|These are word documents, describing the
different methods of Hess Law!
Exam questions (in word format)
Experimental Measurement of Enthalpy Change
Need to understand the difference between the two terms heat and temperature. While they are related, they are not the same thing!
So a bucket full of water at 50ºC would have the same temp. as a 250ml beaker of water at the same temperature, but the heat content of the bucket would be bigger.
We need a new term.
Specific Heat Capacity, is the amount of heat required to raise the
temperature of 1g of a substance by 1K.
This new term gives us a new equation to learn:
Heat transfer = mass x specific heat capacity x temperature change
Heat transfer = m c DT
Units are absolutely essential. Specific Heat Capacity is normally given symbol c in units J g-1 K-1. As a result m must be in grams and DT can either be in ºC or K.
Calorimetry was invented to study the heat transfer to water in this way using a calorimeter.
This is basically a method identical to that alcohol's investigation!
Heat gained by water in calorimeter = m x c x DT
= 150g x 4.2 J g-1 K-1 x 20ºC
= 12,600 J Heat produced by burning 0.9g of ethanol = 12.6kJ
A simple calorimeter.
Since 0.9g is 0.9/46 mol ethanol:
Heat produced by burning 1 mol of ethanol = 12.6 kJ / (109/46) kJ mol-1=
Answer is much smaller than the accepted value of -1371 kJ mol-1.
For accurate methods look up in Fullick p114, look up and make a brief note including diagram - important to know what is inside the bomb.
This is normally called molar enthalpy of solution, or in this case we can be more specific and call it molar enthalpy change of neutralisation.
Heat absorbed by solution = m x c x DT
No. moles HCl = 1.0 mol dm-3 ´ (50cm3/1000cm3)
DHN = -54.6 kJ mol-1
Go through temperature correction method - if necessary - for slower reactions where there will be a greater heat loss. This is the method we did in class with the graph which actually gets some pretty impressive results!
Most enthalpy changes cannot be measured directly by experimental methods. An indirect method must be used.
These calculations are based on The first Law of Thermodynamics
Energy cannot be created or destroyed. It can only be converted from one form to another.
From this two important deductions are made:
1. If the enthalpy change of a reaction is known, then the enthalpy change of the reverse reaction is the same but with the sign changed
C (graphite, solid) + O2(g) ® CO2(g) DH = -393 kJ mol-1
CO2(g) ® C (graphite, solid) + O2(g) DH = +393 kJ mol-1
NB The positive sign for an endothermic change is necessary!