energetics

Enthalpy change is the amount of heat energy taken in or given out during any change in a system provided the pressure is constant.

The standard enthalpy change of formation of a compound is the enthalpy change when 1 mole of the compound is formed from its elements under standard conditions (298K and 100kpa), all reactants and products being in their standard states.

The standard enthalpy of combustion of a substance is defined as the enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions. (298K and 100kPa), all reactants and products being in their standard states

soot (carbon), carbon monoxide and water. It will be less exothermic than complete combustion.

• 100 kPa pressure • 298 K (room temperature or 25oC) • Solutions at 1mol dm-3 • all substances should have their normal state at 298K

• washes the equipment (cup and pipettes etc) with the solutions to be used • dry the cup after washing • put polystyrene cup in a beaker for insulation and support • Measure out desired volumes of solutions with volumetric pipettes and transfer to an insulated cup • clamp thermometer into place making sure the thermometer bulb is immersed in a solution • measure the initial temperatures of the solution or both solutions if 2 are used. Do this every minute for 2-3 minutes • At minute 3 transfer second reagent to cup. If a solid reagent is used then add the solution to the cup first and then add the solid weighed out on a balance. • If using a solid reagent then use ‘before and after’ weighing method • stirs mixture (ensures that all of the solutions is at the same temperature) • Record temperature every minute after addition for several minutes

• energy transfer from surroundings (usually loss) • approximation in specific heat capacity of the solution. The method assumes all solutions have the heat capacity of water. • neglecting the specific heat capacity of the calorimeter- we ignore any energy absorbed by the apparatus • reaction or dissolving may be incomplete or slow. • density of solution is taken to be the same as water

• Using q = m x cp x T calculate energy change for quantities used • Work out the moles of the reactants used • Divide q by the number of moles of the reactant not in excess to give H • Add a sign and unit (divide by a thousand to convert Jmol-1 to kJmol-

• Energy losses from the calorimeter • Incomplete combustion of fuel • Incomplete transfer of energy • Evaporation of fuel after weighing • Heat capacity of calorimeter not included • Measurements are not carried out under standard conditions as H2O is gas, not liquid, in this experiment

Hess’s law states that total enthalpy change for a reaction is independent of the route by which the chemical change takes place

reactants-products

products-reactants

The mean bond energy is the enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules. H = Σ bond energies broken - Σ bond energies made

If the results are worked out experimentally using a calorimeter the experimental results will be much lower than the calculated ones because there will be significant heat loss. There will also be incomplete combustion which will lead to less energy being released. Remember that calculated values of enthalpy of combustions will be more accurate if calculated from enthalpy of formation data than if calculated from average bond enthalpies. This is because average bond enthalpy values are averaged values of the bond enthalpies from various compounds.

The activation energy, EA, is defined as the minimum energy which particles need to collide to start a reaction

Particles can gain energy through collisions