Mine's a double

How do you know which values to double when doing Born-Haber Cycles?

Simple really.

In a compound like CaCl2, you always have to deal with 2 chlorines so any value which refers to chlorine will need to be doubled to make it refer to 2 chlorines.

In this case that will be the enthalpy of  atomisation

1/2 Cl2 (g) --> Cl (g) needs to be doubled

and

enthalpy of  electron affinity

Cl + e-  --> Cl- needs to be doubled.

The possibility is in something like Na2O, now you are dealing with 2 sodiums so you would need to double the sodium values, i.e.

enthalpy of  atomisation

Na (s) --> Na(g) needs to be doubled

and

enthalpy of  ionisation (energy)

Na   --> Na+ + e- needs to be doubled

It is worth being careful though as every now and again they pre-double some things for you in the table, so have a read and double only if it refers to one Na (or whatever) and you need it to be 2.

It's at time like this that I wish blogspot did superscript and subscript.

IT'S A BOMB!!!!!!

Ever wondered what exactly a bomb calorimeter is? Well, this is a bomb calorimeter.

It is basically a really, really, really well insulated container full of water that contains another container inside it that can burn any chemical to completion without losing any heat (well, very, very, very little heat).

Not impressed? Thought it would be a bit more James Bond? 'Fraid it is more Borne-Haber than Bourne Identity (Google it)

What is the point of it?
By using a Bomb Calorimeter, you can very accurately measure the enthalpy of combustion because no heat is lost to the environment, it is all used to raise the temperature of the water.

What do you need to know about it for CH1?
Very little, only that it is an improvement over using a polystyrene coffee cup and a plastic lid with a thermometer pushed through it

Hold on, just had a thought, if you used a bomb calorimeter to keep your coffee in, it would stay hot forever! Why hasn't somebody else thought of this? Infinity Coffee! It could be a big seller, you heard it here first.