chemistry

• relative atomic mass or relative molecular / formula mass


  Ratio of the average mass of the atoms in a sample of an element to the mass of one atom of carbon-12
• Electrolysis


  The decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current
• Electrolyte


  The molten or aqueous substance that undergoes electrolysis
• Describe the transfer of charge during electrolysis


  1. Movement of electrons in the external circuit

  2. Loss or gain of electrons at the electrodes

  3. Movement of ions in the electrolyte
• Products formed during electrolysis of


  • Molten lead(II) bromide
  • Concentrated aqueous sodium chloride
  • Dilute sulfuric acid
• Metals or hydrogen are formed at the cathode and non-metals (other than hydrogen) are formed at the anode
• Metal objects are electroplated to improve their appearance and resistance to corrosion
• Hydrogen-oxygen fuel cell


  Uses hydrogen and oxygen to produce electricity with water as the only chemical product
• Hydrogen-oxygen fuel cells


  • Advantages and disadvantages compared to gasoline/petrol engines in vehicles
• Exothermic reaction


  Transfers thermal energy to the surroundings leading to an increase in the temperature of the surroundings
• Endothermic reaction


  Takes in thermal energy from the surroundings leading to a decrease in the temperature of the surroundings
• Reaction pathway diagrams can show exothermic and endothermic reactions
• Enthalpy change, ∆H


  The transfer of thermal energy during a reaction. ∆H is negative for exothermic reactions and positive for endothermic reactions
• Activation energy, Ea


  The minimum energy that colliding particles must have to react
• Bond breaking is an endothermic process and bond making is an exothermic process
• Physical change


  A change in the physical properties of a substance without any change in its chemical composition
• Chemical change


  A change that results in the formation of one or more new substances with different properties
• A catalyst increases the rate of a reaction and is unchanged at the end of a reaction
• Collision theory


  • Number of particles per unit volume
  • Frequency of collisions between particles
  • Kinetic energy of particles
  • Activation energy, Ea
• A catalyst decreases the activation energy, Ea, of a reaction
• Reversible reaction


  A chemical reaction that can proceed in both the forward and reverse directions
• Describe how changing the conditions can change the direction of a reversible reaction for

  1. The effect of heat on hydrated compounds

  2. The addition of water to anhydrous compounds
• Equilibrium


  In a closed system, when the rate of the forward reaction is equal to the rate of the reverse reaction and the concentrations of reactants and products are no longer changing
• Symbol equation for the production of ammonia in the Haber process: N2(g) + 3H2(g) ⇌ 2NH3(g)
• Sources of hydrogen (methane) and nitrogen (air) in the Haber process
• Typical conditions in the Haber process: 450 °C, 20 000 kPa / 200 atm and an iron catalyst
• Symbol equation for the conversion of sulfur dioxide to sulfur trioxide in the Contact process: 2SO2(g) + O2(g) ⇌ 2SO3(g)
• Sources of sulfur dioxide (burning sulfur or roasting sulfide ores) and oxygen (air) in the Contact process
• Typical conditions for the conversion of sulfur dioxide to sulfur trioxide in the Contact process: 450 °C, 200 kPa / 2 atm and a vanadium(V) oxide catalyst
• Oxidation


  • Loss of electrons
  • Increase in oxidation number
• Reduction


  • Gain of electrons
  • Decrease in oxidation number
• Redox reactions involve gain and loss of electrons
• Reducing agent


  A substance that reduces another substance and is itself oxidised
• Aqueous solutions of acids contain H+ ions and aqueous solutions of alkalis contain OH- ions
• Neutralisation reaction between an acid and an alkali to produce water: H+ (aq) + OH- (aq) → H2O (l)
• Acids


  Proton donors
• Bases


  Proton acceptors
• Strong acid


  Completely dissociated in aqueous solution
• Weak acid


  Partially dissociated in aqueous solution
• Hydrochloric acid is a strong acid: HCl (aq) → H+(aq) + Cl-(aq)