3.6 - Enthalpy Changes for Solids & Solutions

Question 1

What does Hess’ law state?

Answer 1

the total enthalpy change of a reaction is independent of the route taken

Question 2

What is the crystal coordination number of sodium chloride?

Answer 2

6:6

Question 3

What is meant by lattice enthalpy? (of an ionic compound)

Answer 3

the enthalpy change when one mole of ionic compound is formed from its gaseous ions under standard conditions

Question 4

Since lattice enthalpy of a compound cannot be determined directly, what must be used instead?

Answer 4

must be found indirectly using a Born-Haber cycle

Question 5

What is meant by the standard enthalpy of formation?

Answer 5

the enthalpy change that takes place when one mole of a compound in its standard state is formed from its elements in their standard states under standard conditions

Question 6

What is meant by the standard enthalpy of atomisation?

Answer 6

the enthalpy change accompanying the formation of one mole of gaseous atoms from the element in its standard state

Question 7

What is meant by the first ionisation energy of an element?

Answer 7

the enthalpy change accompanying the complete removal of one mole of electrons from one mole of gaseous atoms, forming one mole of gaseous 1+ ions

Question 8

What is meant by the first electron affinity?

Answer 8

the enthalpy change accompanying the addition of one electron to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions

Question 9

When constructing a Born-Haber cycle, which way do the arrows face for an endothermic reaction and an exothermic reaction?

Answer 9

• draw a scale arrow up for an endothermic reaction (+)
• draw a scale arrow down for an exothermic reaction (-)

Question 10

When constructing a Born-Haber cycle, what should the order always be?

Answer 10

1. atomisation of the metal
2. ionisation of the metal
3. atomisation of the non-metal
4. electron affinity of the non-metal (which is exothermic)

Question 11

What is the enthalpy change associated with water molecules hydrating oppositely charged molecules?

Answer 11

the enthalpy change of solution △Hθ sol.

Question 12

What are two factors that affect lattice breaking and enthalpy of solution?

Answer 12

• size of the ion
• charge on the ion

Question 13

How does decreasing the size of the ion affect the lattice breaking and enthalpy of solution?

Answer 13

the smaller the ion, the closer together they are meaning the attraction is greater.

This means that more energy is required for lattice breaking and because smaller ions have a higher charge density, they can attract water molecules more strongly. This makes the enthalpy of hydration more exothermic (more negative).

Smaller radius
Higher charge density
Stronger attraction
Higher Lattice Energy.

Question 14

How does increasing the charge of the ion affect the lattice breaking and enthalpy of solution?

Answer 14

Larger radius
Lower charge density
Weaker attraction
Lower Lattice Energy

Question 15

What is meant by the standard enthalpy of solution?

Answer 15

the enthalpy change that occurs when 1 mole of a substance dissolves in a solvent under standard conditions

Question 16

What is the enthalpy change of a solution?

(i.e. how can it be found?)

Answer 16

the sum of the enthalpy of lattice breaking and the enthalpy of hydration

Question 17

Is lattice breaking endothermic or exothermic?

Answer 17

endothermic

Question 18

Is hydration endothermic or exothermic?

Answer 18

exothermic

Question 19

What is the equation to find the enthalpy change of solution?

Answer 19

△Hθ sol = △Hθ lat. breaking + △Hθ hydration

Question 20

Why would knowing the lattice breaking and hydration values allow you to determine of the solid is soluble?

Answer 20

because if the △Hθ hydration is greater than the △Hθ lat. breaking then the solid will dissolve.

The more exothermic the △Hθ sol. value, the more soluble the salt is

Question 21

How do you calculate the enthalpy change of a reaction?

Answer 21

△ᶠHθ(for all products) - △ᶠHθ(for all reactants)

Question 22

What are the standard conditions?

Answer 22

298K (25 degrees celsius)
1 mol dm⁻³ concentration
101kPa or 1atm pressure

Question 23

What is meant by the standard enthalpy of hydration?

Answer 23

the enthalpy change when one mole of an ionic compound in solution is formed from ions of the elements in the gas phase

Question 24

What does a Born-Haber cycle involve? (What happens generally)

Answer 24

the energy cycle involves turning all the initial elements into gas phase atoms, then into ions and then combining these back to form a solid or a solution