AS Chemistry - Enthaply Change

Enthalpy Change = the heat energy transferred in a reaction at constant pressure.

units - KJ mol-1

Standard Conditions:

• 100kpa

•  298K

Exothermic Reaction - gives out energy, negative (combustion, respiration)

Endothermic Reaction - takes energy in, positive (photosynthesis, thermal decomposition)

Reactions = braking bonds

You need energy to make bonds - endothermic

You give out energy when bonds are formed - exothermic

Calculating bond enthalpies:

Using average bond enthalpies: not accurate because the bond enthalpies are averages based on bond strength in many different compounds.

Enthalpy = total energy given out - total energy released

Standard enthalpy change for a reaction.

The heat energy transferred in a reaction under standard conditions.

Standard enthalpy change of formation.

The energy change when 1 mole of compound is formed from its elements in their standard states under standard conditions.

Standard enthalpy change of combustion.

The energy when 1 mole of substance is completely burnt in oxygen under standard conditions.

2 things must know 

• change in temperature
• number of moles of whats reacting
  

                      q = mcT

q = heat loss or gain in joules

m = mass of water

c = specific heat capacity of water 4.18

T = change in temperature

If answer is positive its an endo, if negative then exo reaction.

Not accurate because

• fuel may not have been burnt completely

• heat excapes from the calormeter

Hess's Law - if given values for enthalpy of combustion