1. Do magnesium and chlorine form an ionic bond or a covalent bond? Explain why or why not.
2. Using the answer from #1, what was learned about the polar nature of the water molecule, explain what causes magnesium chloride to dissolve in seawater.
3. If you heat a container full of seawater, the water will evaporate leaving behind the salts such a magnesium chloride. Considering this evidence, what kind of mixture is seawater?
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1) Magnesium and chlorine form an ionic bond. Covalent bonds are formed when two or more atoms share electrons between them. Ionic bonds are when atoms gain or lose electrons to become charged species (ions) that share an electrostatic interaction called an ionic bond. Magnesium forms a positive ion (cation) with a 2+ charge and chlorine forms a negative ion (anion) with a 1- charge. So two chlorine anions form an ionic bond with one magnesium cation for form MgCl2, a neutral chemical compound. Metals and non-metals often form ionic bonds because of the difference in their electronegativity values. Electronegativity is basically a measure of how strongly an element will attract electrons toward it. Chlorine (a non-metal) has an electronegativity value of 3.0 and magnesium (a metal) has an electronegativity value of 1.3. This makes a difference of 3.0 - 1.3 = 1.7. In general, any electronegativity difference of 1.6 or more is considered an ionic bond, and MgCl2 falls into this category.
2) The general rule of thumb about solubility is that "like dissolves like." In other words, polar solids tend to dissolve in polar solvents and non-polar compounds tend to dissolve in non-polar solvents. We know that MgCl2 is an ionic solid, so we know that it is a polar compound since it is made up of ions. Water (H2O) is a polar solvent. The electronegativity difference between hydrogen and oxygen is about 1.4. This makes water a polar covalent molecule where it still has covalent bonds but there are partial charges on the atoms, hence the polar nature of water. MgCl2 dissolves in water because the individual ions in the solid (Mg2+ and Cl-) coordinate with the partial charges in the water molecules. The magnesium cations coordinate with the partially negatively charged oxygen atoms and the chlorine anions coordinate with the partially positively charged hydrogen atoms. Since numerous water molecules can surround each ion and separate them, this causes them to solvate, or dissolve in the solvent.
3) There are two basic kind of mixtures, homogeneous and heterogeneous. Heterogeneous mixtures are composed of two distinct phases, like sand in water. Homogeneous mixtures are composed of a single phase. Seawater is an example of a homogeneous mixture. The MgCl2 and other ionic solids are completely dissolved in the water, thus forming a single phase to the eye (liquid water). In fact, all ionic solutions are considered homogeneous mixtures.
1) When a metal and a nonmetal bond, the metal ion "gives" its valence electrons to the anion to form an ionic bond. Magnesium is a metal (group 2/alkaline earth metals), while chlorine is a nonmetal (group 7/halogen). Therefore, because magnesium and chlorine do not share electrons, they do not form a covalent bond.
2) Ionic bonds are very polar because of the difference in electronegativity between magnesium and chlorine. That being said, the question states that MgCl2 dissolved in water. A simple rule to remember is: "like dissolves like." Nonpolar substances will dissolve in nonpolar solvents, while polar substances will dissolve in polar solvents. Therefore, water must be polar like MgCl2.
3) The mixture is aqueous and homogenous. The chemical symbol for this would be MgCl2 (aq).
1. Magnesium and chlorine form an ionic bond. This can be determined in two distinct ways. First, you can compare the electronegativity values of both the elements. Since ionic bonds require a transfer of electrons, the difference in electronegativity values must be high so that one element can easily lose an electron and one element can easily gain it. If the difference between two electronegativity values is 1.7 or greater, the two elements will form an ionic bond. In this case, the EN value of Cl is 3.0 and the En value of Mg is 1.2. 3.0 - 1.2 = 1.8, thus making it an ionic bond. In addition, we can also look at the two elements on the periodic table and notice that chlorine is a non-metal with a charge of -1 and magnesium is a metal with a charge of +2. This means that the two combine to form an ionic bond, specifically giving us the compound MgCl2.
2. We learned from question number 1 that there is a big difference in electronegativity between Mg and Cl, thus making it similar to a polar covalent bond. In addition, we know that water is also a polar molecule because of the difference in EN values between O and H. When determining solubility, what is generally true is that "like dissolves like" meaning that polar molecules dissolve in other polar molecules and non-polar molecules dissolve in other non-polar molecules. Since the ionic compound behaves like a polar molecule, it is safe to say that MgCl2 dissolves in water since it is also polar. Inside the solution of water, the MgCl2 breaks apart into individual Mg+2 and Cl-1 ions. The partially negative oxygen atoms surround the Mg ions and partially positive hydrogen atoms surround the Cl ions, making it dissolve in the water.
3. Since the MgCl2 is not visible when it is dissolved but is left behind when heated up tells us that it forms a homogeneous mixture. In a heterogeneous mixture, the substances involved can easily be seen and separated; however, in a homogeneous mixture, this is not true, thus making MgCl2 dissolved in wtaer homogeneous.
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