2. Thermodynamics Flashcards
Define enthalpy of solution
1 mole of ionic solid dissolves in an amount of water large enough so that the dissolved ions are well separated and do not interact with each other
Hess cycle for enthalpy of solution?
ionic solid—-^Hsol—–>dissolved ions
LEF hydration enthalpys
gas ions
Arrows go up from gas ions
Define lattice enthalpy of formation
Enthalpy change when 1 mole of a solid ionic compound is formed from its constituent ions in the gas phase
What will increase lattice enthalpy?
small ions and/or high charges means stronger attraction means greater lattice enthalpy
Define born-haber cycle
A cycle that includes all the enthalpy changes in the formation of an ionic compound
born-haber cycle
what does it look like?
gaseous metal ions and gaseous non-metal atoms
u d
ionisation enthalpy d
u electron affinity enthalpy
gaseous atoms d
u gaseous ions
atomisation enthalpy d
u d
elements in normal states d
d lattice enthalpy of F
enthalpy of formation d
solid ionic compound
What is the experimental lattice enthalpy value?
Born-Haber cycle
How is theoretical lattice enthalpy value calculated?
Using an equation that considers size, charge and arrangement of ions
Why do the theoretical and experimental lattice enthalpy values differ?
Theoretical assumes perfectly ionic structure whereas in many ionic compounds ions are distorted to some extent
What is distortion (lattice enthalpy)?
In ionic compounds, ions are often distorted/ polarised
Define covalent character (lattice enthalpy)
When an ionic compound has distortion in its ions- a large amount
larger=more covalent character
What does it mean if there is a big difference between the theoretical and experimental lattice enthalpy values?
The ionic compound has great ionic character
What does the magnitude of lattice enthalpy signify?
The overall strength of the ionic bonding
What is one property of compound with covalent character?
They have low solubility or are insoluble in water
Define entropy (S)
The degree of inherent disorder in a system
What does a more positive entropy signify?
More disorder
What is the second law of entropy?
Over time entropy will naturally increase
What is the third law of entropy?
The entropy of a substance is zero at absolute zero and will increase with temperature
Why does entropy increase with temperature?
Particles vibrate/move faster so there is more disorder so greater entropy
Which of the states has the highest entropy?
Gases then liquids then solids
Why is there a bigger increase in entropy in the conversion of a liquid to a gas rather than a solid to a liquid?
Gases have such a greater disorder that liquids or solids that it will significantly increase the entropy
What compounds have a low entropy? Think structure
Those with a more ordered structure
What is the entropy equation?
entropy = sum products - sum reactants
What is gibbs free energy change?
It combines the two thermodynamic factors of enthalpy change and entropy
What is the gibbs free energy equation?
Delta G = enthalpy change - (Temp x entropy change)
What entropy and enthalpy value is most favourable?
Lower enthalpy
Higher entropy
What does the gibbs value need to be for the reaction to be feasible?
It needs to be lower or equal to zero
Define feasibility and what does it depend on?
A reaction can take place- depends of temperature
What is another name for feasibility?
Spontaneous
Why might a feasible reaction might not take place?
It has a too high activation energy
If enthalpy is positive and entropy is positive what is the effect on gibbs value? hint: at different temps
Lower temperature = positive value
Higher temperature = negative value
If enthalpy is positive and entropy is positive what is the effect on feasibility at different temperatures?
Lower temperature = not feasible
Higher temperature = feasible
If enthalpy is positive and entropy is negative what is the effect on gibbs value at different temperatures?
It is always positive
If enthalpy is positive and entropy is negative what is the effect on feasibility at different temperatures?
It is never feasible regardless of temperature
If enthalpy is negative and entropy is negative what is the effect on gibbs value at different temperatures?
Lower temperature = negative value
Higher temperature = positive value
If enthalpy is negative and entropy is negative what is the effect on feasibility at different temperatures?
Lower temperatures = feasible
Higher temperature = not feasible
If enthalpy is negative and entropy is positive what is the effect on gibbs value at different temperatures?
Always negative
If enthalpy is negative and entropy is positive what is the effect on feasibility at different temperatures?
Feasible at all temperatures
When are melting and boiling feasible?
When delta G = zero
How do you work out the temperature range for feasibility?
Set G as zero
Rearrange
Divide enthalpy by entropy
