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Chemistry LibreTexts

Electrolytes

Skills to Develop

  • Identify what electrolytes are.
  • Distinguish between strong and weak electrolytes.
  • Explain what happens when electrolytes dissolve in water.
  • Give the equilibrium constant expression for ionizaton.
  • Explain ion product of water, autoionization of water, and pH.
  • Calculate ionization percentage of weak electrolytes.
  • Explain metathesis reactions.

One of the most important properties of water is its ability to dissolve a wide variety of substances. Solutions in which water is the dissolving medium are called aqueous solutions. For electrolytes, water is the most important solvent. Ethanol, ammonia, and acetic acid are some of the non-aqueous solvents that are able to dissolve electrolytes.

Electrolytes

Substances that give ions when dissolved in water are called electrolytes. They can be divided into acids, bases, and salts, because they all give ions when dissolved in water. These solutions conduct electricity due to the mobility of the positive and negative ions, which are called cations and anions respectively. Strong electrolytes completely ionize when dissolved, and no neutral molecules are formed in solution.

For example, \(\ce{NaCl}\), \(\ce{HNO3}\), \(\ce{HClO3}\), \(\ce{CaCl2}\) etc. are strong electrolytes. An ionization can be represented by

\(\mathrm{NaCl_{\large{(s)}} \rightarrow Na^+_{\large{(aq)}} + Cl^-_{\large{(aq)}}}\)

Since \(\ce{NaCl}\) is an ionic solid (s), which consists of cations \(\ce{Na+}\) and anions \(\ce{Cl-}\), no molecules of \(\ce{NaCl}\) are present in \(\ce{NaCl}\) solid or \(\ce{NaCl}\) solution. The ionization is said to be complete. The solute is one hundred percent (100%) ionized. Some other ionic solids are \(\ce{CaCl2}\), \(\ce{NH4Cl}\), \(\ce{KBr}\), \(\ce{CuSO4}\), \(\ce{NaCH3COO}\) (sodium acetate), \(\ce{CaCO3}\), and \(\ce{NaHCO3}\) (baking soda).

Small fractions of weak electrolytes' molecules ionize when dissolve in water. Some neutral molecules are present in their solutions. For example, \(\ce{NH4OH}\) (ammonia), \(\ce{H2CO3}\) (carbonic acid), \(\ce{CH3COOH}\) (acetic acid), and most organic acids and bases are weak electrolytes. The following ionization is not complete,

\(\mathrm{H_2CO_{3\large{(aq)}} \rightleftharpoons H^+_{\large{(aq)}} + HCO^-_{3\large{(aq)}}}\)

In a solution, \(\ce{H2CO3}\) molecules are present. The fraction (often expressed as a %) that undergos ionization depends on the concentration of the solution.

On the other hand, ionization can be viewed as an equilibrium established for the above reaction, for which the equilibrium constant is defined as

\(\mathrm{\mathit K = \dfrac{[H^+] [HCO_3^-]}{[H_2CO_3]}}\)

where we use [ ] to mean the concentration of the species in the [ ]. For carbonic acid, K = 4.2x10-7. You can generalize the definition of K here to give the equilibrium constant expression for any weak electrolyte.

Pure water is a very weak electrolyte.

The ionization or autoionization of pure water can be represented by the ionization equation

\(\mathrm{H_2O \rightleftharpoons H^+ + OH^-}\)

and the equilibrium constant is

\(\mathrm{\mathit K = \dfrac{[H^+] [OH^-]}{[H_2O]}}\)

For pure water, \(\ce{[H2O]}\) is a constant (1000/18 = 55.6 M), and we often use the ion product, Kw, for water,

\(\mathrm{\mathit K_w = \mathit K [H_2O] [H^+] [OH^-]}\)

The constant Kw depends on temperature. At 298 K, Kw = 1x10-14. If there is no solute in water, the solution has equal concentrations of \(\ce{[H+]}\) and \(\ce{[OH-]}\).

\(\mathrm{[H^+] = [OH^-] = 1\times10^{-7}}\),

and

\(\mathrm{pH = -\log [H^+] = 7}\).

Note that only at 298 K is the pH of water = 7. At higher temperatures, the pH is slightly less than 7, and at lower temperatures, the pH is greater than 7.

Electrolytes in Body Fluids

Our body fluids are solutions of electrolytes and many other things. The combination of blood and the circulatory system is the river of life, because it coordinates all the life functions. When the heart stops pumping in a heart attack, the life ends quickly. Getting the heart restarted as soon as one can is crucial in order to maintain life.

The primary electrolytes required in the body fluid are cations (of calcium, potassium, sodium, and magnesium) and anions (of chloride, carbonates, aminoacetates, phosphates, and iodide). These are nutritionally called macrominerals.

Electrolyte balance is crucial to many body functions. Here's some extreme examples of what can happen with an imbalance of electrolytes: elevated potassium levels may result in cardiac arrhythmias; decreased extracellular potassium produces paralysis; excessive extracellular sodium causes fluid retention; and decreased plasma calcium and magnesium can produce muscle spasms of the extremities.

When a patient is dehydrated, a carefully prepared (commercially available) electrolyte solution is required to maintain health and well being. In terms of child health, oral electrolyte is need when a child is dehydrated due to diarrhea. The use of oral electrolyte maintenance solutions, which is responsible for saving millions of lives worldwide over the last 25 years, is one of the most important medical advances in protecting the health of children in the century, explains Juilus G.K. Goepp, MD, assistant director of the Pediatric Emergency Department of the Children's Center at Johns Hopkins Hospital. If a parent provides an oral electrolyte maintenance solution at the very start of the illness, dehydration can be prevented. The functionality of electrolyte solutions is related to their properties, and interest in electrolyte solutions goes far beyond chemistry.

Electrolytes and Batteries

battery.gifSolutions of electrolytes are always required in batteries, even in dry cells. The simplest battery consists of two electrodes. The figure here illustrates a copper-zinc battery. The left hand is a zinc electrode. The zinc atoms have a tendency to become ions, leaving the electrons behind.

\(\mathrm{Zn_{\large{(s)}} \rightarrow Zn^{2+}_{\large{(aq)}} + 2 e^-}\).

As the zinc ions going into the solution, anions move from the copper cell to the zinc cell to compensate for the charge, and at the same time, electrons go from the \(\ce{Zn}\) electrode to the \(\ce{Cu}\) electrode to neutralize the copper ions.

\(\mathrm{Cu^{2+}_{\large{(aq)}} + 2 e^- \rightarrow Cu_{\large{(s)}}}\)

In dry cells, the solution is replaced by a paste so that the solution will not leak out of the package. In this cell, the \(\ce{Zn}\) and \(\ce{Cu}\) electrode has a voltage of 1.10 V, if the concentrations of the ions are as indicated.

Chemical Reactions of Electrolytes

When solutions of electrolytes are combined, the cations and anions will meet each other. When the ions are indifferent of each other, there is no reaction. However, some cations and anions may form a molecule or solid, and thus the cations and anions change partners. These are called metathesis reactons, which include:

  • Solid formation (or precipitation) reactions: the cations and anions form a less soluble solid, resulting in the appearance of a precipitate.
  • Neutralization reactions: \(\ce{H+}\) of an acid and \(\ce{OH-}\) of a base combine to give the neutral water molecule.
  • Gas formation reactions: When neutral gaseous molecules are formed in a reaction, they leave the solution forming a gas.

Redox reactions are also possible between the various ions. In fact, the battery operations involve redox reactions.

Confidence Building Problems

  1. In a battery consisting of \(\ce{Zn}\) and \(\ce{Cu}\) electrodes, the reaction is

    \(\mathrm{Zn + Cu^{2+} \rightarrow Zn^{2+} + Cu}\)

    and the battery can be represented as

    \(\mathrm{Zn | Zn^{2+} || Cu^{2+}| Cu}\).

    where || means a salt bridge, and \(\ce{CuSO4}\) is used to provide \(\ce{Cu^2+}\). In the salt bridge, what ions will move toward the \(\mathrm{Zn | Zn^{2+}}\) cell?

    1. any cation
    2. \(\ce{Cu^2+}\)
    3. \(\ce{Zn^2+}\)
    4. \(\ce{Na+}\)
    5. any anion

Hint: e. any anion

Skill:
Explain ion movement in a solution of electrolytes.

  1. Do the positive ions move in the salt bridge?

Hint: yes

The two types of ions move in opposite directions.

Skill:
Explain ion movement in a solution of electrolytes.

  1. What solution should be used for the \(\ce{Cu}\) electrode?
  1. any zinc salt
  2. any copper salt
  3. any chloride
  4. any salt
  5. an acid
  6. a base

Hint: b. any copper salt
Any salt can be used for the \(\ce{Zn}\)-electrode. But for the \(\ce{Cu}\) electrode, \(\ce{CuSO4}\) or \(\ce{CuCl2}\) is commonly used.

Skill:
Apply chemical knowledge to battery setups.

  1. Which of the following will you use as the salt bridge?
    1. solid \(\ce{NaCl}\)
    2. concentrated \(\ce{NaCl}\) solution
    3. \(\ce{HNO3}\) solution
    4. concentrated \(\ce{H2SO4}\) solution
    5. deionized water
    6. any solid salt

Hint: b. \(\ce{NaCl}\) solution

A salt solution is usually used, but solutions of acids and bases will be all right. \(\ce{NaCl}\) solution is economical and easy to handle.

  1. A solution of which one of the following will best conduct electricity? All solutions have the same concentration in M.
    1. alcohol
    2. ammonia
    3. sugar
    4. acetic acid
    5. table salt

Hint: e. table salt

Skill:
Distinguish strong and weak electrolytes.

  1. Which one of the following solutions has the highest pH?
    1. 0.10 M \(\ce{NaCl}\) solution
    2. 0.10 M \(\ce{HCl}\) solution
    3. Water at 273 K (freezing point of water)
    4. Water at 293 K (room temperature)
    5. Water at 373 K (boiling point of water)

Hint: c. water at low temperature

See pH.

Skill:
Define and estimate pH.

  1. When solutions of electrolytes are combined, the cations and anions exchange partners. These reactions are called:
    1. combustion reactions
    2. redox reactions
    3. oxidation reactions
    4. reduction reactions
    5. metathesis reactions

See Redox.

Skill:
Explain metathesis reactions.

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