Ch 17 - Thermodynamics

Question 1

what is entropy

Answer 1

the randomness of a system

Question 2

give an example of a reaction where the entropy increases

Answer 2

when the products have more moles in total than the reactants
or when the products have more moles of gas than the reacants

Question 3

what is the entropy change of the universe, how can you calculate it

Answer 3

the change in entropy of the system plus the surroundings.
it can only increase, thus for a reaction to be feasible the entropy change of the universe must be greater or equal to zero

Question 4

what does it mean for a reaction to be feasible

Answer 4

that it can happen, it is possible at a certain temperature

Question 5

what is the equation for gibbs free energy

Answer 5

G = H - TS

Question 6

what is entropy change denoted by

Answer 6

delta S

Question 7

what is enthalpy change denoted by

Answer 7

delta H

Question 8

for a reaction to be feasible, what must the value of deta G be

Answer 8

negative

Question 9

what is gibbs free energy

Answer 9

a measure of a way to determine a reactions feasibility

Question 10

at what values of H and S is a reaction always feasible, refer to the equation for gibbs free energy

Answer 10

when H is negative and S is positive, G will always be negative we multiply S by -T to give a negative value, and negative plus negative is always negative

Question 11

at what values of H and S is a reaction never feasible, refer to the equation for gibbs free energy

Answer 11

when H is positive and S is negative, G will always be positive.
we multiply S by -T to give a positive value, and positive plus positive is always positive

Question 12

why do some reactions not occur, even if G is negative

Answer 12

some reactions may have a very high actiavtion energy which means that only very few particles have enough energy to react (refering to maxwell boltzmann distributions), therefore it may take centuries for a reaction to go to completion, and we say that it doesnt occur becaus the rate of reaction is really slow

Question 13

when using the equation for gibbs free energy to plot a graph, what are the axis, gradient and y-intercept

Answer 13

G = S - TH in the form y=mx +c is G = (-H)T + S
this means that the x axis is T, the Y-axis is G, the y intercept is the enthalpy change and the gradient is entropy change times by -1

Question 14

what is the x intercept on a GFE graph

Answer 14

the minimum temperature at which a reaction is feasible

Question 15

how can you calculate the minimum temperature at which a reaction is feasible

Answer 15

T = H/S
make sure that all units are in either j or kj

Question 16

what are the limitations when finding the minimum temperature at which a reaction is feasible using T = H/S

Answer 16

a negative value will mean that a reaction is either always feasible or never feasible
the units need to be the same
we dont know if the reaction is feasible above or below this temperature

Question 17

how can you tell if a reaction is feasible above or below the temperature when G = 0

Answer 17

if the value of S is negative then it is feasible below that temperature, and if its positive then above that temperature.
(this is because of the gradient of the graph - visualise it)

Question 18

using the graph of gibbs free energy againts T, when will a reaction be feasible

Answer 18

when the line is below the X axis (this means that G is negative)

Question 19

without a graph, how can you tell if a reaction is feasible above or below a certain temperature

Answer 19

if S is negative and H is positive, then the reaction is feasible below the temperature
if S is positive and H is negative, then the reaction is feasible above the temperature

Question 20

if T when G = 0 is negative, how can you tell if its always or never feasible

Answer 20

if H is negative and S is positive then the reaction is always feasible
if its the opposite then the reaction is never feasible

Question 21

what is the exam procedure when given a reaction, its feasbility at a temperaature, and its enthalpy change

Answer 21

determine if the value of G is negative or positive, if its feasible then its negative.
determine the sign of the entropy change of the reaction and give a reason. look at the number of moles and the state symbols on either side of the equation.
compare the sign of the entropy to the sign of the enthalpy and state whether TS must be more positive or negative than H

Question 22

how can you determine the sign of the entropy change of a reaction given only the equation

Answer 22

determine the number of moles on each side, if there are more moles of products than reactants, the “randomness” has increased so the entropy is likely to be positive
if there are more moles of gas on the products side then the randomness has increased and the entropy is likely to be positive

Question 23

define electron affinity

Answer 23

the energy required to add one mole of electrons to one mole of gaseous atoms to form one mole of one minus ions

Question 24

which electron affinities are exothermic and endothermic

Answer 24

the first is always exothermic, every other one is endothermic

Question 25

why is the first electron affinity always exothermnic

Answer 25

because the electron experiences a greater attraction to the nucleus than repulsion from the electon cloud, thus energy is released when the electon is attrqacted to the nucleus

Question 26

why are EAs after the first always endothermic

Answer 26

because the ion becomes increasingly negative so the electon experiences a greater repulsion than attraction and energy is needed to overcome the repulsion

Question 27

define the enthalpy of atomisation

Answer 27

ttheenergy required to produce one mole of gaseous atoms of an element under standard conditions

Question 28

give an example of enthalpy of atomisation for sodium and chlorine

Answer 28

sodium: Na(s) ---> Na(g) chlorine: 0.5Cl2(g) ----> Cl(g)

Question 29

is enthalpy of atomisation endo or exo and why

Answer 29

always endothermic because you need to supply energy to break the bonds between atoms

Question 30

define lattice enthalpies

Answer 30

the enthalpy change when one mole of a solid ionic compound is formed form its gaseous ions under standard conditons

Question 31

what does the value of lattice enthalpy indicate for an ionic compound

Answer 31

a more negative value will indicate stronger ionic bonds, leading to a more stable compound

Question 32

what is stability of an ionic compound affected by

Answer 32

the charges and the sizes of constituent ions the more charged and smaller ions, the more stable the compound