Video Transcript
The bar chart below shows the melting points of three ionic compounds. Which of the following assignments is correct? (A) Where X is MgF2, Y is MgO, and Z is NaF. (B) Where X is MgO, where Y is NaF, and Z is MgF2. (C) Where X is NaF, Y is MgF2, and Z is MgO. (D) Where X is MgO, Y is MgF2, and Z is NaF. Or (E) where X is NaF, Y is MgO, and Z is MgF2.
The bar chart provided deals with melting points. The melting point of a substance is the temperature at which there is enough energy in the form of heat to break the attractions between the particles changing the state from solid to liquid. On the chart, the melting point is measured in degrees Celsius. In the case of an ionic compound, the particles present are positively charged ions and negatively charged ions, which are very attracted to each other. This force holding ionic compounds together is called electrostatic attraction, which is the attractive force between positive and negative charges.
This attractive force must be broken to melt the compound, which requires great amounts of heat to be added for an ionic compound to change from a solid state to a liquid state, overcoming the strong electrostatic attractions and disrupting the orderly lattice structure. We can see that for ionic compounds, this will generally require extremely high temperatures by looking at the 𝑦-axis of the chart provided.
We must identify the answer choice that assigns the weakest electrostatic attraction and therefore lowest melting point of the three compounds as ionic compound X and the compound with the strongest electrostatic attraction and therefore highest melting point of the three compounds to ionic compound Z, with ionic compound Y being assigned to the compound with a melting point above that of compound X but below that of compound Z. To determine this, we must look at the factors that affect the strength of electrostatic attractions in ionic compounds.
One factor impacting electrostatic attraction is the charge state on the ions present in the compound. Generally, ions with a charge of one plus or one minus have weaker electrostatic attractions than ions containing a higher charge state, such as ions with a three plus or three minus charge. With this in mind, let’s take a look at a couple of common ionic compounds. Table salt, or sodium chloride, is used commonly on food as a flavor enhancement and is composed of biologically important sodium ions, which are essential for nerve and muscle function, as well as chloride ions, which are crucial ions in many systems of the body, such as the circulatory system. This very important ionic compound contains one plus ions and one minus ions, and its melting point is approximately 800 degrees Celsius, which relative to other household compounds is quite high.
Let’s compare this to another useful ionic compound, magnesium chloride, which has many important medical and industrial uses and in some cases is used for fireproofing or fire extinguishing. Magnesium chloride contains magnesium ions and chloride ions, with magnesium ions having a two plus charge and chloride ions having a one minus charge. Due to the greater charge on the magnesium ion compared to the sodium ion, in sodium chloride, magnesium chloride has a much higher melting point of approximately 1400 degrees Celsius. Using this example, we can see that as the charges of the ions in an ionic compound increase, the melting point generally increases as well.
Let’s now look at our answer choices. Each of our answer choices contain MgF2, or magnesium fluoride, as well as MgO, or magnesium oxide, and NaF, or sodium fluoride. By identifying the charges of the ions in each of the compounds, we can determine the relative strengths of the electrostatic attractions and thus the relative melting points. Let’s begin with magnesium fluoride, which is composed of magnesium and fluoride ions. When in an ionic compound, magnesium will lose its two valence electrons forming an ion with a two plus charge, while fluoride ions are formed when fluorine gains one valence electron forming an ion with a one minus charge. We should also note that the subscript of two in MgF2 indicates that in the ionic compound, for every one magnesium ion, there are two fluoride ions present.
Next, magnesium oxide is also composed of magnesium ions as well as oxide ions, which have a two minus charge. As when forming an ion, oxygen gains two valence electrons. Finally, in sodium fluoride, there are sodium ions, which have a one plus charge, and fluoride ions, which have a one minus charge. We can now compare the values of the charges of each set of ions. Sodium fluoride contains ions with the lowest charge states with charges of one plus and one minus. These electrostatic attractions would be the weakest, giving this ionic compound the lowest melting point of the three. Therefore, we can assign sodium fluoride as ionic compound X.
Let’s next look at the charges of the ions in magnesium oxide. Of the three compounds, magnesium oxide contains ions with the highest charge states, with magnesium with a charge of two plus and oxide with a charge of two minus. This compound would have the highest melting point of the three due to the strong electrostatic attraction between the ions of higher charge states. We can assign magnesium oxide as ionic compound Z.
Finally, magnesium fluoride contains magnesium with a charge of two plus and fluoride with a charge of one minus. This would likely mean that magnesium fluoride would have a melting point between that of sodium fluoride and magnesium oxide. We can assign magnesium fluoride as ionic compound Y. So, the only answer choice that matches our assignments of the three compounds is answer choice (C), where the correct assignments for the three ionic compounds are X is NaF, Y is MgF2, and Z is MgO.
