Calorimerty i think.... ^.^

[dusty_girl1144]

here is the questions for u chem people out there....

q1: in the calorimeter manual, the calorimeter constant is found to be lower then the calculated value. give a possible explanation for this discrepancy. therefore outline a more suitable methods for calibrating the calorimeter

q2: calculate the energy content of food sample (i dont get what formular im actually meant to use...)
  mass of food : 1.07g
  initial temp : 20.87
  final temp: 26.41
is it mean to be the enthalpy change? :s thats the only formular with an "m" component


q3: what is the heat combustion for the reaction of butane
2g and 100KJ of energy heat used.


thanxs everyone

[cara.mel]

q1 - This means that more energy was used when calculating it than is needed to heat it - some was lost somewhere somehow. I don't remember all the errors but having lid off etc are some. Other people can give better answers

q2 - You need more info than those 3 numbers

q3 - n(C4H10) = m/Mr = 2/58
so 1/29 moles releases 100kJ of energy
=> 1 mole releases 29 *100kJ of energy => delta H = -2900kJ (using ratios)

[dusty_girl1144]

thanxs caramel. but what more info do u need for q 2?

[cara.mel]

Something to do with the calibration factor of what you burnt it in.

Essentially it is saying 'I burnt 1.07g of food and the thing went up by 5 degrees'. But that is useless because from that you can't tell at all how much energy was needed to raise it 5 degrees (imagine having some tiny calorimeter and then one as big as a room, very different amounts of energy would be needed to raise each by the same amount)

[Pandemonium]

you need the calibration results.

eg.
5 volts
2.1 amperes
124 seconds

increase in calorimeter temperature of 5 degrees.

CALIBRATION
Cf = VIt/dT (dT = delta T)
Cf = 5x2.1x124/5 = 260.4J/degreesC

DETERMINING ENERGY CONTENT IN SAMPLE
E(released/absorbed) = Cf.dT (this dT is from the sample)
etc. etc.