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# thermodynamic question watch

1. Standard enthalpy of atomisation of potassium +90 First ionisation enthalpy of potassium +420 Bond dissociation enthalpy of chlorine +244 Lattice enthalpy of potassium chloride -706 Standard enthalpy of formation of potassium chloride -436
1. Use the data below to calculate the electron affinity of chlorine:

I got the answer as 388 while the answer sheet says -362 kj mol^-1. Can anyone explain step by step why this is please?
2. You've just summed all of the numbers and reversed the sign.

For a start you need to apply the definition of bond enthalpy.

Then you need to have another look at the definition for enthalpy of formation.
3. (Original post by Pigster)
You've just summed all of the numbers and reversed the sign.

For a start you need to apply the definition of bond enthalpy.

Then you need to have another look at the definition for enthalpy of formation.
how do i apply the definition of bond enthalpy? I understand that bond enthalpy is breaking of bonds but is it the same as atomisation of chlorine?

4. I did:

0= 436+90+420+(0.5 x 244)+(-706)
Ans= -436-90-420-122+706
Ans= -362
5. (Original post by hayilaas)

I did:

0= 436+90+420+(0.5 x 244)+(-706)
Ans= -436-90-420-122+706
Ans= -362
can i just ask why -436 is negative instead of being positive?
6. (Original post by elvss567)
can i just ask why -436 is negative instead of being positive?
It’s the value for the standard enthalpy of formation of potassium chloride as you said above. The value would be positive it potassium chloride is being dissociated
7. (Original post by hayilaas)
It’s the value for the standard enthalpy of formation of potassium chloride as you said above. The value would be positive it potassium chloride is being dissociated
thankyou
8. (Original post by elvss567)
how do i apply the definition of bond enthalpy? I understand that bond enthalpy is breaking of bonds but is it the same as atomisation of chlorine?
They aren't the same.

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