CHEMISRTY PROJECT: ACID-BASE EQUILIBRIA

NAME: FELISAH HOSEINSUBJECT: CHEMISTRYTEACHER: MISS NICOLE ABDULTOPIC: CHAPTER 9:ACID-BASE EQUILIBRIA

OBJECTIVES

• Ionic product of water• Acid-base indicators • pH changes during titrations

• The Ionic Product Of Water, Kw

The important equilibrium in water• Water molecules can function as both acids and

bases. One water molecule (acting as a base) can accept a hydrogen ion from a second one (acting as an acid). This will be happening anywhere there is even a trace of water - it doesn't have to be pure.

• A hydroxonium ion and a hydroxide ion are formed.

• However, the hydroxonium ion is a very strong acid, and the hydroxide ion is a very strong base. As fast as they are formed, they react to produce water again.

• The net effect is that an equilibrium is set up.

• At any one time, there are incredibly small numbers of hydroxonium ions and hydroxide ions present. The hydroxonium ions present in pure water turns out to be 1.00 x 10-7 mol dm-3 at room temperature.

• You may well find this equilibrium written in a simplified form:

• This is OK provided you remember that H+

(aq) actually refers to a hydroxonium ion.

Defining the ionic product for water, Kw

• Kw is essentially just an equilibrium constant for the reactions shown. You may meet it in two forms:

• Based on the fully written equilibrium . . .

• . . . or on the simplified equilibrium:

• Little of the water is ionised at any one time, that its concentration remains virtually unchanged - a constant. Kw is defined to avoid making the expression unnecessarily complicated by including another constant in it.

The value of Kw• Like any other equilibrium constant, the

value of Kw varies with temperature. Its value is usually taken to be 1.00 x 10-14 mol2 dm-6at room temperature. In fact, this is its value at a bit less than 25°C.

The pH concept Self Ionization of water

• A sample of pure water will contain a small quantity of ions (H+ and OH- ) produced from the self- ionization of water.

• These ions exist only for a brief time period before rejoining to form water molecules.

The following equation describes this process:

H2O(L) H+ (aq) + OH- (aq)

the equilibrium constant for the self – ionization of water can be expressed as:

K = [H+ ] [OH- ] [ H2O]

Note :However , since the concentration of liquid

water is constant and so large relative to the concentration of ions, it can be omitted from the equilibrium expression.

At 25 degrees Celsius, the equilibrium expression can be rewritten as :

Kw = [H+ ] [OH- ] =[H3O +] [OH- ] = 1.0 x 10-14

Where the subscript “w” in the equilibrium constant expression stands for “water”.

Because the amounts of hydrogen and hydroxide ions are equal in water (and any neutral solution) we can write:

[H+ ] = [OH- ] = 1.0 x 10-7 MAn increase in one would result in the

decrease in the other.

Acid-Base IndicatorsWhat is an Acid – Base indicator?

An acid-base indicator is any substance whose colour is dependent upon the pH of the solution, they themselves are weak acids.

Different indicators have different ranges

Examples of Acid-Base indicators

Methyl orangeLitmusBromothymol bluePhenolphthalein

Diagram showing Phenolphthalein in acid (left) and Phenolphthalein in base

(right)

• pH CHANGES DURING TITRATIONS

• Titrations involve the addition of one solution to another in order to find out how much of the two solutions just react with each other.

• When you carry out a simple acid-base titration, you use an indicator to tell you when you have the acid and alkali mixed in exactly the right proportions to "neutralize" each other. When the indicator changes colour, this is often described as the end point of the titration.

• An indicator is used to tell when the end point is reached.

• At the end point the indicator changes colour.

• In titration, when the number of moles of acid equals the number of moles of base, this is known as the Equivalence point

• A plot of the changes in pH as an acid is added to an alkali or vice versa is known as a titration curve

The pH Scale

pH stands for the power of hydrogen ion concentration in solution. The numeral values for pH range from 0 to 14 and can be calculated using the following equation:

pH = - log [H+ ] Recall: [H+ ] = 1.0 x 10-7

pH = - log [H+ ] = -log (1.0 x 10-7 ) = 7

Titration curves for a strong acid against a strong base

Running acid into base

Running alkali into the acid

Titration curves for a strong acid against a weak base

Running acid into the alkali

Running alkali into the acid

Titration curves for a weak acid against a strong base

Running acid into the alkali

Running alkali into the acid

Titration curves for a weak acid against a weak base