OCR AS Chemistry F331: Enthalpy

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Enthalpy
Exothermic reaction: gives out energy and heats the surroundings
Endothermic reaction: takes in energy and cools the surroundings
During an exothermic reaction, the products end up with less energy than the reactants had ­ but
the surroundings end up with more, and get hotter.
We can measure the energy transferred to and from the surroundings as enthalpy change.
Enthalpy changes can be shown on enthalpy level diagrams.
Enthalpy level diagram for an exothermic reaction
Reactants
CH4 + 202
Enthalpy
H-
Products
2H2O + CO2
Progress of reaction
Enthalpy changes are negative therefor it is an exothermic reaction
Enthalpy level diagram for an endothermic reaction
Products
CaO + CO2
Enthalpy
Reactants
H+
CaCO3
Progress of reaction
Enthalpy change is positive so it's an endothermic reaction

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Enthalpy (H)
We cannot measure the enthalpy of a substance so we measure the change of
enthalpy when a reaction occurs.
The enthalpy change in a chemical reaction gives the quantity of energy
transferred to or from the surroundings, when the reaction is carried out in a
open container.
Standard Conditions
H varies according to the conditions. In particular, it is affected by
temperature, pressure and the concentration of solutions.
So we choose certain standard conditions to refer to.…read more

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Enthalpy Cycles ­ Enthalpy Changes of Combustion
H going this way....
C (s) + 2H2 (g) CH4 (g)
.... Is the same as H
going this way
CO2 (g) + 2H20 (l)
This shows an enthalpy cycle, also known as an energy cycle.
There is both a direct and indirect way to turn graphite (C) and hydrogen (H2) into
methane (CH4)
We can't measure the enthalpy change for the direct route.…read more

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If you know the enthalpy changes involved in two parts of the cycle, you can work out the
enthalpy change in the third.
So referring to the enthalpy cycle shown below, we can measure H2 and H3 , we can find
H3, which is the enthalpy change that cannot be measure directly.…read more

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Putting in the values of the enthalpies of combustion, which we can measure, we get:
Hf (CH4) = Hc (C) + 2Hc (H2) ­ Hc (CH4)
= -393kJ mol -1 + 2(-286)kJ mol -1 ­ (-890) kJ mol -1
= - 75 kJ mol -1
Using the enthalpy cycle has enabled us to find a value for an enthalpy change which we
could not find directly…read more

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Enthalpy Cycles ­ Enthalpy Changes of Formation
Enthalpy changes of formation are useful for calculating enthalpy changes you can't find
directly.
You need to know H for all the reactants and products that are compounds.
Note: the face value of H for elements is zero. E.g. oxygen = 0
H1
NH3 (g) + HCl (g) NH4Cl (s)
H 2 H3
1/2 N2 (g) + 2H2 (g) ½ Cl2 (g)
The equation for this is:
H1 = -H2 + H3
Hf (NH3) = 46.…read more

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Working out H2:
H2 = NH3 (g) + HCl (g)
So enter the values:
(-46.1) + (-92.3) = -138.4kj mol -1
Working out H3:
H3 = NH4Cl (s)
So enter the values:
-315 kj mol -1
So, using the equation, we can now enter the values:
H1 = -H2 + H3
H1 = --138.4 + -315 = -176.…read more

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Measuring Enthalpy Changes
You can measure enthalpy changes simply in a laboratory.
Energy can be arranged to be transferred to or from water surrounding the reaction vessel.
Exothermic reaction water gets hotter
Endothermic reaction water gets cooler
To find the enthalpy of combustion of a flammable
liquid, you burn it by using a calorimeter.
As the fuel burns, it heats the water.…read more

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Limitations and Uncertainties
Experimental problems with calorimetry
Some heat will be absorbed by the container, rather than going towards heating up
the container
Some heat is always lost to the surroundings during the experiment
Experimental problems with flammable-liquid calorimetry
Some combustion may be incomplete ­ which means less energy will be given out
Some of the flammable liquid may escape by evaporation…read more

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