The Ions in Acids & Alkalis

04 April 2023
The Ions in Acids & Alkalis
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PhysicsSecondary EducationAge 12,13

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04 April 2023
The Ions in Acids & Alkalis

Slide 1 - Diapositive

Content objectives
  • I can describe the characteristic properties of bases in terms of their reactions with (a) acids; (b) ammonium salts.
  • I can state that bases are oxides or hydroxides of metals and that alkalis are soluble bases.
  • I can state that aqueous solutions of acids contain H+ ions and aqueous solutions of alkalis contain OHions.
  • I can describe how to compare hydrogen ion concentration, neutrality, relative acidity, and relative alkalinity in terms of colour and pH using universal indicator paper.

Slide 2 - Diapositive

Properties of Bases and Alkalis
  • pH values of above 7
  • A water-soluble base is referred to as an alkali.
  • bitter taste (almost all are NOT edible) 
  • In basic (alkaline) conditions: red litmus paper turns blue methyl orange indicator turns yellow; thymolphthalein indicator turns blue

Slide 3 - Diapositive

Acids and Bases
  • acids + water= hydrogen ions (H+) - this makes it acidic
  • alkalis (soluble base) + water = hydroxide ions (OH) - this makes it alkaline

Slide 4 - Diapositive

What is a neutralisation reaction?
  • when an acid reacts with an alkali
  • the H+ ions react with the OH ions to produce water

Slide 5 - Diapositive

Example

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Net ionic equation
  • The net ionic equation of acid-alkali neutralisations, and what leads to a neutral solution, since water has a pH of 7, is:
H+ + OHH2O

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03 April 2023
The Ions in Acids & Alkalis

Slide 8 - Diapositive

How do I know the degree of acidity and alkalinity?

Slide 9 - Diapositive

How do I know the degree of acidity and alkalinity?
  • The pH scale is a numerical scale that is used to show how acidic or alkaline a solution is, in other words, it is a measure of the number of ions present in the solution.

Slide 10 - Diapositive

Slide 11 - Diapositive

Hydrogen Ion Concentration & pH
  • The more hydrogen ions, the stronger the acid, but the lower the pH.
  • The more hydroxide ions in a solution, the higher the pH.
  • pH is a measure of the concentration of H+ ions in solution, but they have an inverse relationship.

Slide 12 - Diapositive

Logarithmic
  • each change of 1 on the scale represents a change in concentration by a factor of 10
Example 1
  • an acid with a pH of 3 has ten times the concentration of H+ ions than an acid of pH 4

Slide 13 - Diapositive

Example 2
  • An acid with a pH of 2 has 10 x 10 = 100 times the concentration of H+ ions than an acid with a pH of 4

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03 April 2023
The Ions in Acids & Alkalis

Slide 15 - Diapositive

The Universal indicator
  • Universal indicator is a mixture of different indicators which is used to measure the pH.
  • A drop is added to the solution and the colour is matched with a colour chart which indicates the pH which matches specific colours.

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Typical reactions of bases
Bases + acids
  • neutralisation reaction occurs
  • Acid + Base → Salt + Water

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Typical reactions of bases
Alkalis and ammonium salts
  • Ammonium salts undergo decomposition when warmed with an alkali.
  • Even though ammonia is itself a weak base, it is very volatile and can easily be displaced from the salt by another alkali.
  • A salt, water and ammonia ar e produced.

Slide 19 - Diapositive

NH4Cl + NaOH →NaCl + H2O + NH3

This reaction is used as a chemical test to confirm the presence of the ammonium ion (NH4+).

Example 

Slide 20 - Diapositive

Test for Ammonium Ion (NH4+)
Procedure
  • Alkali is added to the substance with gentle warming followed by the test for ammonia gas using damp red litmus paper.
  • The damp litmus paper will turn from red to blue if ammonia is present.

Slide 21 - Diapositive