Topic 13 Energetics II Flashcards
Why is the first electron affinity exothermic?
New force of attraction (FOA) between e- and nucleus made
What is the first electron affinity equation
X (g) + e- -> X- (g)
Why is second (and third and onwards) electron affinity endothermic?
Because adding e- and negative ions have repulsion
So energy needs to be put in to overcome
What is the definition of electron affinity
Enthalpy change when one mole of electrons are added to one mole of atoms in the gaseous state to form one mole of anions
What is the definition of standard enthalpy of atomisation
Enthalpy change when one mole of gaseous atoms are formed from the element in its constituent state under standard conditions
What is the enthalpy of atomisation? + general equation
Exothermic
X (s) -> X(g)
enthalpy change when 1 mol of an element in its standard state is atomised to produce 1 mol of gaseous atoms.
What is the lattice enthalpy of formation?
Enthalpy change When one mole of a solid compound is formed from its gaseous ions
What is the standard enthalpy of formation?
Enthalpy change when one mole of a substance is formed from its constituent elements in standard conditions
Why are theoretical and actual lattice energies different sometimes?
Bcs the theory assumes the ions are perfectly spherical but they may not be in reality
What is the enthalpy of solution?
enthalpy change when 1 mole of solid ionic compound is dissolved in water
write the equation of enthalpy of solution of NaCl
NaCl (s) + aq -> Na+ (aq) + Cl- (aq)
Suggest why calcium oxide is insoluble in water.
Because the lattice is so strong, lots of energy is needed to overcome
Why is the second I.E. of calcium more endothermic than its first I.E.?
Decreased ionic radius
So e-s are pulled closer to the nucleus so more energy is needed to overcome
how do sizes of ions affectlattice enthalpies?
- cations: the higher charge density/smaller = more perfectly spherical and ionic
- anions: the bigger/lower charge den, the more covalent
- evaluate impact to efoa and magnitude
what are the cation anion combination for the perfect sphere shape lattice? (theoretically)
large, low charge den cation
small high charge den anion
what are the cation anion combination for the perfect covalent lattice? (theoretically)
small, high charge den cation
large, low charge den anion
explain the interactions between water molecules and fluoride ions when the fluoride ions become hydrated. (2 marks)
- water is polar (δ+) so attracted to F- ions
- Efoa between them (stronger than H bonds)
suggest why the hydration of chloride ion is an exothermic process. (2 marks)
- water is polar, got δ+ to form efoa with Cl-
By describing the nature of the attractive forces involved, explain why the value for the enthalpy of hydration for the chloride ion is more negative than that for the bromide ion. (3 marks)
- chloride is a smaller ion
- so higher charge density
- so stronger efoa
Explain why there is a difference between the hydration enthalpies of the magnesium and sodium ions. (2 marks)
Mg2+ charge = smaller and higher charge density than Na+ with 1+ charge
so attracts polar water molecules more strongly
What is the definition of enthalpy of hydration?
The enthalpy change when 1 mole of a gaseous ion is completely dissolved in water into aqueous ions under standard conditions
A reaction is exothermic and has a negative entropy change. This reaction is feasible when it is above or below a certain temperature? Why?
Below
As heat is released and has less disorder, le chetelier favours endothermic reaction.
ASK B TO CLARIFY
Write the equation of the process when potassium nitrate dissolves in water. (1)
KNO3 (s) -> K+ (aq) + NO3- (aq)
What is the requirement for a reaction to be feasible?
Total entropy must be positive / increase
Explain why evaporation of water is spontaneous even though the change is endothermic.
Refer to change in arrangement of water molecules and entropy change.
Gases are more disordered than liquids
System increase in enthalpy
Magnitude of change in system > in surrounding
When talking about entropy change, mention…
Disorder
Ways of arrangement of molecules
Why is there a bigger jump in entropy change from liquids to gas than solids to liquids?
Difference of disorder is larger between liquids to gas than solids to liquid
Explain why the equilibrium yield is unchanged if a catalyst is used in a reaction
CH4 (g) + H2O(g) -> CO(g) + 3H2 (g)
Catalysts only lower activation energy
Both forward and backwards reactions are equally increasing
∆Stotal =
∆Surr + ∆Syst
∆Syst =
Or
∆H
Products - reactants
∆Surr =
-∆H / T
T=
(Tipping temperature)
∆H / ∆Syst
∆G =
∆H - Tx∆S
S from System
When is a reaction feasible in terms of Gibbs value?
If
∆G </= 0 , feasible
∆G > 0 , not feasible
How to determine if reaction is feasible (temperature range) in terms of T and ∆Surr?
When endothermic, it is feasible when temp > T
When exothermic, it is feasible when temp < T
According to le chetelier, also think of the X graph in booklet
If reaction is not feasible (ΔStot is negative) , we say that the reactants are…
Thermodynamically stable,
They will not react
If the reaction is feasible (ΔStot is positive), we say that the reactants are…
Thermodynamically unstable
They will react
If a reaction should take place but it doesnt, it is due to…
Either
- slow kinetics (activation energy)
- non standard conditions
If a non feasible reaction takes place, it is likely that…
It is not under standard conditions
ΔSyst=
Products - reactants
Why can’t we just double the enthalpy for C-C to get C=C? (2 marks)
- C=C is weaker than 2x C-C
- C=C consists of a sigma and pi bond, not two sigmas
Name two properties of ions that affect their enthalpy of hydration. (2 marks)
Ionic radius
Ionic charge
Explain in terms of entropy why this reaction is feasible. (2 marks)
In context
E.g. More particles / gaseous on RH than LHS
So Ssyst increases
Give 2 reasons why the calculated enthalpy change from bond enthalpy is different to the standard enthalpy change value. (2 marks)
- mean bond enthalpy is not specific to the element
- different moles
State why the entropy change of the surroundings for reaction X is highly positive, in terms of bond strengths of the reactants and products. (1 mark)
Highly positive = very exothermic
More energy released to form bonds than energy taken in to break bonds
Bonds of products are stronger than in reactants
If Ssurr is positive, the reaction must be exothermic or endothermic? Why?
Exothermic
As energy released forming bonds in products is greater than energy needed to break
Hence bonds in products must be stronger than bonds in reactants
