Transcript Document

AS Chemistry
Unit 2, Module 3, Energy
Enthalpy 4
Standard enthalpy of
Combustion
Objectives - Enthalpy
2.3.1 Enthalpy Changes, Candidates should be able to:
• (a) explain that some chemical reactions are
accompanied by enthalpy changes, exothermic (ΔH,
negative) or endothermic ΔH, positive);
• (f) define and use the terms:
– (i) standard conditions,
– (ii) enthalpy change of reaction,
– (iii) enthalpy change of formation,
– (iv) enthalpy change of combustion;
Definitions
•
Without referring to your notes, write a
definition of:
Enthalpy change of combustion
The enthalpy change that takes place
when one mole of a substance reacts
completely with oxygen under standard
conditions, all reactants and products in
their standard states.
Standard Enthalpy
Change of combustion
Standard enthalpy change of combustion, ΔHcӨ
Close your text books
Write equations for the standard enthalpy of
combustion of:
• CH4
ΔHcӨ = -890
• H2
ΔHcӨ = -286
• Al
ΔHcӨ = -1676
Write out the equations in full including state
symbols, and the appropriate units.
Experimental determination of
standard enthalpy change of
combustion
What do you need to do to measure a ΔHcӨ?
1. Burn a known mass of fuel
2. Heat a known mass of water
3. Measure the temperature change of the
water
On the paper, draw apparatus to do this.
Experimental determination of
standard enthalpy change of
combustion
What is
wrong with
this
method?
How could
we
improve
it?
Practice…
•
•
2g of propan-1-ol heated 250cm3 of water by
50°C. Find the enthalpy change of combustion
of propan-1-ol
Specific heat capacity water =4.18 J.g-1.K-1
Density of water = 1g.cm-3
Help:
1.
2.
3.
4.
Find heat change of experiment
Find the amount in moles of propanol that reacted
Work out heat loss in kJ.mol-1
Report answer ΔHcӨ
Experimental determination of
standard enthalpy change of
combustion
In these experiments we often end up with a
value for ΔHcӨ that is too low.
Why?
1. There may have been incomplete
combustion
2. There may have been heat loss to the
surroundings (not heating the water)
How could we reduce these?
Experimental determination of
standard enthalpy change of
combustion
A bomb
calorimeter
How does
this reduce
errors?
• Practical 7.6 Nuffield
• Pages 154 - 156
Questions page 195
1) Calculate the enthalpy changes of
combustion for butan-1-ol and decane
using the experimental results below. Give
both answers to three significant figures.
a) 1.51 g of butan – 1 – ol,
CH3CH2CH2CH2OH, heated 300 cm3 of
water by 42.0 oC.
b) 0.826 g of decane heated 180 cm3 of water
from 22.0 oC to 71.0 oC
• Past exam questions