4 - acids and redox

• What do all acids contain?


  All acids contain hydrogen in their formulae
• What happens when an acid is dissolved in water?


  Acids release hydrogen ions as protons, H⁺, into the solution.
• Strong acid


  A strong acid, such as hydrochloric acid, HCl, releases all its hydrogen atoms into solution as H⁺ ions and completely dissociates in aqueous solution.
• Weak acid


  A weak acid, such as ethanoic acid, CH₃COOH, only releases a small portion of its available hydrogen atoms into solution as H⁺ ions. A weak acid partially dissociates in aqueous solution.
• What is classified as a base?


  Metal oxides, metal hydroxides, metal carbonates and ammonia, NH₃, are classified as bases.
• What does a base do?


  A base neutralises an acid to form a salt
• Alkali


  An alkali is a base that dissolves in water releasing hydroxide ion (OH⁻) into the solution.
• What happens in neutralisation of an acid?


  An acid's H^+ ions react with a base to form a salt and neutral water. The H⁺ ions from the acid are replaced by metal or ammonium ions from the base.
• Formula of hydrochloric acid

  HCl
• Formula of sulfuric acid

  H₂SO₄
• Formula of nitric acid


  HNO₃
• Formula of ethanoic acid (vinegar)

  CH₃COOH
• Ionic equation of a neutralisation reaction?


  H⁺ (aq) + OH⁻ (aq) → H₂O (l)
• What is formed when an acid is neutralised by a carbonate?


  A salt, water and carbon dioxide gas.
• Titration


  A technique used to accurately measure the volume of one solution that reacts exactly with another solution.
• What can titrations be used for?

  1. Finding the concentration of a solution.

  2. Identification of unknown chemicals.

  3. Finding the purity of a substance.
• Why is purity of a substance important?


  Its an important aspect of quality control, especially for compounds manufactured for human use, e.g. medicines, food and cosmetics.

  
  

  A tiny bit of impurity in a drug could cause a lot of harm to a patient.
• Practical: how to prepare a standard solution?


  1. The solid is first weighed accurately.

  
  

  2. The solid is dissolved in a beaker using less distilled water than will be needed to fill the volumetric flask to the mark.

  
  

  3. This solution is transferred to a volumetric flask. The last traces of the solution are rinsed into the flask with distilled water.

  
  

  4. The flask is carefully filled to the graduation line by adding distilled water a drop at a time until the bottom of a meniscus lines up exactly with the mark. If too much water is added, the solution will be dilute and must be prepared again. View the graduation mark at eye level for accuracy.

  
  

  5. Finally, the volumetric flask is slowly inverted several times to mix the solution thoroughly. If this step is omitted titration results are likely to be inconsistent.
• What apparatus is used to titrate and acid against a base?

  A pipette or burette.
• Where is each burette reading recorded?

  To the nearest half division. With the bottom of a meniscus on a mark or between two marks.
• Where is each burette reading measure to?


  The nearest ±0.05cm³, so the reading always has two decimal places, the last place being either a 0 or a 5.
• Practical: acid-base titration procedure


  1. Add a measured volume of one solution to a conical flask using a pipette.

  
  

  2. Add the other solution to a burette, and record the initial burette reading to the nearest 0.05cm³.

  
  

  3. Add a few drops of an indicator to the solution in the conical flask.

  
  

  4. Run the solution in the burette into the solution in the conical flask, swirling the conical flask throughout to mix the two solutions. Eventually the indicator changes colour at the end point of the titration. The end point is used to indicate the volume of one solution that exactly reacts with the volume of the second solution.

  
  

  5. Record the final burette reading. The volume of solution added from the burette is called the titre, which is calculated by subtracting the initial burette reading from the final burette reading.

  
  

  6. A quick, trial titration is carried out to first to find the approximate titre.

  
  

  7. The titration is then repeated accurately, adding the solution dropwise as the end point is approached. Further titrations are carried out until two accurate titres are concordant - agreeing to within 0.10cm³.
• Oxidation number


  Oxidation number is based on a set of rules that apply to atoms, and can be thought of as the number of electrons involved in bonding to a different element.
• Oxidation number of elements

  In a pure element, any bonding is to atoms of the same element. So in H₂, O₂, P₄, S₈, Na and Fe the oxidation number of atom of the element is 0.
• Oxidation number of compounds and ions

  Each atom in a compound has an oxidation number. An xodiation number has a sign, which is placed before the number. Oxidation number of an element is numerically the same as the ionic charge.
• How to work out oxidation numbers?


  Sum of the oxdiation = total charge.
• Two types of redox rections?

  1. Addition and reduction of oxygen.

  2. Loss and gain of electrons.
• OILRIG


  Oxidation is loss (of electrons).

  Reduction is gain (of electrons).
• Redox in terms of oxidation numbers


  - Reduction is a decrease in oxidation number.

  - Oxidation is an increase in oxidation number.
• Difference between alkali and base


  Alkalis are also a base. Bases which are soluble in water are known as alkali. Bases do not dissolve in water. All alkalis are bases but not all bases are alkalis.