Thermodynamics 2 Flashcards
What do you call it when you measure heat change at constant pressure
Enthalpy change
Define standard molar enthalpy of formation /\fH
The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions, all reactants and products being in their standard states.
Define standard molar enthalpy change of combustion /\cH
The enthalpy change when one moles of substance is completely burnt in oxygen.
Define standard enthalpy change of atomisation /\atH
The enthalpy change which accompanies the formation of one mole of gaseous atoms from the element in its standard state under standard conditions.
Define first ionisation energy IE
The standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions each with a single positive charge.
Define first electron affinity /\eaH
The standard enthalpy change when a mole of gaseous atoms is converted into a mole of gaseous ions, each with a single negative charge.
Define second ionisation energy
Second ionisation energy is the loss of a mole of electrons from a mole of singly positively charged ions.
Define second electron affinity /\eaH
The enthalpy change when a mole of electrons is added to a mole of gaseous ions each with a single negative charge to form ions each with two negative charges.
Are first and second electron affinities positive or negative enthalpy changes
1) first electron affinity is a negative enthalpy change
2) second electron affinity is positive.
Define lattice enthalpy of formation /\LH
The standard enthalpy change when one mole of solid ionic compound is formed from its gaseous ions.
Is lattice enthalpy of formation a positive or negative enthalpy change and why
-Lattice enthalpy of formation is always negative.
- This is because when a lattice forms, new bonds are formed which causes energy to be given out as it is exothermic.
Define lattice enthalpy of dissociation
Lattice enthalpy of dissociation is the standard enthalpy change when one mole of solid ionic compound dissociated into its gaseous ions.
How are the lattice dissociation enthalpy and lattice enthalpy of formation of a compound linked.
- They have the same numerical value.
- Lattice enthalpy of formation is the negative version and lattice dissociation is positive.
Define enthalpy of hydration /\hydH
The standard enthalpy change when water molecules surround one mole of gaseous ions.
Define enthalpy of solution /\solH
The standard enthalpy change when one mole of solute dissolves completely in sufficient solvent to form a solution in which the molecules or ions are far enough apart not to interact with each other.
Define mean bond enthalpy
The enthalpy change when one mole of gaseous molecules each breaks a covalent bond to form two free radicals, averaged over a range of compounds.
Describe the enthalpy changes involved in forming a compound from its constituent elements under’t standard conditions, all products and reactants being in their standard states.
1) Atomisation
2) Ionisation
3) Electron affinity
4) Lattice formation
Why might an exam question with a Born-Haber cycle have more stages than you think
- It may show the same stage for each compound as separate.
- eg. You could write the atomisation energy for sodium and chlorine together as one step or you could have the atomisation of sodium first, then atomisation of chlorine next as a separate step.
Describe Hess’ law
Hess’ law states that the total energy (or enthalpy) change for a chemical reaction is the same whatever route is taken, provided that the initial and final conditions are the same.
What is a Born-Haber cycle
A thermochemical cycle that includes all of the enthalpy changes involved in the formation of an ionic compound.
What begins a Born-haber cycle
- Born-Haber cycles are constructed by starting with the elements in their standard states.
- All elements in their standard states have zero enthalpy by definition.
Why does a compound having larger ions lead to smaller lattice enthalpies
Because the opposite charges do not approach each other as closely when the ions are larger.
Why does lattice enthalpy increase when the charge on the ions increases (for ions of similar size)
Because ions with double the charge give out roughly twice as much energy when they come together.
How are the ions in an ionic compound arranged when hydrated
- The positive ions are surrounded by the negative ends of the dipole of the water molecule.
- The negative ions are surrounded by the positive ends of the dipoles of the water molecules.
What trends do we see in enthalpy change of hydration
- we see the same trends as lattice enthalpy:
- more negative for more highly charged ions
- Less negative for bigger ions
What three processes make up dissolving an ionic compound in water
1) breaking the ionic lattice to give separate gaseous ions- the lattice dissociation enthalpy has to be put in.
2) Hydrating the positive ions (cations) - the enthalpy of hydration is given out.
3) Hydrating the negative ions (anions)- the enthalpy of hydration is given out.
Why might there be large discrepancy between the theoretical value for lattice formation enthalpy and the experimental value
The bond in question may have some covalent character
Describe why Zinc Selenide has some covalent character
- The Zn+ ion is relatively small and has a high positive charge
- The SE2- ion is relatively large and has a high negative charge.
- the small Zn2+ ion can approach closely to the electron clouds of the Se2- and distort them by attracting them towards it.
- The Se2- is easy to distort because its large size means the electrons are far from the nucleus and its double charge means that there is plenty of negative charge to distort.
- This distortion means that there are more negative electrons between the Zn and Se nuclei.
- This represents a degree of electron sharing or covalency.
- The Se2- ion is said to be polarised.
What are the factors which increase polarisation (seen in molecules with covalent character)
- positive ion (cation)- small ion, high charge.
- Negative ion (anion)- Large size, high charge.
What is entropy (thermodynamics)
The randomness of a system, expressed mathematically.
Which state has the highest entropy
Gases
What are the two key factors that drive spontaneous chemical processes/ govern the feasibility of a chemical reaction
1) Enthalpy change
2) Entropy change
How can the entropy change of a reaction be calculated
By adding all of the entropies of the products and subtracting the sum of the entropies of the reactants.
What is Gibbs free energy G
- A quantity that’s combines entropy and enthalpy change to determine how feasible a reaction is.
If the change in G, /\G for a reaction is negative, what does this mean
The reaction is feasible
if the change in G /\G is positive, what does this mean
The reaction is not feasible.
What does the letter G represent in thermodynamics
Gibbs free energy
What is the equation for change in G, /\G
- /\G=/\H-T/\S
- /\G=change in Gibbs free energy
-/\H= enthalpy change
-T= temperature - /\S=entropy change
Why might some reactions be feasible at times and not feasible at other times
- /\G depends on temperature
- A reaction therefore may be feasible at one temperature but not another.
- So an endothermic reaction can become feasible when temperature is increased if there is a large enough positive entropy change.
What does /\G being zero mean
- This is the point where the reaction is just feasible.
How does Gibbs free energy relate to equillibrium
- When /\G=0, the reaction has just become feasible.
- In a closed system the equilibrium exists around this temperature (/\G=0) in which both products and reactants are present.
What is an example of a situation where /\G=0 is useful and allows us to measure entropy change
- A compound melting/boiling point
- Both reactants and products are present and this is a state of change so /\G=0
- Therefore we can sub in temperature and enthalpy change to work out entropy change.
How do Kinetic factors affect the predictions we make using the Gibbs free energy equation
- You may predict that a certain reaction should occur spontaneously due to the entropy and enthalpy changes but the reaction may take place so slowly that for practical purposes it does not occur at all.
- there is a large activation energy barrier for the reaction.
Why is graphite at room temperature described as thermodynamically unstable but kinetically stable
- The entropy and enthalpy values for its reaction with oxygen show that the reaction is feasible- so thermodynamically unstable.
- However, kinetics tells us that room temperature is too low for this reaction to take place- it takes so long it effectively doesn’t occur at all- this is kinetically stable.
What are the units for entropy change
KJK^-1mol^-1
What are the units for Gibbs free energy change
kJmol^-1
What are the units for enthalpy change
kJmol^-1
How do you convert a temperature in Celsius to kelvins
Add 273
How do you convert an entropy change in JK^-1mol^-1 into kJK^-1mol^-1
Divide by 1000
