3.6 Enthalpy Changes For Solids And Solutions

Question 1

Enthalpy change of reaction

Answer 1

The enthalpy change when the number of moles of the substances in the equation react under standard conditions

Question 2

What are standard conditions

Answer 2

298 K
1 atm
1 moldm-3

Question 3

Enthalpy change of atomisation

Answer 3

Enthalpy change to form one mole of atoms in the gas phase under standard conditions

Question 4

Enthalpy change of lattice formation

Answer 4

Enthalpy change that occurs when one mole of an ionic compound is formed from ions of the elements in the gas phase under standard conditions

Question 5

Enthalpy change of lattice breaking

Answer 5

Enthalpy change that occurs when 1 mole of an ionic compound is broken up into ions of the elements in the gas phase under standard conditions

Question 6

Enthalpy change of hydration

Answer 6

Enthalpy change that occurs when one mole of gaseous ions is surrounded by water molecules to make a solution under standard conditions

Question 7

Enthalpy change of solution

Answer 7

Enthalpy change that occurs when 1 mole of an ionic compound dissolves in water to form a solution under standard conditions

Question 8

First ionisation energy

Answer 8

Energy required to remove 1 mole of electrons from one mole of atoms in the gaseous state

Question 9

1st electron affinity

Answer 9

Enthalpy change that occurs when one mole of gaseous 1- ions are formed from gases atoms

Question 10

How does the attraction occur between water and ionic crystals

Answer 10

Water is highly polar
The positive ends of the polar water molecules are attracted to negative ions in the crystal
The negative ends of the polar water molecules are attracted to positive ions in the crystal

Question 11

In terms of energy what is needed for a lattice to be able to be broken down

Answer 11

The energy supplied by the hydration process must be larger than the lattice breaking energy

Question 12

If the hydration energy is larger than the lattice breaking energy then

Answer 12

The substance dissolves as the /\H*solution is negative

Question 13

If the hydration energy is less than the lattice breaking energy then

Answer 13

The substance is relatively insoluble as the /\H*Solution is positive

Question 14

What does the exothermicity of /\fH* indicate

Answer 14

It is negative which means energy is given out in the formation of the compound from its elements
The ionic compound is more stable than the elements from which it is formed
The more negative it is the more stable the ionic compound

Question 15

What does the endothermicity of /\fH+ indicate

Answer 15

It is positive which means the ionic compound is unstable compared with elements from which it is formed
It can exists but energy is needed to convert elements to compound
Once formed they tend not to decompose as this is too slow

Question 16

What does the solubility of a compound mainly depend on

Answer 16

The enthalpy of solution being exothermic
That means the enthalpy of hydration must be greater than the enthalpy of lattice breaking