Thermodynamics and kinetics (inorganic) Flashcards
Standard enthalpy of formation definition
The enthalpy change when one mole of a substance is formed from its elements at standard states.
Standard states/conditions
1 atmosphere pressure
1 mol l-1 concentration
298K temperature
What does a nought mean in enthalpy formulas
It means at standard conditions.
298K at degrees Celsius
25 degrees Celcius
Enthalpy of combustion definition
The enthalpy change of a when one mole of a substance completely burns in oxygen.
Standard enthalpy of formation symbol
ΔHf°
Standard enthalpy calculation formula
ΔH° = ΣΔHf°(products)- ΣΔHf°(reactants)
(Take into account mole ratio)
Enthalpy of formation of elements
Equals 0
Where do you find enthalpies of formation
In the data booklet with the enthalpies of combustion.
Entropy symbol
S
Entropy
The measure of disorder in a system
When does entropy increase
When temperatures rise
Where a substance melts, boils or dissolves (change of state)
Solids entropy
Low due to atoms being closely packed together.
Gas entropy
High due to atoms being able to move freely.
Liquid entropy
Medium due to irregular arrangement of atoms.
When does entropy increase in state changes
Right after the melting or boiling point there is a rapid increase in the entropy, then entropy flattens off again.
First law of thermodynamics
Energy can’t be destroyed or created, only changed from one form to another.
Second law of thermodynamics
The entropy of a reaction system and its surroundings increases for a spontaneous process.
Third law of thermodynamics
The entropy of a perfect crystal at 0K is zero.
Kelvin to degrees
0 degrees is 273 kelvin
Change in standard entropy
ΔS°= ΣS°(products) - ΣS°(reactants)
When is a reaction feasible
When free energy (ΔG) is <0
Free energy formula
ΔG = ΔH - TΔS
Where t is in kelvin and s is in J K-1 mol -1
Temperature where a reaction becomes feasible
T= ΔH -ΔG/ΔS
Standard free energy of formation formula
ΔG° = ΣΔG°f(products) - ΣΔG°f(reactants)
ΔG at equilibrium
ΔG= 0
ΔG when forwards reaction is favoured
ΔG° <0
ΔG when reverse reaction is favoured
ΔG°>0
why are standard states no longer required at eqiulibrium.
When the reaction starts there is no longer 1 mol l-1 concentration due to products being formed, therefore standard conditions aren’t used.
Where does equilibrium occur in free energy
When the G value of the mixture is lowest
How does free energy indicate equilibrium position
A reaction is feasible if the free energy is less than 0, if the free energy of a forwards reaction is negative the forwards reaction will be favoured since the reverse reaction will have positive free energy.
The reverse is also true so if the reverse reaction free energy is less and 0 the forwards reaction free energy will be positive and the reverse reaction will be favoured.
Order of reaction if the rate doubles when concentration of reactant doubles
1st order
Order of reaction if rate quadruples when concentration doubles
2nd order
Order of reaction if rate remains constant when concentration of reactant doubles
Zero order
Reaction rate formula
Rate =k[A]^m[B]^n
Overall order of a reaction
Add the rate orders
Calculating rate constant k
Produce rate equation
Rearrange for k
Substitute in values
Find the units
Zero order on graph
Straight line downwards
1st order in graph
Flat exponential line downwards
2nd order graph
Steep exponential line downwards
Rate determining step rule
A reaction cannot go faster than the slowest step of the reaction.
Rate determining step
The slowest step in the reaction.
Finding an order of a multi step reaction
Look at the slowest step and the reactants in that stage, the look at the number of moles and that is your reaction order.
Free energy in equilibrium explanation
If ΔG <0 for forwards reaction then this reaction is feasible but the reverse reaction is not feasible. And vice versa.
This means the forwards reaction will occur which will change reactants into products and standard conditions will be lost since there is no longer 1mol l-1 of either reactants or products.
As reactants are converted into products entropy (disorder) increases which will decrease the ΔG value since ΔG =ΔH - TΔS. (Since we are subtracting a bigger and bigger entropy value)
ΔG will decrease until a minimum value which is where equilibrium is achieved.
