Thermodynamics - Born Haber Cycles

Question 1

What is the symbol for enthalpy change?

Answer 1

Delta H

Question 2

What are standard conditions?

Answer 2

298K and 100kPa and all solutions at a concentration of 1moldm-3 and all substances present in their standard states

Question 3

What is enthalpy change?

Answer 3

The heat energy change at constant pressure

Question 4

Define enthalpy of lattice dissociation

Answer 4

The enthalpy change when one mole of an ionic compound is separated into its component gaseous ions

Question 5

Is lattice dissociation endo or exothermic and why?

Answer 5

Endothermic because breaking bonds requires energy

Question 6

Define enthalpy of lattice formation

Answer 6

The enthalpy when one mole of an ionic compound is formed from its constituent ions in the gaseous state

Question 7

Is lattice formation exo or endothermic?

Answer 7

Exothermic bc making bonds releases energy

Question 8

Define enthalpy of formation

Answer 8

The enthalpy change when one mole of a compound is formed from its elements when all the reactants and products are in their standard states under standard conditions

Question 9

Define enthalpy of atomisation of an element

Answer 9

The enthalpy for the formation of one mole of gaseous atoms from the element in its standard state

Question 10

Define bond dissociation enthalpy

Answer 10

The enthalpy change when one mole of a covalent bond is broken under standard conditions in the gaseous state

Question 11

Is bond dissociation enthalpy endo or exothermic?

Answer 11

Endothermic

Question 12

Define first ionisation enthalpy

Answer 12

The enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to produce one mole of gaseous ions with a single positive charge

Question 13

Define second ionisation enthalpy

Answer 13

The enthalpy change when one mole of electrons is removed from one mole of gaseous ions with a single positive charge to form one mole of gaseous ions with a 2+ charge

Question 14

Define third ionisation enthalpy

Answer 14

The enthalpy change when one mole of electrons is removed from one mole of gaseous ions with a 2+ charge to form one mole of gaseous ions with a 3+ charge

Question 15

Define first electron affinity

Answer 15

The enthalpy change when one mole of gaseous atoms forms one mole of negative ions with a single negative charge

Question 16

Define second electron affinity

Answer 16

The enthalpy change when one mole of gaseous ions with a single negative charge forms one mole of gaseous ions with a double negative charge

Question 17

What two assumptions are made by the perfect ionic model?

Answer 17

Bonding in a compound is 100% ionic

The ions are perfect spheres and are not distorted

Question 18

What does a difference in experimental lattice enthalpy and calculated lattice enthalpy show?

Answer 18

The compound has some covalent character

Question 19

How do you calculate the enthalpy of solution?

Answer 19

Enthalpy of lattice dissociation + enthalpy of hydration

Question 20

Define enthalpy of solution

Answer 20

The enthalpy change when one mole of solute dissolves in water

Question 21

Define enthalpy of hydration

Answer 21

The enthalpy change when one mole of gaseous ions is converted into one mole of aqueous ions

Question 22

Are enthalpy of hydration values exo or endothermic and why?

Answer 22

Exothermic bc energy is released when ions are attracted to the polar water molecules

Question 23

What does the value of the lattice enthalpy show?

Answer 23

A measure of the strength of the ionic bond

Question 24

What does the strength of the ionic bond depend on?

Answer 24

Charge of the ion and their size

Question 25

What is mean bond enthalpy?

Answer 25

The enthalpy needed to break one mole of a covalent bond averaged across many compounds containing that bond

Question 26

Why is the bond enthalpy for diatomic molecules not averaged?

Answer 26

Because the bond only occurs in the diatomic molecules

Question 27

Why are the values for second electron affinities positive?

Answer 27

Because there is repulsion between the negative ion and the negative electron being added

Question 28

In a Born Haber cycle, what do arrows going up represent?

Answer 28

Endothermic processes

Question 29

In a Born Haber cycle, what do arrows going down represent?

Answer 29

Exothermic processes

Question 30

Why do smaller ions have a higher lattice enthalpy value?

Answer 30

Smaller so they pack more tightly together

Question 31

Why do ions with a higher charge have a higher lattice enthalpy value?

Answer 31

Form stronger ionic bonds bc more attraction between the ions

Question 32

Why is there a more negative enthalpy of hydration for smaller ions with the same charge?

Answer 32

Bc higher charge density so negative charge on the ion attracts the H+ on the water more strongly

Question 33

What is the first law of Thermodynamics?

Answer 33

Energy cannot be created or destroyed

Question 34

What is enthalpy?

Answer 34

The amount of chemical energy in a system at a constant pressure

Question 35

What is the standard state of a substance?

Answer 35

Its pure, most stable form at 100kPa at a given temperature

Question 36

What is the standard enthalpy change?

Answer 36

The enthalpy change under standard conditions of 298K and 100kPa with all reactants and products in their standard states

Question 37

What is the standard enthalpy change of formation of an element?

Answer 37

0

Question 38

What does Hess’s Law state?

Answer 38

If a reaction can take place by more than one route, the overall enthalpy change is the same, regardless of the route, as long as the final and initial conditions are the same