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Measuring enthalpy change

For Paper 3B. When it asks you to work out the enthalpy of neutralisation, do you do a thermometric titration (alkali in burette, acid in polystyrene cup, add 5cm3 at a time, record temp, draw graph, extrapolate, use ∆H=4.18 x mass x ∆T)? Or do you take it as an instantaneous reaction and measure temp of acid, measure temp of alkali, find the mean temp, add them together, find the final temperature and use the temperature change for that? :confused: I'm sorry if I'm not making any sense....!

Also, for Edexcel do you need to know techniques for recrystallisation? If so what details?

And what on earth is water of crystallisation? I do kind of know what it is, and it crops up often, but what is it? When you calculate Mr does the water count as part of it? Does the water react? is the metal and the water chemically bonded? By what bonds?

So damn nervous about this paper 3B... :s-smilie:
Water of crystallisation is the term given to the molecules of water that are used to build up a crystal lattice in some ionic compounds.

In copper II sulphate the blue crystals would be impossible without using water molecules to act as 'scaffolding' within the structure CuSO4.5H2O
When this is heated the water molecules are driven off and the blue crystals become a white powder.

As most crystals are made by evaporation of an aqueous solution, it makes sense that water molecules can incorporate themselves into the ionic crystal structure. Water, after all, has an oxygen atom with two lone pairs capable of behaving as a Lewis base; it bonds easily to metal ions as evidenced by complex ions in transition metal chemistry.

The metal ions and the water molecules are bonded by dative coordinate bonds, and yes, if you are calculating the RMM then you have to take into account the water of crystallisation when weighing hydrated crystals out.
Reply 2
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Tes
4
Excalibur
For Paper 3B. When it asks you to work out the enthalpy of neutralisation, do you do a thermometric titration (alkali in burette, acid in polystyrene cup, add 5cm3 at a time, record temp, draw graph, extrapolate, use ∆H=4.18 x mass x ∆T)? Or do you take it as an instantaneous reaction and measure temp of acid, measure temp of alkali, find the mean temp, add them together, find the final temperature and use the temperature change for that? :confused: I'm sorry if I'm not making any sense....!


Choose a volume of acid and work out what volume of alkali will react with that. Taking into account conc. Then add one to the other and measure temp change. If you added 5cm3 at a time you'll get lots of heat loss.
:P
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Excalibur
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Thanks charco :smile:

BlakDog - so when do you do the thermometric titration one?
Reply 4
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Tes
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Never ints so inaccurate heat wise. If you needed to do something like that you perform a titration to find the volume of acid/alkali to react it with then perform a thermochemical reaction using this volume. You may use a burette to measure the volume accurately but it should be added all at once to avoid heat loss. :P

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