Enthalpy Change Definitions

Question 1

Define the standard enthalpy of formation

Answer 1

• the enthalpy change when ONE mole of a substance is formed
• from its elements under standard conditions
• all reactants + products being in their standard conditions

Question 2

What is the standard enthalpy of formation of all elements in their standard states?

Answer 2

Zero

Question 3

Why is the enthalpy of formation of Na(s) zero?

Answer 3

• element is already in its natural state at room temp

Question 4

Why is the enthalpy of formation of liquid Na not zero?

Answer 4

• not naturally a liquid at room temp
• Na (l) is not standard state of Na

Question 5

Define standard enthalpy of combustion

Answer 5

• the enthalpy change when one mole of a substance is completely burnt
• in excess oxygen
• under standard conditions, all reactants + products being in standard states

Question 6

Why must all standard enthalpies of combustion have negative values?

Answer 6

• burning a substance in oxygen is always exothermic

Question 7

What enthalpy of combustion do substances like water, carbon dioxide and most other oxides have?

Answer 7

Zero as they cannot be combusted

Question 8

Why may the enthalpy of combustion of a compound be difficult to measure?

Answer 8

• incomplete combustion
• heat loss to surroundings

Question 9

Define the standard enthalpy of hydration

Answer 9

• standard enthalpy change when one mole of gaseous ions
• is converted into one mole of aqeous ions

Question 10

Why is the enthalpy of hydration always exothermic?

Answer 10

The bonds in enthalpy of hydation are made of water molecules.

Question 11

The bonds in enthalpy of hydation are made of water molecules, what does this mean?

Answer 11

• enthalpy of hydration is always exothermic

Question 12

Describe hydration of the ions in terms of standard enthalpy of hydration

Answer 12

• ions bond with water molecules
• pos ions att to neg oxygen in water molecules
• neg ions att to pos hydrogens in water molecules