Question

Which ion in each of the following pairs would you expect to be more strongly hydrated? Why? a. \(\mathrm{Na}^{+}\) or \(\mathrm{Mg}^{2+}\) b. \(\mathrm{Mg}^{2+}\) or \(\mathrm{Be}^{2+}\) c. \(\mathrm{Fe}^{2+}\) or \(\mathrm{Fe}^{3+}\) d. \(F^{-}\) or \(B r^{-}\) e. \(\mathrm{Cl}^{-}\) or \(\mathrm{ClO}_{4}^{-}\) f. \( \mathrm{ClO}_{4}^{-}\) or \(\mathrm{SO}_{4}^{2-}\)

Short answer

The more strongly hydrated ions in each pair are: a. Mg2+ because it has a smaller size and higher charge than Na+. b. Be2+ because it has a smaller size than Mg2+ (and the same charge). c. Fe3+ because it has a smaller size and higher charge than Fe2+. d. F- because it has a smaller size than Br- (and the same charge). e. Cl- because it has a smaller size than ClO4- (and the same charge). f. SO4^2- because it has a higher charge than ClO4- (with similar sizes).

Step-by-step solution

Compare Charges

Na+ ion has a charge of +1 while Mg2+ ion has a charge of +2. Mg2+ ion has a higher charge than Na+ ion.

Compare Sizes

Na+ ion has a larger size than Mg2+ ion, because in general, as we move from left to right across a period, the ionic size decreases.

Determine Which Ion is More Strongly Hydrated

Mg2+ ion has a higher charge and a smaller size compared to Na+ ion, so it should be more strongly hydrated. #b. Mg2+ or Be2+#

Compare Charges

Both Mg2+ and Be2+ have a charge of +2.

Compare Sizes

Be2+ ion has a smaller size than Mg2+ ion because it is in a higher period and to the left of Mg2+.

Determine Which Ion is More Strongly Hydrated

Since both ions have the same charge, but Be2+ is smaller than Mg2+, Be2+ should be more strongly hydrated. #c. Fe2+ or Fe3+#

Compare Charges

Fe2+ ion has a charge of +2, while Fe3+ ion has a charge of +3. Fe3+ ion has a higher charge than Fe2+ ion.

Compare Sizes

Fe3+ ion has a smaller size than Fe2+ ion because, in general, when ions lose more electrons, their size decreases because of a larger effective nuclear charge.

Determine Which Ion is More Strongly Hydrated

Fe3+ ion has a higher charge and a smaller size compared to Fe2+ ion, so it should be more strongly hydrated. #d. F- or Br-#

Compare Charges

Both F- and Br- have a charge of -1.

Compare Sizes

F- ion has a smaller size than Br- ion because it's in a higher period.

Determine Which Ion is More Strongly Hydrated

F- ion has the same charge but a smaller size compared to Br- ion, so it should be more strongly hydrated. #e. Cl- or ClO4-#

Compare Charges

Both Cl- and ClO4- have a charge of -1.

Compare Sizes

ClO4- ion has a larger size than Cl- ion, due to the presence of four oxygen atoms in the ion.

Determine Which Ion is More Strongly Hydrated

Cl- ion has the same charge but a smaller size compared to ClO4- ion, so it should be more strongly hydrated. #f. ClO4- or SO4^2-#

Compare Charges

ClO4- ion has a charge of -1, while SO4^2- ion has a charge of -2. SO4^2- ion has a higher charge than ClO4- ion.

Compare Sizes

Both ClO4- and SO4^2- ions are polyatomic ions and have a similar size.

Determine Which Ion is More Strongly Hydrated

SO4^2- ion has a higher charge compared to ClO4- ion, so it should be more strongly hydrated.

Key concepts

Ionic Charge and Hydration

When discussing the hydration of ions, the ionic charge plays a pivotal role in determining the strength of hydration. Hydration, in the context of chemistry, refers to the process where water molecules surround and interact with an ion. It is an essential concept for understanding numerous biochemical and industrial processes. Ions with higher charges attract more water molecules due to stronger electrostatic forces. For example, in the case of e. e. e. ions compared to the e. The higher charge density, which is the charge relative to the size of the ion, affects the electrostatic attraction between the ion and water molecules. Simply put, the water molecules 'stick' more effectively to ions with higher charge densities, such as e. over the e. . Therefore, e. The more tightly held the water molecules are around an ion, resulting in a more 'strongly hydrated' ion.

Ionic Size and Hydration

While ionic charge is a critical factor in hydration, ionic size also notably influences how strongly an ion is hydrated. Typically, smaller ions have a stronger hydration due to the greater charge density. This means that for ions with identical charges, we often find that the smaller ion will have a closer and more substantial shell of water molecules around it. Delving deeper into the step-by-step solution given for example b., the reason e. ion is more strongly hydrated than the e. ion is precisely because of its smaller size, despite the two ions carrying the same charge. The difference in hydration strength can be visualized as smaller ions holding their hydration 'cloak' tighter compared to their larger counterparts. This concept is essential for understanding phenomena such as the transport across cell membranes, where ion size and hydration significantly influence which ions pass more easily.

Hydrated Ions in Chemistry

Hydrated ions are omnipresent in chemical reactions, particularly in aqueous solutions. In the world of chemistry, an ion's behavior is often dictated by its hydration shell. The hydration shell is the layer of water molecules that encapsulates an ion in solution. The strength of this hydrated shell has direct implications in areas ranging from biological systems, where hydration affects enzyme function and nerve impulse transmission, to industrial systems, such as when minerals are dissolved in water for processing. In each of the comparison exercises—from a. e. to f.—the concepts of ionic charge and size are applied to predict which ions will be more strongly hydrated. For instance, when comparing e. and e. The stronger hydration of e. due to its higher charge highlights how ions of different charges but similar sizes interact differently with water molecules. Understanding the principles behind ion hydration allows scientists and students alike to anticipate the behavior of ions in a solution and thus deduce the outcomes of chemical reactions.