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# 2 Quick Questions watch

1. 1) Why is the magnitude of the lattice enthalpy of three Group halids decreasing in the order NaF > NaCl > KCl?

2) Butan-1-ol was converted into butyl propanoate by the reaction with an excess of propanoic acid. In the reaction, 6.0g of the alcohol gave 7.4g of the ester the percentage yield of ester was...

A) 57
B) 70
C) 75
D) 81

(would appreciate the method with the answer for this question)

Thanks!
2. Soz, for the repetition... here was me thinking that the message didn't post at all because i got a message saying that the server was having problems... then i find out it got posted 3 times, lol! Wierd...
3. http://www.thestudentroom.co.uk/show...51&postcount=2

4. 2.= 70%/b
5. Have i added up wrong again? lol
Sorry about that, the supposed maths student that i am, but at least if u wanted to know that question but for butyl pentanoate u'd get it right
6. 1. NaF to NaCl involved a larger anion, and lattice enthalpy increases as the sum of the ionic radii increases. this is because the force of attraction is weaker when the charges are further apart (the two ions), and so less energy can be gained from the formation of the bond.

NaCl to KCl is the same, except this time the cause of the larger ionic radii sum is the larger cation. Again this reduces lattice enthalpy.

Cheers... just check if i get it...

the smaller the ionic radius of the anion e.g. F-, the stronger the forces of attraction,

so, when metals with a high charge to size ratio (Al3+) polarised negative ions with a high atomic radius (Cl-) and you effectively have covelent bonding... this would be weaker than AlF3? Because the atom radius is smaller therefore the charges are closer together and stronger... is that right???
7. lattice enthalpy increases as the sum of the ionic radii increases
no, it's the other way round - lattice enthalpy INcreases as the ionic radii DEcrease.

the rough sum is Z(+)Z(-)/r^2

where z(+) is the magnitude of the charge on the positive ion
where z(-) is the magnitude of the charge on the negative ion
r is the sum of the ionic radii (i.e. the distance between the nuclei)

so as 'r' gets smaller so the lattice enthalpy gets larger
8. schoolboy error!
9. so the lattice enthalpy increases as the two charges are closer together in the lattice, which is when the sum of the ionic radii are smallest,

yeh, sounds good to me... cheers charco for putting things right
10. You really should use terms like 'more exothermic' or 'less exothermic' rather than increase or decrease which are ambiguous. Dont know if they penalise it in the exams or not but our teachers have made a big point about it. Annoying really because a lot of the books talk about increase (i.e. more energy released) when in numerical terms it's decreasing. Just a minor point really.

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