Chemical changes topic 4

• pH Scale


  Goes from 0 to 14
• pH


  Measure of how acidic or basic a solution is
• The lower the pH of a solution

  The more acidic the solution
• The higher the pH of a solution


  The more basic/alkaline the solution
• Neutral pH is 7
• Acid


  Substance that forms H+ ions in water, has pH less than 7.

  Acids release hydrogen ions (H+) in water which makes the solution acidic
• Base/Alkali


  Substance that forms OH- ions in water, has pH greater than 7
• Neutralisation reaction

  Acid + Base → Salt + Water
• When an acid neutralises a base (or vice versa), the products are neutral (pH 7)
• Indicators can be used to show when a neutralisation reaction is complete
• Neutralisation reactions of strong acids and bases can be used to calculate concentrations by titration
• Lemon juice is acidic
• The pH of an unknown solution is 8
• Titration


  Technique used to find the concentration of an acid or base
• Titration procedure

  1. Fill burette with acid

  2. Add base slowly to acid until colour change

  3. Record volume of acid used

  4. Repeat for consistent results
• Single indicators are used for titrations to detect the end point
• Strong acids fully dissociate in water to produce H+ ions
• Weak acids only partially dissociate in water to produce H+ ions
• pH measures the concentration of H+ ions in a solution
• As pH decreases by 1


  H+ ion concentration increases by a factor of 10
• Strong acids have lower pH than weak acids of the same concentration
• Acid strength (strong/weak) is different to acid concentration
• the dissociation of weak acids is a reversible reaction, which means that the products can react together to reform the acid.
• Which of these describes the strength of an acid?

  The proportion of acid molecules which dissociate into hydrogen ions
• As the concentration of hydrogen ions in a solution increases, what happens to the pH?

  The pH decreases
• what is acid?

  Substance that forms H+ ions in water, has pH less than 7.

  Acids release hydrogen ions (H+) in water which makes the solution acidic
• name three strong acids?

  hydrochloric acid (HCl)

  Sulfuric acid (H2SO4)

  Nitric acid (HNO3)
• For a weak acid, does the position of equilibrium lie to the left or the right?

  left
• what is meant by the strength of an acid?

  the proportion of acid molecules which dissociate into hydrogen ions
• Remember that an acids strength refers to how well it ionises (dissociates) when added to water. Whereas, its concentration refers to how many molecules there are per dm3 of solution.
• metal oxide + acid = salt + water

  metal hydroxide + acid= salt + water

  metal carbonate+ acid=salt + water + carbon dioxide
• During a neutralisation reaction, the positive hydrogen ions from the acid react with the negative hydroxide ions from the alkali to form molecules of water.
• Remember that when an acid and base react, a salt is always formed.

  To work out the formula of the salt, take the positive ion from the base, such as sodium from sodium oxide (Na2O), and the negative ion from the acid, such as chloride from hydrochloric acid (HCl), and combine them together.

  So in the case of sodium oxide (Na2O) reacting with hydrochloric acid (HCl), the salt would be sodium chloride (NaCl).
• When a soluble salt is formed from an acid and an insoluble base, how do you know when an excess of base has been added?

  Some of the reactant will be left unreacted
• Pure metals can be extracted from metal oxides using the element carbon.

  The carbon causes the metal to lose its oxygen, so the metal becomes reduced.

  This produces CO2, and can only work for metals less reactive than carbon.
• what is oxidation?

  Oxidation is the loss of electrons, gain of oxygen
• what is reduction?

  reduction is the gain of electron and the loss of oxygen
• Why do we find pure gold in the ground, but not pure iron?

  Gold is unreactive, so doesn't react with any other elements

  iron is reactive enough to react with oxygen so is oxidised to iron oxide 
• In the electrolysis of a solution, which two ions are present from the water molecules themselves?

  hydrogen(H+) and hydroxide(OH-)
• In the electrolysis of aqueous solutions, what is the rule for determining which ion will be oxidised (lose electrons) at the anode (positive electrode)?

  It will always be the OH-, unless there is a halide ion (such as Cl-, or Br-) present