h 10 lonie Bonds, Melting Points Name Name e bonds are electrostatic forces betw

ID: 1043075 • Letter: H

Question

h 10 lonie Bonds, Melting Points Name Name e bonds are electrostatic forces between cations and anions. Ionic bonds are very strong bonds. Most of the chemicals you have used in lab are ionic crystalline solids that were dissolved in water to MW TR form aqueous solutions. During the lab practicum heating will not affect most of the unknowns because the lattice energy is very high, and therefore, the melting point is higher than the of the Bunsen burner. A few of the unknowns did melt and a few may even decompose before melting. The formula for lattice energy is: temperature NM: e Circle Z" and Z and r in the equation to the left For this class the important variables to understand are charge (Z and Z) and average r The general rule for crystalline structures with ionic bonds is that small ions with large charges higher lattice energy adius (r). have Attraction increases as charge increases greater lattice energy .2 Attraction increases as radius decreases .2 reater lattice energy +1 Predict the order of increasing boiling point and the corresponding melting point. Sodium chloride, potassium iodide, sodium oxide, potassium chloride, ammonium iodide, calcium oxide, potassium bromide, magnesium oxide, sodium fluoride (Write formulas below, not names.) Formula Melting point rO Formula Melting point C) 993 1,132 2,572 2,852 551 681 734 770 801 Ch 10 Crystalline Solids, Lattice Energy, Melting Point

Explanation / Answer

Formula

Melting point (0C)

NH4I

551

681

KBr

734

KCl

770

NaCl

801

NaF

993

Na2O

1132

CaO

2572

MgO

2852

The melting point of a substance is the temperature at which the material changes from a solid to a liquid state. Pure crystalline substances have a clear, sharply defined melting point.

Ionic compounds have high melting points.

The electrostatic attraction (ionic bond) between cations and anions is strong.
It takes a lot of energy to overcome this attraction in order to allow the ions to move more freely and form a liquid.

The factors which affect the melting point of an ionic compound are:

· The charge on the ions.

· Size of the ions.

(i) Charge on the Ions

In general, the greater the charge, the greater the electrostatic attraction, the stronger the ionic bond, the higher the melting point.

From the above table it is clear that MgO has a higher melting point than NaCl. Why?

MgO has a higher melting point than NaCl because 2 electrons are transferred from magnesium to oxygen to form MgO while only 1 electron is transferred from sodium to chlorine to form NaCl.

(ii) The size of the ions.

Smaller ions can pack closer together than larger ions so the electrostatic attraction is greater, the ionic bond is stronger, the melting point is higher. This is also evident from the above table.