5.2 Energy

Question 1

Define lattice enthalpy

Answer 1

formation of 1 mol of ionic lattice from gaseous ions, ∆LEH, under standard conditions

Question 2

What is the use of lattice enthalpy?

Answer 2

Measure of the strength of ionic bonding in a giant ionic lattice

Question 3

Define enthalpy change of solution

Answer 3

enthalpy change of solution is dissolving of 1

mol of solute in water, ∆solH

Question 4

Define enthalpy change of hydration

Answer 4

dissolving of 1 mol of gaseous ions in water, ∆hydH

Question 5

Give the equation for lattice enthalpy change of NaCl

Answer 5

Na+(g) + Cl-(g)—> NaCl(s)

Question 6

Give the equation for the enthalpy change of hydration of Na+(g) + Cl-(g)

Answer 6

Na+(g) + Cl-(g) —> Na+(aq)+ Cl-(an)

Question 7

Give the equation for the enthalpy change of solution of NaCl(s)

Answer 7

NaCl(s) —> Na+(aq) + Cl-(aq)

Question 8

What happens in terms of bonding when an ionic lattice dissolves in water?

Answer 8

Bond between ions break to give gaseous ions (endothermic)

Bonds between gaseous ions and water are made (exothermic)

Question 9

Describe two things that affect lattice enthalpy

Answer 9

Ionic Charge-higher charge, the more energy released when an ionic lattice forms. This is due to stronger electrostatic attraction

Size-smaller the ionic radii of the ions involved, the more exothermic the lattice enthalpy. Smaller ions have higher charge density and their smaller ionic radii mean that the ions can sit closer together in the lattice. Meaning attraction between ions are greater.

Question 10

Give the first electron affinity of chlorine in

Na+(g) + Cl(g) + e-

Answer 10

Na+(g) + Cl(g) + e- —>Na+ + Cl-

Question 11

Define first electron affinity

Answer 11

energy released when 1 mole of gaseous atoms each acquire an electron to form 1 mole of gaseous 1- ions.

Question 12

Give the equation for the atomisation of chlorine in

Na(s) + 1/2Cl2(g)

Answer 12

Na(s) + 1/2Cl2(g) —> Na(s) + Cl(g)

Question 13

What is entropy?

Answer 13

Measure of the dispersal of energy in a system which is greater, the more disordered the system

Question 14

Explain the difference in entropy of a system of solids liquids and gas

Answer 14

Solids: lowest entropy

Gas: have the most random arrangements so highest entropy

Question 15

What else other then state results in more entropy?

Answer 15

More particles, more ways they and their energy can be arranged

So entropy increases as number of moles increase

Question 16

How do you calculate the entropy change of a system?

Answer 16

Entropy of products - reactants

Question 17

What is the feasibility of a process dependent on?

Answer 17

The feasibility of a process depends upon the entropy change and temperature in the system, T∆S, and the enthalpy change of the system, ∆H

Question 18

How do you calculate the free energy of a system?

Answer 18

∆G = ∆H – T∆S (the Gibbs’ equation)

A process is feasible when ∆G has a negative value

Question 19

What are the limitations of predictions made by ∆G about feasibility, in terms of kinetics.

Answer 19

Even if ∆G shows a reaction is theoretically feasible, the activation energy might be really high, or the reaction happens so slowly you don’t notice it happening at all.

Question 20

How do you calculate when the reaction is just feasible?

Answer 20

When ∆G is zero

Question 21

Do substances like disorder?

Answer 21

Yes, they’re more energetically stable, so they try to increase entropy