topic 13 Flashcards
what is the definition of enthalpy change of formation?
the energy change when one mole of a substance if formed from its elements in their standard states
what is the definition of enthalpy change of atomisation?
the energy change when one mole of gaseous atoms is formed from an element in its standard state
what is the definition of electron affinity?
the energy change when each atom in one mole of atoms in the gaseous state gains an e-
what is the definition of ionisation energy?
the energy required to remove one e- from an atom in the gaseous state
what is the definition of enthalpy change of standard lattice?
the energy change when one mole of an ionic solid if formed from its gaseous ions
what does the born haber cycle show?
shows the enthalpy change needed to form one mole of ionic solid from atoms of elements in standard state
what are the factors affecting magnitude of lattice energy and explain why they affect LE?
- size of charge
inc ionic charge, inc lattice energy
-inc charge density
-inc electrostatic force of attraction
-so more energy is released (lattice energy value is more exothermic/ negative)
- size of ion
inc size of ion means greater ionic radius
distance between nuclei inc
attraction dec
less lattice energy released (value is less exothermic)
- number of cation to anion interactions
the more interactions per molecule, the greater the lattice energy
more exothermic
why is there a difference between experimental and theoretical lattice energy of an ionic compound?
theoretical lattice energy assumes that there is 100% ionic bonding
experimental values suggest that bonding within an ionic lattice isnt purely ionic
difference between theoretical and experimental lattice energies tells you the extent of covalent character in an ionic compound
what is lattice energy of dissociation?
ionic compound is broken up when water is added- lattice energy of dissociation
endothermic- bond breaking
lattice dissociation enthalpy isthe enthalpy change needed to convert 1 mole of solid crystal into its scattered gaseous ions
what is enthalpy of hydration?
water is attracted to ions- enthalpy of hydration
the amount of energy released on dilution of one mole of gaseous ions
what is the effect of ion size of enthalpy of hydration?
- inc charge, inc charge density
water molecules more attracted
- inc enthalpy of hydration
- same charge ion→ smaller ion
smaller ionic radius, higher charge density
what does it mean when a reaction is spontaneous?
when a reaction is spontaneous total entropy is greater than 0
what does it mean when something has a positive entropy value?
Reaction is feasible
when does entropy increase?
water evaporation l → g
-solid dissolving s → l as ions can move in any direction
-gases mixing through the air (diffusion)
what two factors govern the feasibility of a reaction?
- enthalpy change which should be negative (a negative value indicates an exothermic reaction)
- the entropy change (ΔS) which should be positive for the reaction to be spontaneous
how do you convert from celcius to kelvin?
C → K you add 273 to C
how do you work out entropy change of the system?
sum of entropy of products - sum of entropy of reactants
how do you work out entropy of the surroundings?
-(change in enthalpy/ temp (in kelvin))
how do you work out total entropy
entropy change of the system + entropy change of the surroundings
why does total entropy have a positive value even if the entropy of the system is negative/
the entropy of the surroundings has to be more positive than the entropy of the system (if negative) for the total entropy to have a positive value
what is gibbs free energy?
when you have an exothermic system which releases energy, and the energy of the surroundings have been accounted for, any left over energy will be considered free energy
how do you calculate gibbs free energy
ΔH - TΔS
What does a positive gibbs value mean?
the reaction is not feasible
how do you calculate temperature using entropy?
T= ΔH/ΔS
what is the difference between born haber cycle and hess’s cycle?
born haber measures enthalpy changes only for ionic solids whereas hess’s cycle measures enthalpy change for any reaction
how do you calculate enthalpy of solution?
ΔHsol= ΔHhyd - ΔHlatt
why may an endothermic reaction only be feasible at high temps but not low temps?
ΔG= ΔH - TΔS, temperature needs to be high in order to make TΔS more positive than ΔH, to outweigh the positive ΔH to make ΔG negative, meaning that the reaction is feasible
why may an endothermic reaction only occur at high temps and not low temps?
has a high enthalpy change
ΔG= ΔH - TΔS, temperature needs to be high in order to make TΔS more positive than ΔH, to outweigh the positive ΔH to make ΔG negative, meaning that the reaction is feasible
why may an exothermic reaction only occur at low temps?
small enthalpy change
TΔS would be a high negative value, making ΔG positive, meaning that the reaction would no longer be feasible at high temps
why may an exothermic reaction be feasible at all temps?
both entropy and enthalpy are negative, meaning that the reaction will always be feasible. also because ΔS is positive, so at high temps TΔS is always positive, so ΔG will always be negative, meaning that the reaction is always feasible
why may a reaction that is calculated to be feasible not occur?
- may be due to activation energy being too high because the reactants are kinetically stable
- very few collisions result in reaction
rate of reaction may be too slow
- very few collisions result in reaction
- non standard conditions
- if ΔG is positive then you can change the reaction conditions to make reaction feasible**
why may a reactions till be feasible even if ΔG is positive?
as some of the reactants can be converted to products
what is another way to calculate ΔG ?
-RT lnK
what is another way to calculate the equilibrium constant
K= e^(-ΔG/RT)
the negative is for the whole expression in the brackets, not just ΔG or RT
what does it mean if ΔG is negative in terms of feasibility and the equilibrium constant?
- The exponent will be positive
- Equilibrium constants are large in this case (greater than one)
- products favoured
- the larger the value of K the more the products are favoured
what does it mean if K becomes very large?
if K becomes so large and ΔG has become so negative (past a certain value) the reaction goes almost entirely to completion (reactants almost entirely converted to products)
what happens when ΔG is positive in terms of feasibility and the equilibrium constant?
- Reaction isn’t theoretically feasible
- The exponent will be negative
- Equilibrium constants are small (less than one)
- reactants favoured
Why is second electron affinity endothermic?
Energy is required to overcome repulsion between the negatively charged ion and electron
What how do you calculate enthalpy change of hydration
Lattice energy of dissociation + enthalpy change of solution
how do you calculate standard enthalpy change
sum of enthalpy change of products - sum of enthalpy change of reactants
what does it mean if an ion has a bigger ionic radius when talking about lattice energy
it can be more easily polarised so will have a degree of covalent character
makes lattice energy more exothermic
when dissolving sodium chloride/ionic solid in water what changes would you see on the thermometer
no measurable change in temp
thermometer not sensitive enough
energy taken in is small
what would the difference in experimental and theoretical lattice energy be for LiI compared to LiCl and why
the difference between born haber and theoretical lattice energy is greater for LiI than for LiCl
iodide ions have a smaller charge density than chloride ions
therefore iodide ions are more likely to have covalent character
so iodide ion is more likely to be polarised by Li+ than chloride ions would be
what happens to entropy change of surroundings and therefore total entropy change when temperature increases
as temp inc entropy change of surroundings becomes less positive
so total entropy becomes less positive
Draw a Hess’s diagram for enthalpy of solution
