Unit 3.7 - Entropy and the feasibility of reactions Flashcards
What term do we use if a reaction can happen?
Feasible
Spontaneous reactions
Reactions that happen easily, on their own accord without outside interference
Example of a spontaneous reaction
Group 1 + water
Examples of reactions that need happen (are feasible but not spontaneous)
Rusting
Food oxidations
Examples of reactions that are not feasible (don’t happen)
Platinum in water
Br2 + 2Cl-
What affects the feasibility of a reaction?
The change in energy - going down an energy gradient
The activation energy
What do both the change in energy and activation energy affect for a reaction?
Its feasibility
What type of reactions seem to be feasible and what type of reactions less so?
Feasible —> exothermic reactions
Less feasible —> endothermic reactions
Why are endothermic reactions not favourable?
Products are less stable than the reactants
Give 2 examples of spontaneous endothermic reactions
-sodium hydrogencarbonate and hydrochloric acid reacting in the cold
-ionic substances dissolving in water at room temperature
What type of reaction is diamond burning in oxygen?
Very exothermic
Diamond burning in oxygen is a very exothermic reaction, so why doesn’t it happen spontaneously?
The presence of strong covalent bonds in diamond means that it takes a lot of energy to break apart the carbon atoms in diamond in order to free them up to burn with oxygen = very high temperature needed
What is another factor that we need to determine whether a reaction is feasible, on top of just knowing the enthalpy change?
Entropy
What 2 things do we need to know in order to determine whether a reaction is feasible?
Enthalpy change
Entropy
Symbol for entropy
S
What is entropy?
A quantitative measurement of the freedom of molecules - a measure of disorder
S meaning
Entropy
Units of entropy
JK-1mol-1
Name and explain the 3 things that affect the value of entropy
The more disordered a substance, the higher the entropy
Temperature
Entropy will also generally be higher if there are more moles or more complicated molecules
What does a more disordered substance do to the value of the entropy?
Increases it
What is another thing that increases entropy except for being a disordered substance?
Entropy will generally be higher if there are more moles or more complicated molecules
example of entropy increasing for a more complicated molecule
CO2>CO in terms of entropy
Put solid liquid and gas in order of increasing entropy
S (solid) < S (liquid) < S (gas)
why do solids have low entropy values?
Highly ordered particles
Why do gases have the highest entropy values?
Most disorganised particles
Why does a liquid have an intermediate entropy value?
Less ordered particles than a solid, but not as disorganised as the particles of a gas
What does the superscript o symbol mean?
Under Standard conditions
What does a substance being more complex make it?
More entropic
What type of compounds are the most entropic?
More complex ones
Is air quite entropic? Why?
Yes, since it’s composed of a mixture of different gases (complex)
What does each substance at a given temperature and pressure have?
A unique entropy
How do we work out the entropy change for a reaction?
Change in entropy = entropy of all the products - entropy of all the reactants
How can we predict whether entropy will increase or decrease without entropy values?
By counting the number of moles of elements in their different states and remember that S(solid)<S(liquid)<S(gas)
Stages to working out entropy changes
- Write a balances equation to check number of moles
- Work out entropy of products and reactants (remembering to multiply up by the number of moles)
- Use the equation: change in entropy = entropy of all the products - entropy of all the reactants
- CHECK: entropy has increased - we could predict this as a single solid has produced two substances, a solid and a gas
What is the entropy change if entropy increases?
Positive
What does a positive entropy change mean has happened?
The entropy has increased
Second law of thermodynamics
For a reaction to be feasible, the total entropy of the system has to increase, so ΔS must be positive
What conditions have to be true in terms of the entropy of the reaction for a reaction to be feasible?
For a reaction to be feasible, the total entropy of the system has to increase, so ΔS must be positive
What does the change in entropy have to be for the reaction to be feasible?
Positive
What has to happen to the total entropy of a reaction for the reaction to be feasible
Has to increase
When will entropy always tend to increase during a reaction?
In any isolated system that is not in equilibrium
Explain why the entropy will always tend to increase in any isolated system that is not in equilibrium
This is the overall entropy - in different parts of the system, the entropy may decrease, but this is because it has increased in other parts of the system
This includes the entropy change of the surroundings
What does the overall entropy include?
The entropy change of the surroundings
NaCl (s) —> Na+ (aq) + Cl- (aq) ΔHsolultion = +5kJmol-1
For any ionic substance to dissolve in water, the enthalpy change of solution must be exothermic, so why does this salt dissolve in water?
Since there’s an increase in entropy of the system
Therefore, the total entropy of the system is positive
(aq = more disordered than s = increased entropy)
What two things do we need to soldier to work out if a reaction is feasible at a certain temperature?
Need to consider the enthalpy change of the reaction and the change in entropy
Gibbs free energy equation
ΔG = ΔH - (TΔS)
What must ΔG be for a reaction to be feasible?
Negative
ΔG meaning
Gibbs free energy
Units of everything in the Gibbs free energy equation
ΔG = Gibbs free energy in kJmol-1
ΔH = enthalpy change in kJmol-1
ΔS = entropy change in JK-1mol-1
T = temperature in K
Which value do we need to alter in the Gibbs free energy equation and why and how?
Entropy change
In JK-1mol-1
Need it to be consistent, so need to divide it by 1000
What could we be asked in terms of the Gibbs free energy equation?
If a reaction if feasible at a given temperature
At which temperature the reaction becomes feasible
What value does Gibbs free energy have to be for a reaction to take place?
Less than or equal to zero
How do we calculate the temperature at which a reaction can take place?
ΔG = O
So TΔS = ΔH
T = ΔH/ΔS
How do we know which reaction is the most feasible out of two values?
The most negative Gibbs free energy value
What does the value of ΔG having to be for a reaction to occur spontaneously?
Negative
If ΔG is negative, what does the reaction do?
Happens spontaneously
If ΔG is positive, what does the reaction do?
Doesn’t occur spontaneously
How do we know when a reaction will not occur spontaneously?
When ΔG is positive
What are the two conditions that we can use to prove whether or not a reaction is feasible?
ΔG must be negative
The total entropy of the system has to increase so ΔS must be positive
When do we use the total entropy of the system increasing for a reaction to be feasible explanation on why a reaction has occurred?
When we only have information about the states of the elements since we can work out whether entropy has increased/decreased from this
When do we use the fact that ΔG needs to be negative for a reaction to be feasible to prove why a reaction has occurred?
When we have information about ΔG
If the value for the Gibbs free energy of a reaction isn’t negative (and so isn’t feasible), what may be done to attempt to make the reaction occur spontaneously with no interference (make it feasible)?
The temperature may be increased
What’s the best method for working out whether a reaction is feasible when we have a solution reaction? Explain
Electrochemical data (Ecell) since Gibbs free energy data is for solids and liquids. We should be using standard conditions of concentration 1moldm^-3 and 298K temperature too.
What do we need to remember to with entropy changes?
Multiply up molar ratios
