Energetics

An Enthalpy change, ΔH is the overall energy exchanged with the surroundings when a change happens at constant pressure and the final temperature is the same as the starting temperature.The specific heat capacity of a material, c, is the energy needed to raise the temperature of 1 g of the material by 1K. For water c = 4.2 J g^-1 K^-1.The standard enthalpy change of a reaction, ΔH⊖reaction, is the energy transferred when the molar quantities of reactants as stated in the equation react under standard conditions.The standard enthalpy change of neutralisation is the enthalpy change when the amounts of acid and alkali in the equation for the reaction neutralise each other under standard conditions.The standard enthalpy change of combustion of a substance, ΔH⊖c is the enthalpy change when one mole of the substance burns completely in oxygen under standard conditions.The standard enthalpy change of formation of a compound, ΔH⊖f, is the enthalpy change when one mole of the compound forms from its elements under standard conditions with the elements and the compound in their standard (stable) states.The standard enthalpy change of atomisation of an element, ΔH⊖at, is the enthalpy change when one mole of gaseous atoms is formed from the element under standard conditions.Hess's Law says that the enthalpy change in converting reactants to products is the same regardless of the route taken, provided the initial and final conditions are the same.The bond enthalpy of a particular bond is the energy required to break one mole of the bonds in a substance in the gaseous state.

When 4.0 g of ammonium nitrate (NH4NO3) dissolves in 100cm^3 of water the temperature falls by 3.0C. Calculate the enthalpy change per mole when NH4NO3 dissolves in water under these conditions.Amount of NH4NO3 used = mass / M(NH4NO3) =4 / 80 = 0.05 mol.Assuming that the solution has the same specific heat capacity and density as water:Energy take in from the solution = mass x specific heat capacity x temperature change= 100 x 4.2 x 3= 1260 JEnergy taken in per mole of NH4NO3 = 1260 / 0.05 = 25200 J mol^-1= 25 kJ mol^-1The reaction is endothermic, so the enthalpy change for the system is positive.NH4NO3(s)---->NH4NO3(aq) ΔH = +25kJ mol^-1

When 50 cm^3 of 2.0 mol dm^-3 hydrochloric acid is mixed with 50 cm^3 of 2.0 mol dm^-3 sodium hydroxide in a bomb calorimeter at 25C and 1 atmosphere pressure, the temperature rises by 13.7C. What is the enthalpy change for the neutralisation reaction?The equation for the reaction is: HCl(aq) + NaOH(aq) ----> NaCl(aq) + H2O(l)Amount of HCl used = amount of NaOH used= (50 / 1000) x 2 = 0.1 molEnergy given out and used to heat the solution = 100 x 4.2 x 13.7 = 5754 JEnergy given out per mole of acid = 5754 / 0.1 = 57540 J mol^-1ΔHneutralisation = -57.5 kJ mol^-1Now, as the concentrations of the two solutions were effectively 1.0 mol dm^-3 immediately after mixing, the temperature was 25C and the pressure 1 atmosphere, the value of ΔHneutralisation has been obtained under standard conditions.HCl(aq)+NaOH(aq)---->NaCl(aq)+H2O(l)ΔH⊖neutralisation = -57.5 kJ mol^-1This is called a standard enthalpy change of neutralisation.