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Ionic Equilibrium: Definition & Calculations

Ionic Equilibrium: Definition & Calculations
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  • 0:04 Equilibrium
  • 0:39 Ionic Equilibrium in Solutions
  • 2:52 Sparingly Soluble Salts
  • 3:30 Solubility Product Principle
  • 5:20 Ionic Equilibrium Example
  • 6:21 Lesson Summary
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Lesson Transcript
Instructor: Saranya Chatterjee

Saranya has a masters degree in Chemistry and in Secondary Education. She has taught high school, AP chemistry for 2 years and is teaching undergraduate college chemistry for 3 years.

This lesson talks about ionic equilibrium and solubility product of sparingly soluble salts. It also shows how to calculate pH of a solution and solubility product/solubility of sparingly soluble salts.

Equilibrium

Have you ever seen what happens to raisins immersed in a bowl of water for couple of hours? The raisins swell up and the water turns brown. What's going on in there? Well, the water enters the raisins through their skin, and some of the raisin juice oozes out of the raisin into the water and turns the water brown. At some point, an equilibrium is established between the raisin juice and the water.

There can also be other day-to-day examples of establishing this kind of equilibrium. For example, carbonated water in a soda bottle has an equilibrium between carbonic acid to carbon dioxide (the bubbles) and water.

Ionic Equilibrium in Solutions

The equilibrium established between the unionized molecules and the ions in the solution of weak electrolytes is called ionic equilibrium. For example, take acetic acid breaking up into acetate ions and hydrogen ions:

CH3 COOH → CH3 COO- + H+

Chemical substances that can conduct electricity in their aqueous state or in molten state are called electrolytes. In pure water or in an aqueous solution, the product of concentrations of hydrogen and hydroxyl ions is a constant at a given temperature. This is called ionic product of water and is conventionally designated by Kw.

The idea of the ionic product of water can be understood by looking at the autoionization reaction of water that may be expressed as:

H2 O + H2 O = H3 O+ + OH-

Kw = CH3 O+ COH-

The value of Kw at 25°C is 1*10-14.

In the study of acid base equilibria in aqueous solutions, we're primarily interested in the hydrogen ion concentration of a solution. Solutions that we deal with are usually dilute, and the hydrogen ion concentrations are some negative power of 10.

pH is a measure of hydrogen ion concentration. It's a measure of the acidity or alkalinity of a solution. Aqueous solutions at 25°C with a pH less than seven are acidic, while those with a pH greater than seven are basic or alkaline.

pH can be calculated using the following formula.

pH = -log CH3 O+

Let's discuss how to calculate pH with an example.

What will be the concentration of CH3 O+ in a solution having pH = 5.6?

Solution: pH = -log CH3 O+

  • log [H3 O+] = -pH
  • log [H3 O+] = -5.6
  • [H3 O+] = antilog (-5.6) = 2.512*10-6

Sparingly Soluble Salts

Salts having lower solubility in water at ordinary temperature are called sparingly soluble salts. Silver chloride is one example. When excess of a sparingly soluble salt is dissolved in water, part of the salt passes into the solution to make it saturated at room temperature.

It appears that the process of dissolution has stopped, but actually, a dynamic equilibrium is established between the solid salt and the ions in solution. This can be represented as:

AgCl(solid) → Ag+ + Cl-

At equilibrium, the rates of the forward and the backward processes become equal.

Solubility Product Principal

Recall that the law of mass action states that the rate of any chemical reaction is proportional to the product of the masses of the reacting substances, with each mass raised to a power equal to the coefficient that occurs in the chemical equation. Application of the law leads to the solubility product principal, which was proposed by Nernst in 1889 and is of great significance in qualitative and quantitative analysis.

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