FFR: Complex Ions

When an ionic compound dissolves in water, it typically just exists in solution as it’s ions in aqueous forms. For example, NaCl dissolved in water exists as aqueous Na+ and aqueous Cl-. Sometimes, these aqueous ions will react with one another and form complex ions.

When a complex ion is formed, it is the result of a ligand bonding to a central ion or compound. A ligand is a compound that will bind to a dissolved ion, forming a new compound. A the central ion that the ligand bonds to is always going to be a metal ion. The metal can bond with ligands through both ionic and covalent means. If it is an ionic bond, the number of ligands a metal can bond with is determined by the charge number of the atom. For example, iron can exist as Fe2+ or Fe3+, and both of them can bond with the Hydroxide ligand (OH-). This leads to the complexes of Fe(OH)2 and Fe(OH)3, as this type of complexation is ionically bonded.

What other types of Ligands can you think of besides OH-? To help you come up with ideas, here are some properties of most ligands.

• Ligands donate electrons to the metal atom.
• Ligands tend to be negatively charged or neutral.
• Ligands can donate more than one pair of electrons to form a bond.
• Most metals are nearly always bound to one or more ligands.

Complex formation can change the properties of a dissolved metal ion, but rusting also involves oxidation and formation of solid corrosion products. Iron metal loses electrons and is oxidized to positive iron ions; it is not reduced when it becomes positively charged. Oxygen reduction can accompany that process in moist air. Rust is a mixture of iron oxides and oxyhydroxides with varying water content, not simply a shiny metal to which a couple of water ligands were added.

• If the Ligand is an anion, change the name based on how the anion ends:

• Change -ide to -o (phosphide becomes phospho)
• Change the -ate to -ato (sulfate becomes sulfato)
• Change the -ite to -ito (nitrite becomes nitrito)

• If the Ligand is neutral, use the normal name of the ligand EXCEPT for the following:

• Water becomes aqua
• Carbon Monoxide becomes Carbonyl
• Ammonia becomes ammine
• O$_2$ becomes dioxygen
• N$_2$ becomes dinitrogen
• There are others, but these are the most common.

• Number the ligands in the complex ion with the following prefixes: mono (1), di (2), tri (3), tetra (4), penta (5), hexa (6), hepta (7), octa (8).

As for the central metal atom, there are two rules for that. If the complex ion is an anion (there is a negative charge) you will need to suffix the metal’s name with ‘ate’, i.e. aluminate or chromate. Sometimes, the latin name is used instead. These include Iron (Ferrate), Copper (Cuprate), Lead (Plumbate), and Gold (Aurate).

Chapter Problems

1. What is the solubility of $NaCl$ ($K_{sp}=37.3$) in a 1M solution of $MgCl_2$ ?
2. Reviewed rate/equilibrium task: describe how crushing and stirring can speed dissolution of MgCl₂ at a fixed temperature, then explain why that does not necessarily change equilibrium solubility. Smaller pieces expose more surface; stirring transports dissolved material from the interface. To claim a changed saturation amount, supply equilibrium measurements under new specified conditions.
3. You have $15L$ of water, and you add 41mol of $LiBr$ and 10mol of $PbCl_2$ . After bringing the solution to equilibrium, you find out you have $2.5kg$ of the less soluble salt left. What is the new boiling point of the solution?
4. A solution of 0.3M $NaOH$ and 0.75M $MgCl_2$ is saturated with $NaCl$ ($K_{sp}=37.3$). What is the concentration of $NaCl$ in the solution?
5. You have 5L of water, 200g of $KCl$ ($K_{sp}=21.7$), 400g of $KOH$ ($K_{sp}=364$), and 330g of $K_2SO_4$ ($K_{sp}=1.32$). After adding two salts, you are able to completely dissolve them. The third salt is unable to be fully dissolved in the solution. Which salt is left, and how much is left undissolved?
6. Acid rain can occur if the concentration of $HCl$ exceeds $10^{-4}M$ . What would the concentration (in ppm) need to be in the air for acid rain to develop. Assume $P=0.9atm$ , and $T=10^\circ C$ .
7. An evaporation tower is used to strip dissolved gas from water. If a water treatment plant is removing $H_2S$
   ($k_{H,\ 298}=11.49,\ C=2100$) from groundwater, what is the minimum concentration that can be met if they heat up the water during the process to $30^\circ C$ ? In ambient air, H2S has a concentration of $2\cdot10^{-4} mol\%$.
8. How much does the concentration of Carbon Dioxide change throughout the year if pressure is constant at 0.9 atm (remember $CO_2$ makes up $0.03\%$ of the atmosphere)? The maximum temperature is $32^\circ C$ and the minimum temperature is $2^\circ C$ .
9. At what temperature will the solubility of $HCl$ exceed $NH_3$?
10. If seawater is about $3.5\%$ by weight $NaCl$ , the rest being water, what is the freezing point?
11. You have three solutions of the same compound at varying molarities. Name two ways you can easily rank the three solutions in order of increasing concentration if you only know the compound involved in the solution?
12. A chemist decides he wants to invent a new type of windshield washer fluid that won’t freeze in a Russian winter. If he wants to use $MgCl_2$ and Aluminum Sulfate at a $2:1$ ratio by mass, what will the molality of $MgCl_2$ be if winter can reach $-68^\circ C$ ?
13. A beaker of water is held under $0.05atm$ of $NH_3$ gas. What temperature do you need if you want an equilibrium concentration of $60\frac gL \ NH_3$ ?
14. A solution of Lithium Sulfate is being heated. At $80^\circ C$ , Lithium Sulfate is added until it remains as a precipitate in the solution. What would you expect to happen after you take the solution off of heat?
15. If rain forms at a height where $P=0.52atm$, at a temperature of $8^\circ C$, what will the $CO_2$ concentration be in the raindrop, assuming it spends enough time in the cloud to equilibrate and it doesn’t gain any further $CO_2$ along its descent?
16. Reviewed coordination enrichment: explain why metallic copper placed in ordinary aqueous NaCl cannot simply be assumed to become a copper-ion hydrate or complex. An oxidation process must first supply Cu⁺ or Cu²⁺; identify an oxidant and an electron-balanced reaction before discussing those ions’ complex formation. Salt and water alone do not justify the original proposed synthesis.
17. Seawater meets river water at an estuary. The salinity of the seawater begins to infiltrate the river. If the salinity of the water can be shown to be $\left[\text{Salt}\right] = 3.0\cdot\exp\left(-0.0005d\right)$, where $d$ is distance from river mouth in $m$ and $\left[\text{Salt}\right]$ is the concentration of salt as $NaCl$ in $\frac{mol}{kg}$ . At what point up the river will the water freeze if the surface temperature reaches $-1^\circ C$ ?
18. Carbon dioxide is dissolved into the oceans of the world. If the top $1m$ of our oceans were to increase in average temperature from $17^\circ C$ to $18^\circ C$, what would be the mass of CO2 released? The surface area of the oceans is $361\, 000\, 000km^2$.
19. When Ammonia gas dissolves in water, it reacts to form Ammonium and Hydroxide ions. If you have 4 atm of ammonia over water at 25ºC and 30% of the dissolved ammonia dissociates, what is the solubility of Ammonium Chloride (ksp = 30.9)?
20. If a bubbler keeps a fish tank saturated in oxygen, what temperature does the tank need to be kept at to ensure that the concentration is at least 11mg/L?