C10 Thermodynamics

Question 1

what is thermodynamics? what does it allow us to predict?

Answer 1

• a branch of physical chemistry concerned with the transfer of energy
• allows predictions as to whether chemical reactions / processes will ‘go’
• allows for prediction of physical stabilities

Question 2

what is energy?

Answer 2

• the capacity to do work (motion against an opposing force)

Question 3

what are 2 forms that energy can take?

Answer 3

kinetic:
- the energy something possesses due to its motion
- any mass that is moving has kinetic energy

potential:
- the energy something possesses due to orientations / positions of its components
- eg. gravitational

Question 4

what are the 4 laws of thermodynamics?

Answer 4

• zeroth law: thermodynamic equilibrium
• first law: conservation of energy
• second law: principles of entropy
• third law: absolute zero temperature

Question 5

what is meant by system and surroundings?

Answer 5

system:
the bit we are interested in
eg. chemical reaction

surroundings:
everything else and where we make our observations
eg. in air / in a water bath

Question 6

describe an open system

Answer 6

• both matter and energy can be transferred between the system and the surroundings
• mass transfer and heat transfer
• eg. test tube containing hot water

Question 7

describe a closed system

Answer 7

• only energy can be transferred between the system and the surroundings
• only heat transfer
• eg. a stoppered test tube containing hot water

Question 8

describe an isolated system

Answer 8

• no matter or energy can be transferred
• no heat or mass transfer
• eg. stoppered and insulated test tube containing hot water

Question 9

what is internal energy (U)?

Answer 9

• the sum of all the kinetic energy and potential energy contributions of all the components in the system
• total potential and kinetic energy in a system
• impossible to calculate
• can only calculate changes in U

Question 10

how can ΔU be calculated?

Answer 10

U final - U initial = ΔU

Question 11

what 2 things can the internal energy of a system by changed by?

Answer 11

heat
work

Question 12

what formula links work (w), ΔU and heat (q)?

Answer 12

ΔU = w + q

Question 13

describe heat and work in relation to systems

Answer 13

• work (w) and heat (q) are done on or done by the system
• work and heat are what transfer energy into and out of our system

Question 14

describe what it means if q is positive or negative

Answer 14

• q is positive if heat is absorbed by the system
• q is negative if heat is given off by the system

Question 15

describe what it means if w is positive or negative

Answer 15

• w is positive if work is done on the system
• w is negative is work is done by the system

Question 16

what does enthalpy take into account?

Answer 16

• enthalpy (H) accounts for pressure and volume
• heat content of an open system at constant pressure

Question 17

what is the enthalpy change of a reaction?

Answer 17

• energy changes during chemical reactions
• enthalpy change between products and reactants

Question 18

what does Hess’ law state?

Answer 18

• the standard reaction enthalpy is the sum of all the standard reaction enthalpies into which the overall reaction is divided
• can take any path for a reaction (any arrows) and will always get the same answer

Question 19

what is enthalpy of formation of a substance?

Answer 19

• standard enthalpy (per mol) for its formation from its elements in their reference state
• they can be used to work out overall reaction enthalpy changes

Question 20

is bond breaking endothermic or exothermic?

Answer 20

endothermic (energy is taken in)

Question 21

is bond making endothermic or exothermic?

Answer 21

exothermic (energy is release)

Question 22

what is entropy (S)?

Answer 22

• the disorder of a system

Question 23

how do spontaneity and work relate to each other?

Answer 23

• spontaneous processes are driven by entropy and don’t require work to be done
• non-spontaneous processes require work to be done

Question 24

describe entropy changes in isolated, open and closed systems

Answer 24

• isolated: entropy increase is spontaneous
• open/closed: you have to take into account the entropy of the system and its surroundings

Question 25

sodium thiosulphate spontaneously transitions from liquid to solid at room temperature.

this doesn’t make sense because it is a spontaneous DECREASE in entropy. explain this

Answer 25

• chemical process gives out large amounts of heat (exothermic) - transferred to surroundings
• surroundings become hotter and thermal motion is greater = increases their entropy
• dispersion of energy is greater so entropy of surroundings increases
• total entropy of surroundings and system is increasing meaning it can be spontaneous

Question 26

explain the third law / absolute zero

Answer 26

• at 0 Kelvin, all thermal motion is eliminated
• there is no disorder, S = 0
• this is Absolute Zero which we cannot go below

Question 27

what is the equation for Gibbs free energy?

Answer 27

ΔG = ΔH - TΔS

Question 28

what must ΔG be for a reaction to be spontaneous?

Answer 28

less than 0

Question 29

if ΔS is positive and ΔH is negative, what does T have to be (if anything) for the reaction to be spontaneous?

Answer 29

spontaneous at all T

Question 30

if ΔS is positive and ΔH is positive, what does T have to be (if anything) for the reaction to be spontaneous?

Answer 30

only spontaneous at high T

Question 31

if ΔS is negative and ΔH is negative, what does T have to be (if anything) for the reaction to be spontaneous?

Answer 31

only spontaneous at low T

Question 32

if ΔS is negative and ΔH is positive, what does T have to be (if anything) for the reaction to be spontaneous?

Answer 32

not spontaneous for any values of T

Question 33

in biology, what drives non-spontaneous reactions?

Answer 33

ATP

Question 34

what is the formula for equilibrium constant, Keq?

Answer 34

[products] / [reactants]

Question 35

what does the numerical value of Keq tell us about the proportions in a reaction?

Answer 35

• if Keq > 1, products dominate
• if Keq < 1, reactants dominate
• if Keq = 0, reaction is at equilibrium

Question 36

a reaction is spontaneous if ΔG < 0. when else is a reaction spontaneous?

Answer 36

if Keq is much greater than 1
(reaction favours products)

Question 37

what does the Van’t Hoff equation allow us to see?

Answer 37

temperature can have an effect on the Keq

Question 38

state the equation linking Keq and ΔG

Answer 38

ΔG = -RTln(Keq)