which molecules are fixed in a lattice.

Question 25

What is the formula to work out delta S

Answer 25

delta S = s(products) - s(reactants) Units of delta s are J mol-1 K-1

Question 26

If there are fewer gas molecules in products than reactants, then delta S is...?

Answer 26

Negative

Question 27

If gas molecules appear in the products then delta S is...?

Answer 27

Positive

Question 28

Why do gas molecules have higher entropy?

Answer 28

Gas molecules have more ways available to distribute their energy

Question 29

What is the second law of thermodynamics?

Answer 29

The entropy of the universe increases in the course of a spontaneous change.

Question 30

What is the formula to work out the entropy for a particular macro state?

Answer 30

S = k ln(W) Where k is Boltzmann constant and W is the number of microstates

Question 31

How does volume affect entropy?

Answer 31

For gases the entropy increases with increasing volume

Question 32

How does temperature affect entropy?

Answer 32

The higher the temperature the higher the entropy

Question 33

How does physical state affect entropy?

Answer 33

FRom solid to liquid to gas, entropy increases

Question 34

How do number of particles affect entropy?

Answer 34

When all other things are equal, reactions that increase the number of particles in the system tend to have a positive entropy change

Question 35

What is the third law of thermodynamics?

Answer 35

The entropy (S) of a perfect crystalline substance is zero at the absolute zero of temperature

Question 36

What is the standard entropy change for a reaction given by?

Answer 36

Delta S= sum (s of products) - sum(s or reactants)

Question 37

What is the equation for the spontaneity of the universe?

Answer 37

Delta(s univ) = delta(s system) + delta(s surr) > zero

Question 38

Is a process spontaneous if delta Ssystem, delta Ssurr and delta Suniv are positve?

Answer 38

Yes it is a spontaneous process

Question 39

If d(s sys), d(s surr) and d(s univ) are negative is the reaction spontaneous?

Answer 39

No, the process will occur in the opposite direction

Question 40

If d(s sys) is + and d(s univ) is - then is the process spontaneous?

Answer 40

Yes, if d(s sys) is greater than d(surr)

Question 41

If d(s sys) is - and d(s surr) is +, is the process spontaneous?

Answer 41

Yes, if d(s surr) has a larger magnitude than d(s sys)

Question 42

Is d(s univ) is equal to zero, what does this tell you about the reaction?

Answer 42

It is at equilibrium

Question 43

What happens if d(s) + and d(h) -?

Answer 43

Spontaneous at all temperatures

Question 44

What happens if d(s) + and d(H) +?

Answer 44

Spontaneous at high temperatures (where exothermic is relatively unimportant)

Question 45

What happens if d(s) - and d(H) -?

Answer 45

Spontaneous at low temperatures (where exothermicity is dominant)

Question 46

What happens if d(s) - and d(h) is +

Answer 46

Process not spontaneous at any temperatures (reverse process is spontaneous at all temperatures)

Question 47

How do you work out d(s Surr) for change of entropy in surroundings?

Answer 47

D(s surr) = - d(H)/T

Question 48

What is another easier way to determine spontaneous reactions?

Answer 48

-Td(s univ) = -d(H sys) + Td(s sys) is less than 0

Question 49

For the Gibbs free energy equation, what must does a negative d(G) indicate?

Answer 49

Forward reaction is spontaneous

Question 50

What does a positve delta G indicate

Answer 50

Non spontaneous forward reaction or a spontaneous reverse reaction

Question 51

What does delta G = 0 indicate?

Answer 51

Equilibrium

Question 52

What is the equation for Gibbs free energy?

Answer 52

G = H - TS

Question 53

What is the equation for delta G at constant temperature?

Answer 53

D(g) = d(H) - Td(S)