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Help with limiting reagent and bond polarity

order the molecules in order of increasing polarity
CF4
CHCl2F
CH2Br2
CH3I
Can someone explain the answer to me please


the answer:
CF4
CH3I
CH2Br2
CHCl2F
Reply 1
Avatar for Loyale
Loyale
OP
15
Can you also explain a method to work out the limiting reagent
Original post by Loyale
order the molecules in order of increasing polarity
CF4
CHCl2F
CH2Br2
CH3I
Can someone explain the answer to me please


the answer:
CF4
CH3I
CH2Br2
CHCl2F


Which ones don't you understand from that ordering?
Reply 3
Avatar for Loyale
Loyale
OP
15
Original post by langlitz
Which ones don't you understand from that ordering?




I dont understand how to come to the answer and how to order them
Original post by Loyale
Can you also explain a method to work out the limiting reagent


Work out the number of moles of each reactant present. Then find the ratio in which the reactants react to form the product.

e.g.

2HCl+CaCO3→CO2+H2O+CaCl2 2HCl +CaCO_3 \rightarrow CO_2 + H_2O + CaCl_2

So if you had 3 moles of HCl and 1 mole of CaCO3 what's he limiting reagent?

Spoiler

(edited 8 years ago)
Original post by Loyale
I dont understand how to come to the answer and how to order them


Ok I'll try an explain it :smile:

So the electronegativity values are F > Cl > Br > I (electronegativity increases up and across from left to right on the periodic table, this is something you should learn as it will come in very useful). CF4 is non-polar because, although F has the highest electronegativity of any element, they are equally spaced around the carbon. So the pull on it's electrons cancel each other out.

The other ones are fairly intuitive. Increasing the number of electronegative elements will increase the polarity of the molecule.
Electronegativity increases across a period and decreases down a group
CF4 is non polar because the 4 F atoms are equally spaced around the central C atom and so the dipoles cancel.
For the remaining 3 molecules we need to use Pauling's electronegativity values (you can google this if you are unsure of what it is - it's basically just the periodic table but the electronegativity values)
All you have to do is work out the difference between the most electronegative atom in the molecule (the one with the highest value) and the least electronegative atom in the molecule (the one with the lowest value)
By doing this we roughly get the values:
CH3I = 0.46
CH2Br2 = 0.76
CHClF = 1.8
...putting them in that order.

Polarity just how polar a molecule is. So the higher the electronegativity difference the more polar the molecule is.

As for the limiting reagent, it is just the one that is completely used up first in a reaction

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