Topic 8 Energetics I

It is the heat change in a reaction at a constant pressure. It is given the symbol delta H and has the units of kJmol-1

100kPa pressure 298 Kelvin

They are reactions that absorb energy from the surroundings. The products have a higher energy than reactants Delta H is + Example would be thermal decomposition

They are reactions that release energy to the surroundings The products are lower in energy than the reactants Delta H is - Example would be combustion

Standard enthalpy change of reaction Standard enthalpy change of neutralisation Standard enthalpy change of combustion Standard enthalpy change of formation

The enthalpy change of a reaction according to the molar quantities in the equation under standard equations

The enthalpy change when an acid and alkali react to form 1 mole of water under standard conditions

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions

The enthalpy change when 1 mole of a substance is completely burned in oxygen to make CO2 and H20, under standard conditions

It is the amount of energy needed to break 1 mole of a bond type in a molecule in the gaseous state

To break a bond energy needs to be absorbed Bonds are broken in the reactants and this is an endothermic process so delta H is + If more energy is needed to break bonds than energy given out when bonds are formed, the reaction will be endothermic

When bonds are formed, energy is released Bonds are made when products are being produced and this is an exothermic process so delta H is - If more energy is released when bonds are formed than what was needed to break initial bonds, the reaction will be exothermic

Total energy to break bonds - Total energy released forming bonds reactants - products

It is used to work out the enthalpy change of combustion A fuel is burnt to raise the temp of the water by a specific amount. Generally, we weigh the fuel before and after burning to work out the mass of the fuel burnt A lid is placed on top to prevent heat loss and shield winds The energy from the fuel is transferred into the water although some of it is lost to the surroundings too

q = m c deltaT q - Heat energy lost or gained (J) m - mass of water or solution (g) c - specific heat capacity (4.18) delta T - change in temp (K) Then calc moles and then enthalpy = q/n

Can use solutions Have a polystyrene cup to prevent heat loss Add acid first - measure temp Then add alkali/solid, stir and measure temp change

The total enthalpy change of a reaction is independent of the route taken.

If you are given enthalpy of formation, you draw a formation cycle Reactants —–> products at the top and then elements in their standard states at the bottom. Arrows have to be pointing upwards

If you are given combustion data you draw a combustion cycle Reactants —–> products at the top and combustion products at the bottom Arrows have to be pointing downwards