5.5 Enthalpy and Entropy

Question 1

What is Lattice Enthalpy ?

Answer 1

It is the enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions.

Question 2

What is the equation for lattice enthalpy definition ?

Answer 2

Na+ (g) + Cl-(g) = NaCl(s)

Question 3

What is the standard enthalpy of formation definition ?

Answer 3

Enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions.

Question 4

What is the equation for standard enthalpy of formation ?

Answer 4

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

Question 5

What is the definition for the standard enthalpy change of atomisation ?

Answer 5

Enthalpy change when 1 mole of gaseous atoms forms from the element in its standard state.

Question 6

What is the equation for standard enthalpy of atomisation ?

Answer 6

Na(s) = Na(g)
1/2 Cl2 (g) = Cl(g)

Question 7

What is the first ionisation energy definition ?

Answer 7

Enthalpy change when 1 electron is removed from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions.

Question 8

What is the equation for first ionisation energy ?

Answer 8

Na (g) = Na+ (g) + e-

Question 9

What is the second ionisation energy definition ?

Answer 9

Enthalpy change when 1 electron is removed from each ion in 1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ ions.

Question 10

What is the definition for first electron affinity ?

Answer 10

Enthalpy change when 1 electron is added to each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1- ions.

Question 11

What is the equation for first electron affinity ?

Answer 11

Cl- (g) + e- = Cl (g)

Question 12

What is the definition for second electron affinity ?

Answer 12

Enthalpy change when 1 electron is added to each ion in 1 mole of gaseous 1- ions to form 1 mole of gaseous 2- ions.

Question 13

What is the Born-Haber Cycle ?

Answer 13

Lattice enthalpy cannot be calculated directly, so requires it be calculated indirectly through creating a route for changing elements in their standard states into an ionic lattice.

Question 14

Are second electron affinities endothermic or exothermic ?

Answer 14

They are endothermic as a second electron is being gained by a negative ions, which repels the electron away, so energy must be put in to force the negatively charged electron on to the negative ion.

Question 15

How are ionic compounds dissolved in water ?

Answer 15

Water molecules can break up giant ionic lattice structure and overcoming the strong electrostatic attractions between oppositely charged ions.

Question 16

What is the standard enthalpy change of solution ?

Answer 16

Is the enthalpy change that takes place when one moles of a solute dissolves in a solvent.

Question 17

What is the equation for the standard enthalpy change of a solution ?

Answer 17

NaCl (s) + aq = Na+(aq) + Cl-(aq)

Question 18

Is the standard enthalpy change of a solution endothermic or exothermic ?

Answer 18

Can be endothermic or exothermic depending on the relative sizes of the lattice enthalpy and the enthalpy changes of nhydration. .

Question 19

What is the enthalpy change of hydration ?

Answer 19

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

Question 20

What are the equations for enthalpy change of hydration ?

Answer 20

Na+(g) + aq = Na+ (aq)
Cl-(g) + aq = Cl- (aq)

Question 21

What are some properties of ionic compounds ?

Answer 21

High melting and boiling points, soluble in polar solvents, conduct electricity when in molten or in aqueous solution.

Question 22

How does charge of the ion affect lattice enthalpy values ?

Answer 22

The greater the charge of the ions, the greater the attraction between oppositely charged ions, so lattice enthalpy is more exothermic for ions with a larger charge ?

Question 23

How does charge of the ions affect enthalpy of hydration values ?

Answer 23

The greater the charge, the greater the attraction between ions and water molecules so is more exothermic.

Question 24

How does the ionic radius affect lattice enthalpy values ?

Answer 24

The smaller the ionic radius, the less electron shells present so the attraction between oppositely charged ions is greater, so the more exothermic the lattice enthalpy value is.

Question 25

How does the ionic radius affect enthalpy of hydration values ?

Answer 25

The smaller the ionic radius, the greater attraction between ions and water molecules so the more exothermic the values for enthalpy of hydration.

Question 26

What is entropy ?

Answer 26

Used as a measure of how dispersed or spread out a systems energy is. It is often used as a measure of the disorder or randomness of a system.

Question 27

What does a greater entropy value indicate ?

Answer 27

The greater the entropy value, the more dispersed the energy is and the less order there is.

Question 28

What are the general entropy rules for solids, liquids and gases ?

Answer 28

Solids have the smallest entropy values, liquids have greater entropy values than solids, but gases have the greatest entropy values.

Question 29

What is the standard entropy ?

Answer 29

Is the entropy of one mole of a substance, under standard conditions. Always positive.

Question 30

How do you calculate entropy changes ?

Answer 30

Delta S = Sum of products - sum of reactants.

Question 31

What does the term feasibility refer to ?

Answer 31

Used to describe whether a reaction is able to happen and is energetically viable.

Question 32

What is free energy change (Delta G) ?

Answer 32

Is the overall energy change during a chemical reaction. It is made up of enthalpy change and entropy change.

Question 33

What is the Gibbs’ equation ?

Answer 33

G = Delta H - T Delta S

Question 34

When is a reaction feasible ?

Answer 34

For a reaction to be feasible, there must be a decrease in free energy.