4-Chemical changes

• pH scale goes from 0 - 14 and measures the acidity or alkalinity of a solution (concentration of H+ ions there are - more H+, more acidic- each decrease of 1 on the scale increases them by a factor of 10)

  • 0 - 6 are acidic
  • 7 is neutral
  • 8 - 14 are alkaline
• acids are substances with pH of below 7,

  bases are substances with pH of above 7
• pH can be measured with:

  • pH probe electronically measures pH and produces a numerical reading
  • chemical indicator, it will change colour
  • universal indicator is a mixture of chemical indicators, goes from red to blue due to pH
• pros and cons of pH measures

  • pH probe: more accurate, more expensive
  • chemical indicator: less accurate, cheaper
• • acids are substances with pH of below 7, produce hydrogen ions in aqueous solutions (H+/-)
  • bases are substances with pH of above 7, produce hydroxide ions in aqueous solutions (OH+/-)
  • alkalis are a base that dissolve in water to form a solution with the pH of above 7, only some bases are soluble in water
• neutralisation

  • reaction when acid and base react together
  • always produces salt and water
• • strong acids will ionise completely which means all of it's particles will dissociate/ the reactants will completely turn into products
  • weak acids will not fully ionise, a small portion of it's particles will dissociate to release hydrogen ions, the ionisation is reversible and the equilibrium lies to the left- more undissociated particles
• strength and concentration in acids:

  • strength- how much an acid can dissociate
  • concentration- how much acid there is in a certain volume
• examples of strong acids:

  • sulfuric acid - H₂SO₄
  • nitric acid -HNO₃
  • hydrochloric acid - HCl
• examples of bases:

  • sodium hydroxide - NAOH
  • calcium carbonate - CACO₃
• examples of weak acids:

  • ethanoic acid - CH₃COOH
  • citric acid
  • carbonic acid
• neutralisation reactions

  • acid + metal oxides → salt + water
  • acid + metal hydroxides → salt + water
  • the negative ions from the salt combine with the positive ions from the base to form a salt
  • he positive hydrogen ions from the acid react with the negative hydroxide ions from the alkali to form molecules of water
  • acid + metal carbonates → salt + water + carbon dioxide
• required practical- making soluble salts with acid

  1. place fixed volume of dilute sulfuric acid in beaker and heat gently (limiting reactant)
  2. add the solid base which is copper oxide little by little till it stops reacting and there is black powder left over (excess)
  3. filter the excess base using filter paper and a funnel
  4. heat the blue salt solution which is copper sulfate in evaporating basin until crystals start to form
  5. leave the solution to dry overnight so that crystals will form
  6. scrape the crystals onto paper towel and gently pat them dry
• reactivity series of metals from most to least reactive:

  1. potassium
  2. sodium
  3. lithium
  4. calcium
  5. magnesium
  6. carbon
  7. zinc
  8. iron
  9. hydrogen
  10. copper
• reactivity refers to how easily an atom loses or gains electrons to form an ion. The easier, the more reactive it will be
• reactivity of metals with acid

  • metal + acid → salt + hydrogen
  • the most reactive metals will react more violently
  • group 1's (1-3) will react explosively, whiz around, release lots of heat, produce lots of hydrogen
  • group 2's (4-5) will produce lots of bubbles as solid metal disappears in the acid
  • zinc and iron (7-8) will produce less bubbles
  • copper (10) won't react at all
• testing reactivity of metals with acid

  • measure temperature change - most reactive will release most heat
  • has to be a fair test - same mass and surface area of metal, fixed volume of acid with the same concentration
• reactivity of metals with water

  • metal + water → metal hydroxide + hydrogen
  • only the most reactive metals are reactive enough to do this (1-4)
  • magnesium will react slightly
  • zinc, iron and copper won't react at all
• metal displacement reactions

  • more reactive metals can displace less reactive metals
  • Mg + FeSO₄ → MgSO₄ + Fe (Mg is more reactive than Fe)
• OIL RIG - loss and gain of electrons

  • Oxidation Is Loss (of electrons)
  • Reduction Is Gain (of electrons)
• oxidation and reduction of oxygen

  • oxidation is the gain of oxygen
  • reduction is the loss of oxygen
  • when metals oxidate they become metal oxides
• metal oxides

  • when a metal is oxidised, therefore gaining oxygen
  • most metals are metal oxides as oxygen is so abundant in the atmosphere
  • although unreactive metals like gold are found in their pure form as they are too unreactive to react with oxygen
• reduction of metal oxides (removal of oxygen) to get pure metals

  • metal oxide + carbon → pure metal + carbon dioxide
  • this can only be done to metals less reactive than carbon: zinc, iron, copper
  • metals that are more reactive than carbon won't be displaced by it so need electrolysis
• redox reactions

  • when reduction and oxidation take place at the same time
  • as neither can take place by themselves, an atom must gain the electrons the other atom lost
• ionic displacement (redox) reaction equation

  • Ca + FeSO₄ → CaSO₄ + Fe
  • Ca + Fe²⁺ → Ca²⁺ + Fe
  • the SO₄ ions don't change so are not shown in the ionic equation
• electrolysis equipment

  • electrodes made out of carbon that are inert (unreactive)
  • positive electrode is the anode
  • negative electrode is the cathode
  • place the electrodes in electrolyte solution - has ions
  • connect electrodes with wire
  • connect wire to a cell that provides electricity
• electrolysis is used to extract reactive metals from their metal oxides (reduce/reduction) by melting them into their molten ionic compounds and passing electricity through them. It is only used on metals more reactive than carbon as it is expensive due to lots of energy needed to be used
• electrolysis

  • the ions are being oxidised and reduced at the electrodes (in terms of electrons)
  • anode - ions are oxidised
  • cathode - ions are reduced
• aqueous electrolysis

  • cathode will attract the positive ions - metal ion, hydrogen ion
  • it will only discharge the ion of the least reactive element. If there is copper then it will be discharged instead of hydrogen
  • anode will attract the negative ions- hydroxide ion (OH) or halide
  • it will discharge a halide(halogen-group 7) if it is present, if halide isn't present then the hydroxide ion is discharged