2. measuring enthalpy change (CALORIMETRY)

calorimetry experiments

measure the heat given out by reactions

reactions that occur in solutions

• put reactants in a container that insulates heat to reduce heat loss/gain

• use thermometer to measure temperature at regular intervals

you can find out how much energy is given out from combustion reactions by…

…measuring the temperature change it causes as it burns

to calculate the heat energy absorbed by water as fuel burns you need:

• mass of water

• temperature change ΔT

• specific heat capacity of water 4.18 J g^-1K^-1

in practice heat will always be lost to the surrounding

this makes it hard to get accurate results

in practice some fuel is burnt via incomplete combustion

meaning less energy is given out

flammable liquids are often too volatile

so you may loose some of the fuel to evaporation

calculating temperature change

highest temperature recorded - initial temperature: this method is not accurate because of the heat lost from the calorimeter to surroundings

calculating accurate temperature change

• during experiment record temperature at regular intervals stating a few minutes before the reaction starts

• draw 2 lines of bets fit

• extrapolate both lines so they both pass the time when the reaction started

• the distance between both lines is the temperature change

equation to calculate enthalpy change

q=mcΔT

q=mcΔT units

• q=heat lost/gained

• m=mass in g of solution in polystyrene beaker

• c=specific heat capacity of water, 4.18

• ΔT=temperature change

calculating standard enthalpy change of combustion

• calculate the amount of heat lost or gained using q=mcΔT REMEMBER YOU NEED TO CHANGE THE UNITS OF q FROM J TO KJ BECAUSE STANDARD ENTHALPIES OF COMBUSTION ARE ALWAYS IN KJ mol ^-1

• calculate the number of moles of fuel caused the enthalpy change using n=Mass÷Mr

• calculate the standard enthalpy of combustion using Δ_cH^∘=q÷n

ERRORS:

• heat lost to surrounding

• assums all solutions have water’s specific heat capacity

• any heat absorbed by the apparatus is ignored

• incomplete reaction

• density of solution not the same as water

• water evapourated

• room temperature changed

• incomplete conbustion of fuel

IMPROVE ACCURACY

• Enclosed flame

• burn fuel in pure oxygen

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