To work out the enthalpy change you do:
the sum of bond energies in the reactants - the sum of bond energies in the products
Given average bond energies, you can easily work out the sum of bond energies.
For example, if we are told that
C-H = 413
O=O= 498
C=O= 746
H-O=464
Work out the enthalpy change for this reaction: CH4 + 2O2 > CO2 + 2H2O
((4 x C-H) + (2 x O=O)) - ((2x C=O) + (4 x H-O))
Then we substitute in:
(4 x 413) + (2 x 498) - (2 x 746) + (4 x 464) = 2648KJ - 3348KJ = 700KJ enthalpy change
Credit to author of this source: https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&cad=rja&ved=0CFoQFjAE&url=http%3A%2F%2Fchemistryatdulwich.wikispaces.com%2Ffile%2Fview%2FSection%2B4%2Bb%2Benergetics.doc%2F183320535%2FSection%25204%2520b%2520energetics.doc&ei=a72UUZOaHYa_0QXMkoD4Bg&usg=AFQjCNF9yn2bAWyGOPYSO4W1NrJExx7HQw&sig2=_XxB8HEYh8MHVUe5abc6sA&bvm=bv.46471029,d.d2k
Showing posts with label Energetics. Show all posts
Showing posts with label Energetics. Show all posts
Thursday, 16 May 2013
4.15 understand that the breaking of bonds is endothermic and that the making of bonds is exothermic
Breaking bonds requires energy, this takes in heat: endothermic.
Making bonds releases energy, this gives out heat: exothermic.
Making bonds releases energy, this gives out heat: exothermic.
4.14 represent exothermic and endothermic reactions on a simple energy level diagram
Exothermic: lower energy level at end;
Endothermic: more energy at end;
Activation energy is the energy requires to start the reaction.
| gcsescience |
| docbrown |
4.13 understand the use of ΔH to represent enthalpy change for exothermic and endothermic reactions
ΔH is the symbol that represents the amount of energy lost or gained in a reaction.
+ΔH is endothermic (because it gains heat)
-ΔH is exothermic (because it looses heat)
+ΔH is endothermic (because it gains heat)
-ΔH is exothermic (because it looses heat)
4.12 calculate molar enthalpy change from heat energy change
|
ΔH
=
|
T
(C)
x mass of H2O
(g) x 4.2J/g C./
|
|
number
of moles
|
ΔH
is measured in J/mol or
kJ/mol.
- ΔH = molar enthalpy change
- T = temperature increase or decrease caused by the reaction
- mass of H2O = mass of water in grams;
- 4.2J/g C = specific heat capacity of water
4.11 describe simple calorimetry experiments for reactions such as combustion, displacement, dissolving and neutralisation in which heat energy changes can be calculated from measured temperature changes
Measure the temperature at the beginning of the experiment, measure the temperature at the end. How ever much heat has gone up or down is the calorimetry of the reaction.
For example if you have a beaker of water and take its temperature, then burn a piece of bread under it, the change in temperature is the calories (energy) of the bread.
For example if you have a beaker of water and take its temperature, then burn a piece of bread under it, the change in temperature is the calories (energy) of the bread.
4.10 understand that chemical reactions in which heat energy is given out are described as exothermic and those in which heat energy is taken in are endothermic
In an exothermic reaction heat is given out. Because bonds are made which gives out energy.
Think expelled; exit; exo.
In a endothermic reaction heat is taken in. Because bonds are Broken which requires energy.
Think expelled; exit; exo.
In a endothermic reaction heat is taken in. Because bonds are Broken which requires energy.
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