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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…

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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…

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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…

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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…

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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…

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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…

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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 +…

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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…

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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…

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