Chemistry

• Law of conservation of mass: no atoms are lost or made during a chemical reaction, so the mass of the products equals the mass of the reactants
• In a chemical equation, it must be balanced in terms of the numbers of atoms of each element involved on both sides - there must be exactly the same number on each side
• Relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula

  • For example, for HCl: Mr = 1 + 35.5 = 36.5
• In a balanced chemical equation, the sum of Mr of reactants in quantities shown equals the sum of Mr of products in quantities shown (essentially no mass is lost during a reaction)
• Whenever a measurement is made, there is always some uncertainty about the result obtained
• Be prepared to make estimations of uncertainty
• Electrolytes are ionic compounds in the molten state or dissolved in water
• When an ionic substance is melted or dissolved, the ions are free to move about within the liquid or solution, known as an electrolyte
• Electrolysis is a process in which electrical energy from a direct current supply decomposes electrolytes
• During electrolysis, positively charged ions (cations) move to the negative electrode (cathode), and negatively charged ions (anions) move to the positive electrode (anode)
• Ions are discharged at the electrodes producing elements
• • Br- to anode, Br2 (l) is produced
• When ionic compounds are molten, it is simpler to predict the products of electrolysis as there are no ions present except those in the ionic compound:

  • Identify which ions are within the ionic compound
  • The + ions will go to the cathode
  • The - ions will go to the anode
• Oxidation is Loss (of electrons)

  Reduction is Gain (of electrons)

  Reduction occurs at the cathode and oxidation occurs at the anode in electrolysis reactions
• In the electrolysis of copper sulfate solution using copper electrodes, the process can be used to purify copper:

  • Cu2+ ions from the anode move to the cathode, where they gain electrons and are discharged as pure copper
  • Impurities form as sludge below the anode
  • The cathode will increase in mass as it gains pure copper, while the anode will lose mass as copper ions are lost and so are impurities
• In a core practical investigating the electrolysis of copper sulfate solution:

  • With inert electrodes:
  • At the cathode Cu (s) is produced
  • At the anode O2 is produced
  • With copper electrodes:
  • Cu2+ ions deposited as Cu at the cathode are replaced by Cu2+ ions from the anode, maintaining the concentration of Cu2+ ions in the solution
• What is reduction

  The gain of eletrons