Acids And Alkalis

Acids are substances that form hydrogen ions ( ) when dissolved in water.

Example

Hydrochloric acid   gives  (aq) and  (aq) ions when dissolved in water. 

Although there is a wide variety of acids, they have certain general properties:

acids turn litmus from blue to red in colour;

acids are electrolytes, because in solution, they are ionic and therefore conduct electricity;

Acid + metals ----> salt + hydrogen gas

Mg + H2SO4 ---> MgSO4 + H2

Acid + Metal Oxide(Bases) ---> salt + water

CuO + 2HNO3 ---> Cu(NO3)2 + H2O

Acid + Metal Carbonate ---> salt + water + carbon dioxide

CaCO3 + 2HCl ---> CaCl2 + CO2 + H2O

Base and Alkali

Compounds which react with acid to form a salt and water only are called bases.

A soluble base is called an alkali and in aqueous solution it produces hydroxide ions (OH-).

Alkalis are substances that form hydroxide ions (OH-(aq)) in water

Acid + Alkali Salt + Water

NaOH + HCI NaCI + H2O

Alkali + Metal ion Insoluble Metal Hydroxide

2OH- + Ca2+ Cu(OH)2

Alkali + Ammonium Salt Salt + Water + Ammonia Gas

NaOH + NH4 NaCI + H2O + NH3

The Strength of an Acid or Alkali

pH is an abbreviation for power of the Hydrogen and has values from 0 to 14.

Any pH value below seven is acidic; the lower it is, the stronger the acid, and the higher the

hydrogen ion concentration.

Any pH value above seven is alkaline; the higher the value, the stronger the alkali, and the lower the

hydrogen ion concentration.

The value of seven is the neutral value, which means that the solution is neither acidic nor alkaline.

Used Universal Indicator

Strong acids

Strong acids are fully ionised in solution

Weak acids

Weak acids are only partially ionised in solution. Some of the ions recombine, and remain as

molecules. This is shown by the reversible reaction sign in the equation. An example is carbonic acid.

Concentration (g dm-3)

Concentration is the number of moles of solute per liter of solution. A concentration of 10 g dm-3 means

there is 10 g of solute dissolved in1 dm3 of solution.

Strong acid Weak acid

Sulphuric acid H2SO4 Carbonic acid H2CO3

Nitric acid HNO3 Ethanoic acid CH3COOH

Hydrochloric acid HC1 Sulphurous H2SO3

  Citric acid C6H8O7

  Phosphoric acid H3PO4

Molarity (M)

Molarity is probably the most commonly used unit of concentration. It is the number of moles of solute per

liter of solution. A concentration of 2 mol dm-3 means there are 2 moles of solute dissolved in 1 dm3 of

solution.

Dilution

In dilution of solution, we should take note that mole of solute before dilution is equal to the mole of

solute after dilution

M1 = Molarity before dilution

M2 = Molarity after dilution

V1 = Volume before dilution

V2 = Volume after dilution

M1V1 = M2V2

Concentration of Acid

Concentration of hydrogen ion in acid depends on

1. Molarity(concentration) of the acid

2. Strong acid or weak acid

3. Basicity of the acid

Preparing A standard Solution

A standard solution is a solution whose concentration was known.

Usually, a standard solution is prepared by volumetric flask.

In the process, you need to calculate the mass of the solute and the volume of the solvent.

Example 9

You are asked to prepare 250cm3 of the solution of sodium hydroxide, 0.5 moldm-3. Find out the mass of sodium hydroxide that you need to dissolve into the solvent.

[ Relative atomic mass: Na=23, O = 16; H=1] Answer

The mol of NaOH =

Molar mass of sodium hydroxide

=

The mass of sodium hydroxide

=

Molarity and the pH Value of Acid or Alkali

 pH value of acid or alkali is affected by 2 factors:

1. The Molarity

2. Strong or weak acid(Alkali)

NeutralizationIndicator colour in acid pH<7 colour in neutral pH=7 colour in alkali pH >7

Litmus* red 'purple' blue

phenolphthalein colourless colourless pink

methyl orange* pinky red orange about pH 6 yellow

methyl red red orange yellow

bromothymol blue yellow green blue