3.7 entropy and feasibility of reactions Flashcards
apart from enthalpy, what other factor determines whether or not a reaction will take place?
the change in entropy
- a reaction that is exothermic will result in products that are more thermodynamically stable than the reactants
- this is a driving force behind many reactions and causes them to be spontaneous
- however this cannot be reconciled with the fact that many endothermic reactions proceed readily under normal conditions
- it follows that there must be other factors other than the enthalpy change
- the other factor which determine whether or not a reaction takes place is the change in entropy
what is entropy?
- measure of randomness of the particle
- measure of order/disorder in a system (chaos)
what are the units of entropy?
JK^-1mol^-1
the entropy of gases is (greater/less) than that of liquids which is (greater/less) than that of solids
greater
greater
S (gas) > S (liquid) > S (solid)
what does the third law of thermodynamics state?
- at 0K, particles are in a maximum state of order and have zero entropy as they are stationary
lots of order = low entropy
if asked to predict whether a reaction is likely to have an increase or decrease in entropy, what do you do?
- look at the state each substance is in
- as well as how many moles you start and end with in the reaction
- an increase in disorder and entropy will lead to a positive entropy change ∆S = +ve
NH4Cl (s) —> HCl (g) + NH3 (g)
is the entropy change positive or negative?
1 mol —> 2 mol
solid —> gas
∴ more disorder ∴ ∆S=+ve
in general a significant increase in the entropy will occur if:
- there is a change of state from solid or liquid to gas
- there is a significant increase in the number of molecules between products and reactants
systems will always seek to move towards a state of more disorder
what factors could stop reactions occurring so quickly?
a large activation energy
how to calculate enthalpy change (∆S°) quantitatively?
∆S° = ΣS° products - ΣS° reactants
reactions that have a +ve ∆S end up in (more/less) stable products?
more
diamond is described as metastable.
what does metastable mean?
has a high activation energy
what do you calculate to see if a reaction will proceed?
the change in free energy (∆G)
- whether or not a reaction will proceed depends on a balance between entropy and entropy
- these two quantities combine to give a single term known as free energy, G
what is the gibbs equation?
∆G = ∆H - T∆S
∆G = kJmol^-1
∆H = kJmol^-1
T = kelvin
∆S = kJK^-1mol^-1
if ∆G is positive, the reaction (occurs/doesnt occur)?
doesnt occurs / not feasible
if ∆G is negative, the reaction (occurs/doesnt occur)?
occurs
- feasible
a reaction that has increasing entropy (+ ∆S) and is exothermic (- ∆H) will make ∆G (positive/negative) and so will always (be feasible/not be feasible)
negative
be feasible
if ∆G is negative, is there still a possibility that the reaction will not occur?
- yes
- or may occur so slowly that effectively it doesnt happen
- if the reaction has a high activation energy the reaction will not occur
- thermodynamics tells us information about whether a reaction can occur or whether it is possible for a reaction to occur
- can it tell us anything about the rate at which the reaction occurs?
no
how can you predict at what temperature a reaction would become feasible?
- if ∆G tells us whether a reaction is feasible, so ∆G=0 can predict at what temperature a reaction would become feasible
∆G = ∆H - T∆S
0 = ∆H - T∆S
T∆S = ∆H
T = ∆H / ∆S
any phase changes for any reactions such as melting and boiling have a ∆G=?
∆G = 0
calculate the temperature range that this reaction will be feasible
N2 (g) + O2 (g) —> 2NO (g)
∆H = 180 kJmol^-1
∆S = 25 JK^-1mol^-1
∆G = ∆H - T∆S
∆G = 0
T = ∆H / ∆S
= 180 / (25/1000)
= 7200K
the reaction will be feasible when ∆G ≤ 0
so T must be >7200K
- all units must be the same *
how do you graphically determine ∆G?
y = mx + c
∆G = -T∆S + ∆H
∆H = c intercept
-∆S = gradient of graph
when graphically determining ∆G need to consider:
- a positive gradient means ∆S is negative
- when ∆G < 0 then the reaction is spontaneous
- the slope of the line may change as if the products start as a gas and change to liquid the entropy change would be different
when working out the Ecell value for an electrochemical cell, if the value is positive then the reaction is (spontaneous/not spontaneous), with ∆G therefore being (positive/negative)
spontaneous
negative
Zn 2+ (aq) + 2e- —> Zn (s) E = -0.76V LHS
Fe2+ (aq) + 2e- —> Fe (s) E = -0.44V RHS
EMF = RHS - LHS
= + 0.32
= +ve so feasible so ∆G is -ve
if Kc > 1 is ∆G positive or negative?
negative
if Kc < 1 is ∆G positive or negative?
positive
what does the second law of thermodynamics state?
- in any spontaneous reaction the total entropy increases
- so tells us that there us a general tendency towards disorder
what is the standard entropy, S°, of a substance?
the entropy of one mole of that substance under standard conditions
what are the units of gibbs free energy?
kJmol^-1 or Jmol^-1
if:
∆H = +ve
∆S = -ve
∆G = always +ve
what is the feasibility of the reaction?
- the reaction can never be feasible
if:
∆H = -ve
∆S = +ve
∆G = always -ve
what is the feasibility of the reaction?
- the reaction is always feasible
- (it may not happen quickly as the activation energy needs to be considered)
if:
∆H = +ve
∆S = +ve
∆G = -ve at high temperatures
what is the feasibility of the reaction?
- this reaction is opposed by ∆H (endothermic) but favoured by ∆S
- under standard conditions the reaction would not proceed, but if T is high enough to make -T∆S as large as possible (so T∆S > ∆H) then the reaction will proceed
if:
∆H = -ve
∆S = -ve
∆G = -ve at low temperatures
what is the feasibility of the reaction?
- this reaction is feasible whilst -T∆S is a small value
- once T becomes significantly higher the reaction is less likely to occur spontaneously
- the only way to find out is to plug numbers into the equation
what does it mean if a reaction is ‘spontaneous’?
- a spontaneous process is capable of taking place without needing to be driven by an outside source of energy
is a reaction with positive or negative entropy change more likely to be spontaneous?
- positive
is a reaction with positive or negative enthalpy change more likely to be spontaneous?
negative
(exothermic)
what does it mean for a reaction to be ‘feasible’?
- for a reaction to be feasible at a given temperature, the reaction must occur spontaneously
- this means no external energy input is required for the reaction to take place
