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CHEM10006 > 1. Enthalpy, entropy and free energy > Flashcards

1. Enthalpy, entropy and free energy Flashcards

1
Q

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

A

heat or work

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2
Q

What is internal energy?

A

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

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3
Q

What is the difference between potential and kinetic energy?

A

PE is energy that is stored or ready to be released

KE moving objects have energy of motion

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4
Q

How can energy be defined?

A

As the capacity to do work or supply heat

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5
Q

What is enthalpy? H

A

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

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6
Q

What is entropy? S

A

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

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7
Q

What is Gibbs energy? G

A

The energy that is available to do work

G= H - TS

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8
Q

What is the first law of thermodynamics?

A

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

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9
Q

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

A

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

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10
Q

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

A

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

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11
Q

What is the SI unit of energy?

A

1 joule = 1 kg m2 s-2

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12
Q

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

A

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

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13
Q

What is the equation for work?

A

W = -P x delta V

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14
Q

What happens to W if v increases or decreases?

A

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)

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15
Q

What can enthalpy be defined as?

A

H = U + PV

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16
Q

What happens to enthalpy at constant pressure?

A

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

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17
Q

What happens to enthalpy at constant volume?

A

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

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18
Q

What is the difference between heat and temperature?

A

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

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19
Q

Explain the enthalpy of the physical changes of water

A

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

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20
Q

What is the standard state of a substance?

A

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

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21
Q

What is a standard state reaction?

A

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

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22
Q

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

A

Zero

23
Q

What is entropy?

A

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

24
Q

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

A

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.

25
Q

What is the formula to work out delta S

A

delta S = s(products) - s(reactants)

Units of delta s are J mol-1 K-1

26
Q

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

A

Negative

27
Q

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

A

Positive

28
Q

Why do gas molecules have higher entropy?

A

Gas molecules have more ways available to distribute their energy

29
Q

What is the second law of thermodynamics?

A

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

30
Q

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

A

S = k ln(W)

Where k is Boltzmann constant and W is the number of microstates

31
Q

How does volume affect entropy?

A

For gases the entropy increases with increasing volume

32
Q

How does temperature affect entropy?

A

The higher the temperature the higher the entropy

33
Q

How does physical state affect entropy?

A

FRom solid to liquid to gas, entropy increases

34
Q

How do number of particles affect entropy?

A

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

35
Q

What is the third law of thermodynamics?

A

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

36
Q

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

A

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

37
Q

What is the equation for the spontaneity of the universe?

A

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

38
Q

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

A

Yes it is a spontaneous process

39
Q

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

A

No, the process will occur in the opposite direction

40
Q

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

A

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

41
Q

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

A

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

42
Q

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

A

It is at equilibrium

43
Q

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

A

Spontaneous at all temperatures

44
Q

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

A

Spontaneous at high temperatures (where exothermic is relatively unimportant)

45
Q

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

A

Spontaneous at low temperatures (where exothermicity is dominant)

46
Q

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

A

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

47
Q

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

A

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

48
Q

What is another easier way to determine spontaneous reactions?

A

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

49
Q

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

A

Forward reaction is spontaneous

50
Q

What does a positve delta G indicate

A

Non spontaneous forward reaction or a spontaneous reverse reaction

51
Q

What does delta G = 0 indicate?

A

Equilibrium

52
Q

What is the equation for Gibbs free energy?

A

G = H - TS

53
Q

What is the equation for delta G at constant temperature?

A

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

CHEM10006 (13 decks)

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