Enthalpy and Entropy

Question 1

State Hess’s Law

Answer 1

If a reaction can proceed by multiple routes, the total enthalpy change for all routes is equal.

Question 2

Define enthalpy change of atomisation

Answer 2

The enthalpy change that accompanies the formation of ONE MOLE of atoms of an element in the gaseous state from its standard state under standard conditions

Question 3

Define lattice enthalpy

Answer 3

The enthalpy change that accompanies the formation of one mole of ionic lattice from its gaseous elements under standard conditions

Question 4

Define enthalpy change of solution

Answer 4

The enthalpy change when one mole of ionic substance dissolves in water to form one mole of aqueous ions.

Question 5

Define enthalpy change of hydration

Answer 5

The enthalpy change that accompanies one mole of gaseous ions dissolving in water to form one mole of aqueous ions under standard conditions.

Question 6

Define enthalpy change of formation

Answer 6

The enthalpy change that accompanies the formation of one mole of compound from its constituent elements under standard conditions and in standard states

Question 7

Define enthalpy change of neutralisation

Answer 7

The enthalpy change that accompanies the reaction of aqueous acid with aqueous base to form one mole of water under standard conditions

Question 8

What happens when we are using BH cycles to find the lattice enthalpy of CaCl2?

Answer 8

We must multiply the electron affinity and atomisation of Cl by 2 as 2 moles are being formed in each case.

Question 9

In a BH cycle, what is the enthalpy change of formation equal to?

Answer 9

All the clockwise enthalpy changes. Normally, this is all the other enthalpies.

Question 10

Why is the second electron affinity endothermic, whereas the first is exothermic?

Answer 10

Energy is released when the first electron is added, as it is attracted to the nucleus.

For second electron affinity, work must be done to overcome the electrostatic repulsion of the 1- ion and the electron.

Question 11

What is the enthalpy sign for atomisation, and why?

Answer 11

Endothermic (+ve), as energy is required to break the bonds and form a gaseous atom

Question 12

What is the enthalpy sign for ionisation? What is the pattern for successive ionisation energies

Answer 12

Endothermic as more energy is required to overcome the attraction. As electrons are removed from closer to the nucleus, the ionisation energy increases, as they are more strongly attracted.

Question 13

What is the sign for lattice enthalpy, why?

Why do we need a born-Harber cycle to calculate lattice enthalpy?

Answer 13

-ve, as bonds are being formed. Therefore, the more negative, the stronger the bonds and the more stable the lattice, as the electrostatic attraction is greater.

It cannot be measured directly, as we can’t have gaseous ions forming an ionic lattice. Therefore, we measure it indirectly, using Hess’s Law, and known enthalpy changes.

Question 14

Given the enthalpy change for an endothermic reaction, and how much energy is used per unit time, how can you calculate the amount of a reactant or product used in that time?

Answer 14

Total energy/enthalpy change = how many times the reaction can occur in its stoichiometric ratio. Multiplying this by the number of moles reacting tells us total moles of the reactant/product. * by Mr to find mass.

Question 15

What are the three factors affecting the difference in lattice enthalpy between Mg2+ and Na+ and chloride ions?

Answer 15

Mg has higher ionic charge (2+/+)
Mg is smaller ion as fewer orbitals, so atomic radius smaller
stronger bonds due to increased attraction between oppositely charged ions.

Question 16

What is the sign for the enthalpy change of neutralisation?

Answer 16

Negative.

Question 17

What is enthalpy change of solution a combination of?

Answer 17

Lattice dissociation (opposite sign of lattice enthalpy) and hydration of the gaseous ions.

Question 18

Which two factors affect both hydration and lattice enthalpy? How do they affect each?

Answer 18

ionic charge, ionic radius

LE
ionic charge - large - more exothermic
Ionic radius - large - less exothermic

HE
Ionic charge - large - more exo - more attracted to H2O
Ionic radius - large - less exo - less attracted to H2O

Question 19

What do you need to be careful of on enthalpy questions?

Answer 19

Have they given both H and S in terms of KJ mol-1? They probably haven’t.

Question 20

How do you explain whether a reaction is feasible at a given temperature?

Answer 20

G must be less than zero. Consider whether H or S carrier the greater significance at low and high temperatures, and consider the signs of both.

Question 21

How do you calculate the entropy change of a reaction

Answer 21

products-reactants

Question 22

What is the equation for gibbs free energy of a system

Answer 22

/\G=/\H-T/\S

Question 23

When finding the temperature at which a reaction is feasible, what must you remember?

Answer 23

You must calculate /\S and /\H in the same molar ratio, e.g. For water, the formation enthalpy refers to one mole of water.

Question 24

What happens to lattice enthalpy when the anion increases in size e.g. Cl–> I

Answer 24

The lattice enthalpy becomes less exothermic as the bonds are not as strong

Question 25

What happens to hydration enthalpies as the size of the anion increases?

Answer 25

It becomes less negative.

Question 26

What is the factor affecting how negative hydration enthalpies are?

Answer 26

How well the ions attract and bone with water molecules

Question 27

What is the sign on solution enthalpies?

Answer 27

Could be both positive or negative

Question 28

If the enthalpy change of solution is negative, how do you set up the born-haber cycle?

Answer 28

just the same as any other, it doesn’t matter which way the arrows go.

Question 29

What is the equation to solve for a born-haber cycle of solution enthalpies?

Answer 29

/\H(hyd)=/\H(le) + /\H(soln)

Question 30

Define standard enthalpy change of solution

Answer 30

the enthalpy change when one mole of ionic compound is dissolved completely in water under standard conditions

Question 31

What are the two components that make up solution enthalpy?

Answer 31

Hydration enthalpy (-ve) + lattice dissociation enthalpy (+ve)

Question 32

What are the calculations involved in finding enthalpy change of neutralisation?

Answer 32

find the mass of the solutions reacting
Measure the temperature change
use Q=mc/\t to find energy change
find the moles of acid and base reacted, make sure you use the limiting moles
divide energy change by moles
make sure sign is negative, as temperature increase of surroundings means energy LOST by the system