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Which of the following compounds has the highest melting point : $\ce{PCl5}$, $\ce{H2O}$, $\ce{NaCl}$, $\ce{SrCl2}$, $\ce{CaF2}$?

I've asked a question concerning the boiling points and I got an answer, that states that ionic compounds will have higher boiling points than those of dipole-dipole interaction compounds.

So, the one that will have higher melting point will certainly be an ionic compound, either $\ce{NaCl}$, $\ce{CaF2}$ or $\ce{SrCl2}$.

I thought that I should calculate the molar weight but that won't work, because the correct answer is $\ce{CaF2}$ and $\ce{SrCl2}$ has a greater molar weight.

I am really confused.

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@Freddy Please be a little more thorough with your edits and make sure to include the actual question, if removing it from the title. The short summary should not state 'edited body', because this is obvious, but rather what you changed, like 'improved format' or 'corrected title', etc.. – Martin - マーチン Aug 11 '14 at 17:05
@Martin sorry i will take care of it. – Freddy Aug 11 '14 at 17:07
@Jocka.M : Please mark one of the provided answers as accepted so that question doesn't get categorised as unresolved. – ashu Aug 12 '14 at 7:24
both answers are perfect ! Done – Jocka.M Aug 12 '14 at 7:38
up vote 10 down vote accepted

The strength of the ionic bond depends on Coulomb's law for the force acting between two charged particles where larger force translates to a stronger ionic bond. The equation is

$$F = \frac{-k\cdot q_1 \cdot q_2}{r^2}$$

$k$ is a constant; $q_1$ and $q_2$ are the charges on the ions and $r$ is the distance between the ions.

So the larger the charge the greater the force, and the smaller the radius the smaller the distance between the ions (smaller $r$ in the equation) again leading to a stronger force.

So, first you should calculate the charge on each ion in a molecule. As can see, the charge on strontium ($\ce{Sr^2+}$) and calcium ($\ce{Ca^2+}$) ions is more than the charge on sodium ions ($\ce{Na+}$). Also, calcium ions are smaller than strontium ions. Fluorine ions are also smaller than Chlorine ions. So $\ce{CaF2}$ will have higher bond strength which means it will have higher boiling point since stronger bonds need more energy to break.

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it is very clear now.. Thanks :) – Jocka.M Aug 11 '14 at 18:06

You can deduce it from the Coulomb law. As you can see, the bigger the charge of the respective ions, the electrostatic interaction between them is stronger. The charge of calcium and strontium cations is twice as that of sodium. r is the distance between the charges, so for smaller ions the distance would be smaller i.e. the interaction would be greater. Calcium cation is smaller than strontium cation, therefore the correct answer is indeed calcium fluoride.

It can also be explained with more sophisticated equations: Kapustinskii equation or Born-Lande equation. I will let you analyze them.

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Thanks..... :) . – Jocka.M Aug 11 '14 at 13:03

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