chemical changes

• in oxidation a substance gains oxygen and loses electrons
• in reduction a substance loses oxygen and gains electrons
• metals react with oxygen to form metal oxides
• the more easily a metal loses an electron, the more reactive it is, which forms the reactivity series
• in a displacement reaction, a more reactive metal will displace a less reactive metal from a solution of its salt
• the method of extraction depends on the reactivity of metals. unreactive metals such as gold exist as elements in the earth
• metals are often found as metal oxides, metals that are less reactive than carbon will be heated with carbon to form pure metal and carbon dioxide
• a pH with a pH less then 7 are acidic and pH more than 7 is alkaline. this can be measured using a pH probe or universal indicator
• soluble bases are called alkalis. acids are neutralised by bases
• acid + metal hydroxide = salt + water
• acids contain and hydrogen ions ( H+ ) and alkalis contain hydroxide ions ( OH- ). when an acid reacts with an alkali the ions react to form water. this is a neutralisation reaction
• acids can be neutralised by metal oxides and metal carbonates

  • acid + metal oxide = salt + water
  • acid + metal carbonate = salt + water + carbon dioxide
• soluble salts can be made by mixing acids with insoluble bases such as metal oxides, metal hydroxides and metal carbonates
• electrolysis is the use of an electrical current to break down compounds containing ions into their elements. the substance being broken down is the electrolyte
• the electrodes must conduct electricity and are usually made of carbon.
• in electrolysis

  • negatively charged ions move to the anode
  • positively charged ions move to the cathode
• metals that are more reactive then carbon are extracted using electrolysis. this requires lots of heat and electrical energy so is expensive
• cryolite lowers the melting point of a substance so less energy is needed to break it down
• when ionic compounds dissolved in water to form aqueous solutions the water molecules break down to form OH- and H+. at the negative electrode:

  • hydrogen produced is metal is more reactive than hydrogen
  • metal produced if less reactive than hydrogen
• when ionic compounds dissolved in water to form aqueous solutions the water molecules break down to form OH- and H+. at the negative electrode:

  • hydrogen produced is metal is more reactive than hydrogen
  • metal produced if less reactive than hydrogen
  • at positive, oxygen produced unless solution contains halide ions
  • if halide present then the halogen is produced