Energetics

• Most chemical reactions are accompanied by a change in heat energy
• The amount of heat change is given the symbol:

 ∆H 

• For an exothermic reaction, this sign is negative

• For an endothermic reaction, this sign is positive

∆H 

• Energy is given out

• E.g. Reaction between sulfuric acid and magnesium
• There is a rise in temperature
• This is where new bonds are made

• Energy is taken in
• E.g. Reaction between sodium hydrogen carbonate and citric acid
• There is a fall in temperature
• This is where bonds are broken

Heat given out = 

mass of solution x specific heat capacity of solution x temperature rise

abreviated to:

Q=mc∆T 

m=mass of solution  c=specific heat capacity ∆T=temperature rise

• The specific heat capacity of the solution is the amount of heat needed to raise the temperature of one gram of solution by one degree Celsius
• The specific heat capacity of water is 4.2 (joules per gram per degree Celsius)

• For reactions carried out at a constant pressure, the heat change is known as the enthalpy change
• The enthalpy change per mole is called the molar enthalpy change and given symbol ∆T

Most common experiments are those using fuels such as alcohol. The liquid is burnt and the heat generated is used to heat up water. The temperature rise of the water is measured and the heat generated is calculated the same as before. 

• set up equipment as follows

• measure start and finish temp of water
• measure start and finish mass of alcohol

• The energy required to break a bond is called bond energy
• Usually measured in kilojoules per mole of bonds (kJ/mol)

Most common at IGCSE:

C-C = 412 H-H  = 436 O=O = 496 H-Cl  =432 Cl-Cl = 242

C=O = 743 O-H  = 463

Step 1: calculate the sum of energies for the bond broken

Step 2: calculate the sum of the energies of the bonds made

Step 3: calculate ∆H using the formula 

• ∆H = bonds broken - bonds made