5.4 chemical changes

• oxidation is loss of electrons (or gain of oxygen)
• reduction is gain of electrons (or loss of oxygen)
• acids produce hydrogen ions (H+)when dissolved in water
• acids have pH values less than 7
• alkalis produce hydroxide ions (OH-) when dissolved in water
• alkalis have pH values greater than 7
• neutral substances are neither acidic or alkaline, they have pH values equal to 7
• pH scale measures the concentration of H+ ions in solution
• metal + acid > salt + hydrogen
• metal oxide + acid > salt + water
• metal hydroxide + acid > salt + water
• metal carbonate + acid > salt + water + carbon dioxide
• ionic equation of any neutralisation reaction

  acid (H+) + alkali (OH-) > salt + water (H2O)
• in an aqueous solution it is the H+ ions that make the solution acidic
• a strong acid is completely ionised in aqueous solution
• a weak acid is only partially ionised in aqueous solution
• electrolysis always turns ions into atoms
• the cathode is the negative electrode
• the anode is the positive electrode
• metals + oxygen > metal oxides
• metals form positive ions
• a more reactive metal can displace less reactive metals from a compound
• metals less reactive than carbon can be extracted from their oxides by reduction with carbon
• oxidation ionic equation example


  Na > Na+ + e-
• reduction ionic equation example


  Na+ + e- > Na
• acid + base > salt + water
• hydrochloric acid (HCl) produces chlorides (XCl) in alkali reactions
• nitric acid (HNo3) produces nitrates (XNO3) in alkali reactions
• sulfuric acid (H2SO4) produces sulfates (XSO4) in alkali reactions
• pH scale can be measured using universal indicator or a pH probe
• stronger an acid, lower the pH value, higher the H+ concentration in the solution
• as the pH decreases by one unit, the H+ concentration of the solution increases by x10
• when an ionic substance is melted or dissolved, the ions are free to move within the liquid of solution
• the substance being broken down in electrolysis is the electrolyte
• positively charged ions move to the negative electrode (cathode)
• negatively charged ions move to the positive electrode (anode)
• ions are discharged at the electrodes, producing elements
• when a simple ionic compound is electrolysed the metal is produced at the cathode and the non-metal is produced at the anode
• metals that are more reactive than carbon are extracted by electrolysis of molten compounds
• in aqueous solutions, at the negative electrode, hydrogen is produced unless the metal is less reactive than hydrogen