Topic 13 Samiyah

Question 1

what is the definition of enthalpy change of formation?

Answer 1

the energy change when one mole of a substance if formed from its elements in their standard states

Question 2

what is the definition of enthalpy change of atomisation?

Answer 2

the energy change when one mole of gaseous atoms is formed from an element in its standard state

Question 3

what is the definition of electron affinity?

Answer 3

the energy change when each atom in one mole of atoms in the gaseous state gains an e-

Question 4

what is the definition of ionisation energy?

Answer 4

the energy required to remove one e- from an atom in the gaseous state

Question 5

what is the definition of enthalpy change of standard lattice?

Answer 5

the energy change when one mole of an ionic solid if formed from its gaseous ions

Question 6

what does the born haber cycle show?

Answer 6

shows the enthalpy change needed to form one mole of ionic solid from atoms of elements in standard state

Question 7

what are the factors affecting magnitude of lattice energy?

Answer 7

• size of chargeinc 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 ioninc size of ion means greater ionic radius distance between nuclei incattraction decless lattice energy released (value is less exothermic)- number of cation to anion interactionsthe more interactions per molecule, the greater the lattice energymore exothermic

Question 8

why is there a difference between experimental and theoretical lattice energy of an ionic compound?

Answer 8

theoretical lattice energy assumes that there is 100% ionic bondingexperimental values suggest that bonding within an ionic lattice isnt purely ionicdifference between theoretical and experimental lattice energies tells you the extent of covalent character in an ionic compound

Question 9

what is lattice energy of dissociation?

Answer 9

ionic compound is broken up when water is added- lattice energy of dissociationendothermic- bond breakinglattice dissociation enthalpy isthe enthalpy change needed to convert 1 mole of solid crystal into its scattered gaseous ions

Question 10

what is enthalpy of hydration?

Answer 10

water is attracted to ions- enthalpy of hydrationthe amount of energy released on dilution of one mole of gaseous ions

Question 11

what is the effect of ion size of enthalpy of hydration?

Answer 11

• inc charge, inc charge densitywater molecules more attracted- inc enthalpy of hydration- same charge ion→ smaller ionsmaller ionic radius, higher charge density

Question 12

what does it mean when a reaction is spontaneous?

Answer 12

when a reaction is spontaneous total entropy is greater than 0

Question 13

what does it mean when something has a positive entropy value?

Answer 13

Reaction is feasible

Question 14

when does entropy increase?

Answer 14

water evaporation l → g-solid dissolving s → l as ions can move in any direction-gases mixing through the air (diffusion)

Question 15

what two factors govern the feasibility of a reaction?

Answer 15

• 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

Question 16

how do you convert from celcius to kelvin?

Answer 16

C → K you add 273 to C

Question 17

how do you work out entropy change of the system?

Answer 17

sum of entropy of products - sum of entropy of reactants

Question 18

how do you work out entropy of the surroundings?

Answer 18

-(change in enthalpy/ temp (in kelvin))

Question 19

how do you work out total entropy

Answer 19

entropy change of the system + entropy change of the surroundings

Question 20

why does total entropy have a positive value even if the entropy of the system is negative/

Answer 20

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

Question 21

what is gibbs free energy?

Answer 21

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

Question 22

how do you calculate gibbs free energy

Answer 22

ΔH - TΔS

Question 23

What does a positive gibbs value mean?

Answer 23

the reaction is not feasible

Question 24

how do you calculate temperature using entropy?

Answer 24

T= ΔH/ΔS

Question 25

what is the difference between born haber cycle and hess’s cycle?

Answer 25

born haber measures enthalpy changes only for ionic solids whereas hess’s cycle measures enthalpy change for any reaction

Question 26

how do you calculate enthalpy of solution?

Answer 26

ΔHsol= ΔHhyd - ΔHlatt

Question 27

why may an endothermic reaction only be feasible at high temps but not low temps?

Answer 27

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

Question 28

why may an endothermic reaction only occur at high temps and not low temps?

Answer 28

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

Question 29

why may an exothermic reaction only occur at low temps?

Answer 29

small enthalpy changeTΔS would be a high negative value, making ΔG positive, meaning that the reaction would no longer be feasible at high temps

Question 30

why may an exothermic reaction be feasible at all temps?

Answer 30

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

Question 31

why may a reaction that is calculated to be feasible not occur?

Answer 31

• may be due to activation energy being too high because the reactants are kinetically stable - very few collisions result in reactionrate of reaction may be too slow- non standard conditions - if ΔG is positive then you can change the reaction conditions to make reaction feasible**

Question 32

why may a reactions till be feasible even if ΔG is positive?

Answer 32

as some of the reactants can be converted to products

Question 33

what is another way to calculate ΔG ?

Answer 33

-RT lnK

Question 34

what is another way to calculate the equilibrium constant

Answer 34

K= e^(-ΔG/RT)the negative is for the whole expression in the brackets, not just ΔG or RT

Question 35

what does it mean if ΔG is negative in terms of feasibility and the equilibrium constant?

Answer 35

• 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

Question 36

what does it mean if K becomes very large?

Answer 36

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)

Question 37

what happens when ΔG is positive in terms of feasibility and the equilibrium constant?

Answer 37

• Reaction isn’t theoretically feasible- The exponent will be negative- Equilibrium constants are small (less than one)- reactants favoured

Question 38

Why is second electron affinity endothermic?

Answer 38

Energy is required to overcome repulsion between the negatively charged ion and electron

Question 39

What how do you calculate enthalpy change of hydration

Answer 39

Lattice energy of dissociation + enthalpy change of solution