Born Haber Cycles Flashcards
What are these?
Ways of laying out different and gradual changes in enthalpy in order to work out one that cannot be calculated using experiments such as lattice enthalpies (formation)
By following principle of Hess’s law
Hess’s law
The overall enthalpy change of a reaction is the same, independent of the route taken
As long as the reactants you start with are the same as the end products
Upwards arrows in a born haber cycle
Show a route that takes in energy (positive and endothermic) so the species above have taken in energy to be of a higher energy value
Downwards arrows in a cycle
Shows processes that release energy (exo) so the species it points to below are of a lower energy level than the one above
And so these values are negative
Energy levels
Species with higher enthalpy are above those with lower enthalpy
Starting level of a born haber cycle
The molecules of elements in their standard states
What process is done to both the molecules in their standard states?
Enthalpy of atomisation (endothermic process, breaks bonds, is positive value thus upwards arrow)
What do we have after each molecule has been atomised?
Substances in pure elemental gaseous states
Do we need to multiply the enthalpy values given by anything?
Yes potentially if we are trying to obtain 2 moles so double
Also depends what value was given to use, eg for atomisation of Cl2 but we only want Cl, we divide by 2
What do we do to the pure elemental gaseous atoms if we want to form a negative ion (of different charges)
Use the first electron affinity value = forms -1 ion
Use the second electron affinity value = forms -2 ion
What do we do to the pure elemental gaseous atoms if we want to form a positive ion (of different charges)
Use first Ionisation energy = +1
Use second ionisation energy = +2
Successful ionisation/electron affinities vs using multiple moles
EXAMPLE
If we want to form a -2/+2 etc ion then we need to use different values
However if we want to form 2 moles of +1/-1 ion etc we multiply the first ionisation/ea by 2
What do all values in a born haber cycle add to?
0
What happens if you follow the sign in a cycle stage?
Keep the same sign as the enthalpy value
What happens if you go against the arrow in a cycle stage?
The sign is reversed
How can we use the cycles in calculation to work out missing values in stages?
The value for elements in standard states —> compound is the standard enthalpy of formation
According to Hess law this value is equal to if you go a different route: atomise the standard elements, ionise/ add electrons/ calculate lattice enthalpy of formation
So we can work out missing value by calculation
When are electrons written in the cycle?
Following the ionisation to form a metal ion, it is written alongside the products on the above line
Then the next line would be an increase in electrons (successive IE) or the gain of this electron(s) in EA so we can form a negative ion
What is written under the elements in their standard states?
The ionic compound
Standard enthalpy of formation in the cycle
An arrow pointing down (exothermic) to show energy released when making bonds between the pure elements to form an ionic compound
Standard enthalpy of formation in the cycle
An arrow pointing down (to show exothermic reaction where energy was released when ionic bonds were made) from gaseous ions to solid ionic compound
Are first electron affinity values exothermic pos or neg?
TENDS TO BE exothermic so a negative value because energy is released when the atom gains an electron
Are second electron affinities positive or negative?
CAN BE ENDOTHERMIC so positive because energy is needed to be taken in to the (already negative) ion.
To overcome the repulsive forces between negative electron and negative ion
Using Hess law in the equation
Standard enthalpy of formation = total atomisation, EA, IE, lattice enthalpy of formation
