U2 topic 6 (enthalpy changes)

What is the standard enthalpy change of reaction, ΔᵣH°?

Give an example equation

The enthalpy change when molar amounts of reactants, as shown in the balanced equation, react under standard conditions

E.g:
H₂(g) + ½O₂(g) → H₂O(l)

NOTE:

• Reactants in the amounts shown by the balanced equation

• Standard conditions: usually 298 K and 100 kPa

• Can be exothermic or endothermic

What is the standard enthalpy change of neutralisation, ΔneutH°?

The enthalpy change when when one mole of water is produced by the neutralization of an acid with an alkali, under standard conditions.

Key points:

• Must form exactly 1 mol water as product

• ALWAYS exothermic (in our syllabus)

define standard enthalpy change of formation, ΔfH°?
Give an example equation

- The enthalpy changewhen1 mole of a compound
- from its constituent elements intheir standard states

[like Br(l) and Na(s)]

- under standard conditions of 298K and 1atm

E.g:

C(s) + 2H₂(g) → CH₄(g)


NOTE:

• Must form exactly 1 mol product

• Can be exothermic or endothermic

• elements themselves HAVE NO ΔfH°
  (such as O2, H2 etc)
  
  constituent elements = reactant elements that make up the product/compound

What is the standard enthalpy change of combustion, ΔcH°

The enthalpy change when 1 mole of a substance is completely burned in excess oxygen, under standard conditions.

E.g:
1CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)

• Completely burned burned in excess oxygen

• 1 mol of reactant
  (if q has 2 mol, then make sure to give 2ΔcH° aswell, given in picture)
  
  

• ALWAYS exothermic → ΔcH° is negative

What is the standard enthalpy change of atomisation, ΔatH°?

Give an example

The enthalpy change when1 mole ofgaseousatoms is formed from an element in its standard state, under standard conditions.

Examples:
½Cl₂(g) → 1Cl(g)

Na(s) → 1Na(g)

standard state of Br2 = liquid

Product must be 1 mol and gas cuz it is atomisation

ANSWER = B

State whether these are exothermic or endothermic

combustion

nuetralistation
atomisation

combustion = exo

nuetralistation = exo

atomisation = endo

backward reaction in green ink

since we find ΔfH°, the elements on reactant side MUST BE IN STANDARD STATE
so I2 is solid

ALSO final answer ΔfH° must be 1 mol, so we divide ΔrH° by 2