topic 4 - chemical changes

• When metals react with other substances, the metal atoms lose electrons to form positive ions
• The more reactive a metal is:

  • the more vigorous its reactions are
  • the more easily it loses electrons in reactions
• When a metal reacts with water, a metal hydroxide and hydrogen are formed
• When a metal reacts with a dilute acid, a salt and hydrogen are formed
• A metal below hydrogen in the reactivity series will not react with dilute acids
• Many metals react with oxygen to make metal oxides. The reactions are oxidisation reactions because the metal gains oxygen
• Oxygen can be removed from metal oxides in chemical reactions. In this reaction, the metal is reduced as it loses oxygen
• Oxidisation

  Is

  Loss (of electrons)

  Reduction

  Is

  Gain (of electrons)
• Oxidisation is the gain of oxygen. Reduction is the loss of oxygen
• A more reactive metal can displace a less reactive metal from its compounds. For example, magnesium is more reactive than copper so it displaces copper from copper sulfate solution:

  magnesium + copper sulfate —> magnesium sulfate + copper
• A balanced ionic equation can be split into 2 half equations. For example:

  Mg(s) + Cu^2+(aq) —> Mg^2+(aq) + Cu(s)

  Mg(s) —> Mg^2+(aq) + 2e^-

  magnesium atoms lose electrons - oxidisation

  Cu^2+(aq) + 2e^- —> Cu(s)

  copper ions gain electrons - reduction

  Reduction and oxidisation happen at the same time, so the reactions are called redox reactions
• Most metals are extracted from ore found in the earth’s crust
• If a metal is less reactive then carbon, it can be extracted from its compounds by heating with carbon. Copper is an example of this:

  Copper oxide + carbon —> copper + carbon dioxide

  Copper oxide is reduced as carbon is oxidised so this is a redox reaction
• Metals more reactive than carbon are extracted by electrolysis. For example, aluminium is extracted by electrolysis of molten aluminium oxide
• During electrolysis of aluminium, its ions gather at the cathode and gain electrons to form aluminium atoms. At the anode, oxide ions lose electrons and form oxygen gas.
• Acids form acidic solutions in water. Acids produce hydrogen ions in aqueous solution
• Alkalis form alkaline solutions in water. Alkalis produce hydroxide ions in aqueous solutions
• A neutral solution is neither acidic or alkaline
• The pH measures the acidity or alkalinity of a solution. If the pH is 7, the solution is neutral and universal indicator turns green. Alkaline solutions have a pH of over 7 and turn blue/purple. Acidic solutions have a pH of under 7 and turn orange/red.
• Blue litmus paper turns red in acidic solutions and stays blue in neutral and alkaline solutions
• Red litmus paper stays red in acidic and neutral solutions and turns blue in alkaline solutions
• A base is any substance that reacts with an acid to form a salt and water
• Bases that are soluble in water are called alkalis and dissolve in water to form alkaline solutions
• A neutralisation reaction is a reaction between an acid and a base
• Acids take part in reactions in which salts are produced. In these reactions, the hydrogen ions in the acids are replaced by metal ions
• When acids react with metals, the products are a salt and hydrogen
• The reactions of acids with metals are redox reactions. Reduction and oxidisation happen at the same time
• Acids are neutralised by bases, including alkalis, and metal carbonates
• Metal oxides are bases. In general, when an acid reacts with a metal oxide, the products are a salt and water. Alkalis are soluble bases. A salt and water are produced when acids react with alkalis.
• A salt, water and carbon dioxide are produced when acids react with carbonates
• A soluble salt can be prepared by reacting an acid with a suitable insoluble reactant, for example a metal, metal oxide, metal hydroxide or a carbonate
• A dilute solution contains a relatively small amount of dissolved solute
• A concentrated solution contains a relatively large amount of dissolved solute
• Acids in solution are a source of hydrogen ions. The hydrogen ions are produced when the acid dissociates or breaks down to form ions
• Strong acids completely dissociate into ions in solution
• Weak acids only partially dissociate in solution
• The pH of a solution is a measure of its concentration of hydrogen ions. The higher the concentration of hydrogen ions, the lower the pH. The lower the concentration of hydrogen ions, the higher the pH
• A soluble salt can be prepared by reacting an acid with a dilute solution of an alkali such as sodium hydroxide or ammonia. The steps are:

  • carry out a titration to determine volumes that must be mixed
  • mix the acid and alkali in the volumes without an indicator
• Electrolytes are ionic compounds that are in the molten state or dissolved in water. This means the ions are free to move
• Electrolysis is a process in which electrical energy breaks down electrolytes