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.

Enthalpy change that occurs when when 1 mole of a compound is formed from its constituent elements under standard conditions and all reactants and products in their standard states.

the enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions and all reactants and products being in their standard states.

Tempreture-298k(25° c) pressure-100kpa(100,000pa) concentration-1mol dm-3

• energy is transferred into surrounding • The products have less energy than the reactants • negative ΔH

• Heat is taken in from the surroundings to the system, increasing the temperature of the surroundings • positive ΔH • products have more energy than reactants

△fH=(sum of △fH of product)-(sum of △fH of reactant)

ΔcH=(sum of ΔcH of reactant)-(sum of ΔcH of product)

q = mcΔT q=heat exchanged with surroundings (J) m=mass of the substance heated/cooled (g) c=specific heat capacity (J g-1 K–1) ΔT=change in temperature(K or °C )

• Using q = m x cp x ΔT 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 ΔH • A)dd a sign and unit (divide by a thousand to convert Jmol-1 to kJmol-1

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

Calorimetry is the process of measuring the amount of heat given off or taken in during a chemical reaction

1-washes the equipment(cup and pipettes etc)with the solutions to be used 2-dry the cup after washing 3-put polystyrene cup in a beaker for insulation and support 4-Measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup 5-clamp thermometer into place making sure the thermometer bulb is immersed in solution 6-measure the initial temperatures of the solution or both solutions if 2 are used. Do this at regular intervals like every minute for 2-3 minutes until a trend is observed 7-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. 8-If using a solid reagent then use ‘before and after’ weighing method 9-stirs mixture (ensures that all of the solution is at the same temperature) 10-Record temperature every minute after addition for several minutes

• measure & record initial temperature of the solution for a few minutes before addition? • measure &record the temperature after the addition at regular intervals like each minute for 2-3 minute

• plot a graph of temperature against time • extrapolate the temperature curve/line back to the time the reactants were added together to determine ΔT at the point of addition

-incomplete combustion -evaporation -density of solution is taken to be the same as water. -The method assumes all solutions have the heat capacity of water. -energy transfer from surroundings (usually loss)