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 Form 4 Chemistry: Energy changes in chemical and physical processes lessons

Determination or measurement of Enthalpy (heat) changes.

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Answer Text:
Determination\measurement of Enthalpy (heat) changes.
-Calorimeters are sued and have to be insulated to reduce heat loss to the surrounding.
Source of errors:
- Heat loss to the surrounding.
- Absorption of heat by the calorimeter (vessels). Main heat changes under consideration:
- Enthalpy of combustion (ΔHc), Enthalpy of neutralization (ΔH neut), Enthalpy of solution (ΔHsoln) ,Enthalpy of precipitation (ΔH precip), Enthalpy of
displacement (ΔHDisp), Enthalpy of formation (ΔHf)
1. Heat of combustion ΔHc
- Is the heat changes when the mole of a substance is completely burned in oxygen, at one atmospheric pressure.
- Since heat is usually evolved and hence ΔH is usually negative.
Example;
#C(s) + O_2(g) to CO_2(g) ; Δ H= -394 Kjmol^-1#
Apparatus for finding ΔHc of a fuel. Heat evolved = Specific heat capacity of water x
Mass of water x
Temperature rise = CMΔT; joules, where;
C= specific heat capacity.
M = mass of water.
ΔT= temperature change.
= CMΔT; joules,where; C = specific heat capacity
M = mass of water.
ΔT= temperature change.
Examples
1. Assume Volume of water in calorimeter = 100#cm^3# Initial temperature of water = #225^0C#
Final temperature of water= #50.5^0C#
Change in temperature of water = #28.0^0C#
Mass of water= 100g
Mass of lamp before burning= 30.46g
Mass of lamp after burning = 30.06g
Mass of ethanol burnt = 30.46 -30.06= 0.40g
(a). Determine the heat evolved;= Δ H= CMΔ T =4.2 x 100 x 28 =11760 Joules =11.760 KJ
(b). Hence calculate the molar heat of combustion of ethanol;
Moles of ethanol burnt = Mass of ethanol burnt; 0.4 = 0.008695 moles.
RFM of ethanol 46 Thus if 0.008695 moles =11.76 kilojoules,
Then 1mole = ?
1 mole = 11.76 x 1 = -1352.50 KJMol-
0.008695
= - 1352.5 KJMol-1 (negative value as the reaction is exothermic since heat is evolved).
2. When ethanol was burnt in the apparatus shown (in example 1), the results were: mass of fuel burnt, M1=1.50g; mass of water, M2=500g; ΔT= #13.0^0C#.
(C=12; H=1; O=16; SHC of water=4.18KJKg-K-). Find the molar enthalpy of combustion of ethanol.
Compare the experimental value with the listed value of –1368 #KJMol^(-)# and explain any difference.
Solution:
Heat evolved = CMΔT = 4.18 x 500 x13 1000 = 27.17KJ
Molar mass of ethanol,
C2H5OH = [(12 x 2) + (1 x6) + (1 x 16)] = 46
Thus if 1.5g = 27.17KJ
Then 46 g = ?= 46 x 27.17
1.5 = 833.213KJMol-
3. In an experiment to determine the heat of combustion of methanol, CH3OH a student used a set up like the one shown in the diagram below. Study the set up and the data and answer the questions that follow.
Data:
Volume of water =500 #cm^3#
Final temperature of water=#27.0^0C#
Initial temperature of water =#20.0^0C#
Final mass of lamp + menthol = 22.11g
Initial mass of lamp +methanol =22.98g
Density of water-# 1gcm^(-3)#