AC15: Lattice energy

Question 1

When heat is released, is the reaction endo or exo thermic

Answer 1

Exothermic

Question 2

For an exothermic reaction, is the enthalpy change positive or negative

Answer 2

Negative

Question 3

When heat is absorbed is the reaction endo or exo thermic

Answer 3

Endothermic

Question 4

For an endothermic reaction, is the enthalpy change positive or negative

Answer 4

Positive

Question 5

What are the standard conditions for enthalpy changes

Answer 5

• 1 atm
• 298 K / 25 degrees

Question 6

Define the standard enthalpy change of a reaction

Answer 6

The enthalpy change when the number of moles of reactants shown in an equation react together under standard conditions, all reactants being in their standard states

Question 7

Define the standard enthalpy of formation of a compound

Answer 7

The enthalpy change when 1 mole of a compound is formed form its ELEMENTS in their standard states, under standard conditions

Question 8

Define bond enthalpy

Answer 8

The enthalpy change required to break 1 mole of bonds between 2 atoms in the gaseous state

Question 9

Define first ionisation energy of an element

Answer 9

The energy needed to remove 1 electron from every atom in 1 mole of gaseous atoms of an element

Question 10

Define second ionisation energy of an element

Answer 10

The energy needed to remove 1 electron from every ion in 1 mole of gaseous +1 ions of an element

Question 11

Are ionisations endo or exo thermic

Answer 11

Endothermic

Question 12

Why are all ionisations endothermic

Answer 12

Because energy is needed to overcome the attraction between the outer electrons and the nucleus

Question 13

Is the first or second ionisation more endothermic

Answer 13

Second ionisation

Question 14

Why is the second ionisation more endothermic

Answer 14

Because the number of protons remain the same but 1 less electron, so there is less repulsion between the remaining electrons, allowing them to move in closer to the nucleus, where they are more strongly attracted

Question 15

What does Hess’s law state about the enthalpy change in a chemical reaction

Answer 15

Independent of the route taken by the reaction, provided the initial and final conditions are the same

Question 16

Define enthalpy change of atomisation of an element

Answer 16

The enthalpy change when 1 mole of gaseous atoms of an element are formed from the element in its standard state

Question 17

Why does aluminium have a greater enthalpy change of atomisation

Answer 17

Since the number of out shell electrons increases, so there are more electrons contributing to the metallic bond which requires more energy

Question 18

Define the first electron affinity of an element

Answer 18

The enthalpy change when 1 electron is added to each atom in a mole of gaseous atoms of an element

Question 19

Is the first electron affinity of an element that commonly forms negative ions positive or negative

Answer 19

Negative

Question 20

Why is the first electron affinity of an element that commonly forms negative ions negative

Answer 20

Because the electron that is added is attracted to the nucleus more strongly than it is repelled by the other electrons

Question 21

Are second electron affinities positive or negative

Answer 21

Positive

Question 22

Why are second electron affinities always positive

Answer 22

Energy must be put in to overcome the repulsion between the negatively charged X- ion and the e- that is added

Question 23

Define lattice enthalpy

Answer 23

The enthalpy change when 1 mole of a sold ionic compound is formed from its gaseous ions

Question 24

What is lattice enthalpy a measure of

Answer 24

The strength of ionic bonding in a compound

Question 25

Are lattice enthalpies always endo or exo thermic

Answer 25

Exothermic

Question 26

Why are lattice enthalpies always exothermic

Answer 26

Heat is released when ions come together the form ionic bonds

Question 27

If the lattice enthalpy is more exothermic, is the ionic bonding weaker or stronger

Answer 27

Stronger

Question 28

What are the 2 factors that affect the lattice enthalpy

Answer 28

- The size of the ions - The charge on the ions

Question 29

How does the size of the ions affect the lattice enthalpy

Answer 29

Smaller the ionic radii of the ion, the greater their charge density, hence stronger the attraction between the ions and the more exothermic the lattice enthalpy

Question 30

How does the charge on the ions affect the lattice enthalpy

Answer 30

Greater the charge, the greater their charge density, so the stronger the attraction between the ions, and the more exothermic the lattice enthalpy

Question 31

What is the name of the lattice enthalpy cycles

Answer 31

Born-Haber cycle

Question 32

Going down a group do the lattice enthalpies become more or less exothermic

Answer 32

Less exothermic

Question 33

Why do lattice enthalpies become less exothermic going down a group

Answer 33

- The charge on the ion stays the same but the ionic radii increases, which decreases charge density so attraction to oppositely charged ions becomes weaker

Question 34

Do lattice enthalpies become more or less exothermic across a group

Answer 34

More exothermic

Question 35

Why do lattice enthalpies become more exothermic across a group

Answer 35

The charge on the ion increases, and the ionic radii decreases due to the increase in nuclear charge, so there's increased attraction to other ions

Question 36

Define enthalpy change of solution

Answer 36

The enthalpy change when 1 mole of a compound dissolves in a solvent

Question 37

When an ionic solid dissolves, what are the 2 steps in the process

Answer 37

- Lattice breaks, endothermic reaction - Separated ions are hydrated, ion-dipole forces between ions and water, exothermic

Question 38

Define enthalpy change of hydration

Answer 38

Enthalpy change when 1 mole of gaseous ions dissolve to form one more of hydrated ions

Question 39

Is the enthalpy change of solution endo or exo thermic

Answer 39

Either, it depends on the balance between the lattice enthalpy and the hydration enthalpies

Question 40

What 2 factors affect the enthalpy change of hydration of an ion

Answer 40

- Ionic charge - Ionic radii

Question 41

How does ionic charge affect the enthalpy change of hydration of an ion

Answer 41

The greater the charge, the stronger the attraction between the ion and the water molecule, the more exothermic the enthalpy change of hydration

Question 42

How does ionic radii affect the enthalpy change of hydration of an ion

Answer 42

The greater the radii, the weaker the attraction between the ion and the water molecule, so the less exothermic the enthalpy change of hydration

Question 43

What are the bonds that form between ions and water

Answer 43

Ion-dipole forces

Question 44

Explain why the first ionisation energy of sodium is more endothermic that that of potassium (2 marks)

Answer 44

- Sodium has a smaller atomic radius - nuclear attraction increases

Question 45

Explain why the lattice enthalpy of sodium oxide is more exothermic than that of potassium oxide (2 marks)

Answer 45

- Sodium ions have a smaller ionic radius - Sodium ion has a greater attraction to the O2- ion

Question 46

Explain what is meant by the term enthalpy change of hydration (2 marks)

Answer 46

- 1 mole of gaseous ions react - Gaseous ions dissolve in water

Question 47

Predict how the enthalpy changes of hydration of F- and Cl- would differ, explain your answer (2 marks)

Answer 47

- F- more exothermic since F- has a smaller size - F- has a greater attraction to water

Question 48

Define the term lattice enthalpy (2 marks)

Answer 48

The formation of one mole of an ionic compound from its gaseous ions under standard conditions

Question 49

Explain what is meant by the term enthalpy change of solution (1 mark)

Answer 49

1 mole of a compound/ substance/ solid/ solute dissolving in water

Question 50

Student carries out a test with twice the mass of CaCl2. All other quantities are very similar, predict any differences between the temperature change and the calculated value of enthalpy change of solution from the experiment, explain your reasoning (2 marks)

Answer 50

- Temperature change is doubled and the enthalpy change of solution is the same - Twice the energy produced in the same volume and the ration of energy produced to mass or number of moles is the same

Question 51

How would the lattice enthalpies of magnesium chloride and calcium chloride differ, explain your answer (3 mark)

Answer 51

- Lattice enthalpy of magnesium chloride is more exothermic than calcium chloride - Because magnesium ion is smaller than calcium ion - Therefore the attraction between magnesium ion and chloride ion is greater