Acids and bases


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Summary presentation for grade 12 science, chapter 9: acids and bases

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Acids and bases

  1. 1. 1 Everything Science 9. Acids and bases Chemistry Grade 12
  2. 2. 2 Everything Science Revision The Arrhenius definition of acids and bases defines an acid as a substance that increases the concentration of hydronium ions (H3 O+ ) in a solution. A base is defined as a substance that increases the concentration of hydroxide ions (OH− ) in a solution. However, this definition only applies to aqueous solutions (in water). The Brønsted-Lowry definition is much broader. An acid is a substance that donates protons (H+ ) and a base is a substance that accepts protons. In different reactions, certain substances can act as both an acid and a base. These substances are amphoteric substances. Amphiprotic substances are amphoteric substances that are Brønsted-Lowry acids and bases. Water is both amphoteric and amphiprotic. A conjugate acid-base pair refers to two compounds (one reactant and one product) that differ only by a hydrogen ion (H+ ) and a charge of +1.
  3. 3. 3 Everything Science Strength and concentration
  4. 4. 4 Everything Science Ka and Kb Ka and Kb are the equilibrium constants for the reaction of an acid or a base with water. A large Ka and Kb means that the acid or base is strong. A small Ka and Kb means that the acid or base is weak. Strong acid, large Ka Strong base, large Kb Weak acid, small Ka Weak base, small Kb
  5. 5. 5 Everything Science Acid-base reactions When an acid and a base react, they form a salt and water. The salt is made up of a cation from the base and an anion from the acid. An example of a salt is sodium chloride (NaCl), which is the product of the reaction between sodium hydroxide (NaOH) and hydrochloric acid (HCl). The reaction between an acid and a base is a neutralisation reaction. In the reaction between an acid and a metal the products are a salt and hydrogen. In the reaction between an acid and a metal hydroxide or metal oxide the products are a salt and water. In the reaction between an acid and a metal carbontae or metal hydrogen carbonate the products are a salt, water and carbon dioxide.
  6. 6. 6 Everything Science The pH scale The pH scale is a measure of the acidity or alkalinity of a solution. It ranges from 0 to 14. Values greater than 7 indicate a base, while those less than 7 indicate an acid. Kw Auto-protolysis is the transfer of a proton between two of the same molecules. The auto- ionisation of water is one example of auto-protolysis. Water ionises to a small extent. Kw is a measure of this auto-ionisation. Kw is 1 × 10−14 at 25 ° C. We can use Kw to help us calculate pOH. pOH is calculated as: pH = - log[H+ ] = - log[H3 O+ ] pH = 14 - pOH
  7. 7. 7 Everything Science Titrations A simple rule for determining the likely pH of a solution is as follows: An indicator is a compound that is a different colour in a basic solution, an acidic solution, and at the end-point of a reaction. They are used to determine the end-point during a neutralisation reaction.
  8. 8. 8 Everything Science Titrations contd. Titrations are the method used to determine the concentration of a known substance using another, standard, solution. Acid-base titrations are an example. Acid-base titrations are carried out as follows: 1. A carefully measured volume of the solution with unknown concentration is put into a conical flask. 2. A few drops of a suitable burette indicator is added to this solution. 3. The conical flask is placed on a white tile or piece of paper. 4. A volume of the standard solution with unknown solution (known concentration) concentration is put into a burette and is slowly added to the solution in the flask, drop by drop. 5. At some point, adding one more drop will change the colour of the unknown solution to the colour of the end-point of the reaction. 6. Record the volume of standard solution that has been added up to this point. 7. Use the information you have gathered to calculate the exact concentration of the unknown solution. 8. Note that adding more solution once the end-point has been reached will result in a colour change from the end-point colour to that of the acid (if the solution in the conical flask is a base) or of the base (if the solution in the conical flask is an acid).
  9. 9. 9 Everything Science For more practice see: Shortcode: ESCPR
  10. 10. 9 Everything Science For more practice see: Shortcode: ESCPR