Amines are organic compounds that contain an \(\mathrm{NH}_{2}\) group, and water-soluble amines are weak bases in water. For example, the compound methylamine, \(\mathrm{H}_{3} \mathrm{C}-\mathrm{NH}_{2}\), is a weak base. (a) Draw a dot diagram for methylamine. (b) Using dot diagrams, show the equilibrium reaction between methylamine and water. (c) To which side does the equilibrium in part (b) lie? What did we tell you that allowed you to figure out the answer? (d) The similar compound ethane, \(\mathrm{H}_{3} \mathrm{C}-\mathrm{CH}_{3}\), does not act as a weak base. Why can methylamine act as a weak base but \(\mathrm{H}_{3} \mathrm{C}-\mathrm{CH}_{3}\) can't? (Hint: Draw a dot diagram for \(\mathrm{H}_{3} \mathrm{C}-\mathrm{CH}_{3}\).) (e) Is it appropriate to call methylamine an electrolyte? If so, is it weak or strong? Explain.

Step by step solution

01

(a) Drawing dot diagram for methylamine

First, we need to draw a dot diagram (also known as Lewis structure) for methylamine. Methylamine has the chemical formula \(\mathrm{H}_{3} \mathrm{C}-\mathrm{NH}_{2}\), which means it has a nitrogen atom, a carbon atom, and five hydrogen atoms. Here is the dot diagram for methylamine: H | H – N – H | C – H | H The nitrogen atom forms three single bonds with three hydrogen atoms, and the carbon atom forms a single bond with the nitrogen atom as well as three single bonds with three hydrogen atoms.

02

(b) Equilibrium reaction between methylamine and water

Next, we need to show the equilibrium reaction between methylamine (\(\mathrm{H}_{3} \mathrm{C} \mathrm{NH}_2\)) and water (\(\mathrm{H}_{2}\mathrm{O}\)) using dot diagrams. In the equilibrium reaction, methylamine transfers a hydrogen ion to a water molecule, which becomes a hydronium ion (\(\mathrm{H}_{3}\mathrm{O}^{+}\)): \(\mathrm{H}_{3} \mathrm{C} \mathrm{NH}_2 + \mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{H}_{3} \mathrm{C} \mathrm{NH}_{3}^{+} + \mathrm{OH}^-\) The double arrow signifies that the equilibrium reaction can proceed in both forward and reverse directions.

03

(c) Determining equilibrium position and explanation

We need to decide which side of the equilibrium the reaction will favor. Since methylamine is a weak base, it will partially dissociate in water. As a result, the equilibrium position will lie to the left side, which will favor the reactants over the products. We determined this from the fact that methylamine is a weak base, and weak bases have low ionization in water. This information points towards the equilibrium lying towards the side containing methylamine and water, as opposed to the side containing the methylammonium and hydroxide ions.

04

(d) Methylamine as a weak base vs. ethane and explanation

We are asked to explain why methylamine (\(\mathrm{H}_{3} \mathrm{C} - \mathrm{NH}_{2}\)) can act as a weak base, while ethane (\(\mathrm{H}_{3} \mathrm{C} - \mathrm{CH}_{3}\)) cannot. First, let's draw a dot diagram for ethane: H H | | H - C - C - H | | H H Methylamine can act as a weak base because it has a nitrogen atom with a lone pair of electrons. The lone pair of electrons on the nitrogen atom can accept a proton (\(\mathrm{H}^{+}\)) from water, which is why it behaves as a weak base. On the other hand, ethane does not have a nitrogen atom, and all the electrons in its structure are used in single bonds between carbon and hydrogen atoms. Therefore, it has no available lone pairs of electrons to accept a proton from water, which means it cannot function as a base.

05

(e) Methylamine as an electrolyte and its strength

Lastly, we need to determine if methylamine is an electrolyte and if so, whether it is a weak or strong electrolyte. An electrolyte is a substance that, when dissolved in water, becomes capable of conducting electricity due to the presence of free ions. Since methylamine is a weak base and can accept protons from water, it will ionize partially in water to produce a small number of free charged ions: \(\mathrm{H}_{3} \mathrm{C} \mathrm{NH}_2 + \mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{H}_{3} \mathrm{C} \mathrm{NH}_{3}^{+} + \mathrm{OH}^-\) This means that methylamine will be able to conduct electricity when dissolved in water, making it an electrolyte. However, it is only a weak electrolyte because it ionizes only partially in water.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Weak Bases in Water

When we refer to weak bases in water, we're talking about substances that don't completely dissociate in an aqueous solution. Amines, like methylamine ((CH₃NH₂)), are quintessential examples of weak bases. Unlike strong bases, which fully ionize in water releasing a substantial amount of OH^- ions, weak bases only partially ionize. This means they exist in an equilibrium state where the base and its ionized products coexist in solution. The degree of ionization is quite low, so the concentration of hydroxide ions in the solution is relatively small, resulting in a higher pH than strong bases.

Understanding this concept helps students recognize why methylamine doesn't significantly alter the pH of water, and this characteristic also explains its role as a weak electrolyte, as it can't conduct electricity as effectively as strong bases.

Chemical Equilibrium

Chemical equilibrium is achieved in a reaction when the rate of the forward reaction equals the rate of the reverse reaction. In the context of weak bases like methylamine in water, this means both the reactants (methylamine and water) and the products (methylammonium ion and hydroxide ion) are present simultaneously in a dynamic balance. Knowing that methylamine is a weak base helps us understand why the equilibrium lies to the left — favoring the reactants — as there are considerably more original base and water molecules than ionized products. It's essential here to realize that at equilibrium, the concentrations of reactants and products are constant over time, not necessarily equal, reflecting the weak ionization property.

Lewis Dot Structures

Lewis dot structures are diagrams that show the bonding between atoms of a molecule, as well as any lone pairs of electrons that may exist. Methylamine's Lewis dot structure reveals a nitrogen atom with a lone pair, which is the key to its ability to act as a weak base. The lone pair can attract and bond with a hydrogen ion (H⁺) from water.

The presence of these lone pairs also distinguishes compounds that can act as bases from those that cannot, such as ethane, which lacks such a structure. Therefore, drawing Lewis dot structures not only provides the visualization of a molecule's composition but also hints at its possible chemical behavior and reactivity.

Electrolytes in Chemistry

In the realm of electrolytes in chemistry, a substance must contain free ions to conduct electricity in water. Methylamine, when in water, will ionize into methylammonium ions (CH₃NH₃⁺) and hydroxide ions (OH^-), albeit to a much lesser extent than strong electrolytes like table salt (NaCl).

As a weak electrolyte, methylamine's limited ionization reflects on its lower electrical conductivity. This characteristic is vital to understand as it has practical implications, such as electrolyte balance in biological systems and the criteria for substances used in batteries and electrochemical cells.