Chemistry DF

• Molar volume of gases

  One mole of any gas at room temperature and pressure will take up the same volume, 24,000 cm3 or 24 dm3
• Ideal gas law


  • Pressure is proportional to Temperature
  • Volume is proportional to Temperature
  • Pressure and Volume are inversely proportional
• The ideal gas equation is: pV = nRT
• Units for ideal gas equation

  • p = pressure in Pascals
  • V = volume in m3
  • T = temperature in Kelvin
  • n = moles
  • m = mass in grams
• R is the ideal gas constant, equal to 8.31 JK-1 mol-1
• Experimental techniques to measure gas volume

  • Use a gas syringe
  • Measure mass lost on a weighing balance and calculate moles of gas
  • Collect the gas released in an upturned test tube filled with water
• Chemical equations must be balanced before use in calculations
• State symbols

  • (s) - solid
  • (l) - liquid
  • (g) - gas
  • (aq) - aqueous (dissolved in water)
• Shapes of simple molecules and ions

  • Determined by the number of electron pairs around the central atom and the repulsion between them
  • Largest bond angle possible exists between covalent bonds
• Bonding in organic molecules

  • Single bonds are σ bonds with free rotation
  • Double bonds consist of a σ bond and a π bond formed by sideways overlap of p-orbitals
• Enthalpy change (ΔH°)

  Heat energy change under standard conditions of 100 kPa pressure and 298 K temperature
• Bond breaking


  Endothermic process, energy taken in from surroundings
• Bond making


  Exothermic process, energy released to surroundings
• Overall enthalpy change (ΔH) = Sum of bond breaking enthalpies - Sum of bond making enthalpies
• Bond enthalpy


  Averaged value representing the energy required to break one mole of the stated bond in the gaseous state under standard conditions
• Shorter bonds are stronger than longer bonds, and double/triple bonds are stronger than single bonds
• Enthalpy change of reaction (ΔrH°)

  The enthalpy change when quantities of substances in standard states react completely under standard conditions
• Enthalpy change of formation (ΔfH°)

  The enthalpy change when one mole of a substance is produced from its elements under standard conditions
• Enthalpy change of combustion (ΔcH°)

  The enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions
• Enthalpy change of neutralisation (ΔneuthH°)

  The enthalpy change when solutions of acid and alkali react together under standard conditions to produce one mole of water
• Standard conditions


  Solution concentrations of 1.00 mol dm-3, pressure 100 kPa and a stated temperature (generally 298 K)
• Calorimetry


  Experimental method for finding enthalpy change by measuring temperature change over time as a reaction occurs
• q = mcΔT, where q = energy change (J), m = mass (g), c = specific heat capacity (J g-1 °C-1), ΔT = temperature change (°C)
• Specific heat capacity


  The energy required to raise 1g of the substance by 1K without a change of state
• Hess's law states that the overall enthalpy change for a reaction is the same regardless of the route taken
• q


  Energy change of a substance of known mass
• Specific heat capacity


  Energy required to raise 1g of the substance by 1K without a change of state
• Calculating enthalpy change per mole (J mol-1) of substance


  Using energy value (q)
• Energy in a reaction system must be conserved, as it cannot be created or destroyed
• The overall enthalpy change for a reaction is the same, regardless of the route taken
• Hess's Law


  Used to determine the enthalpy changes for reactions that cannot be found directly using an experimental method
• Triangular cycle method with an intermediate product

  Direction of the arrows indicates whether the values should be added or taken away
• Enthalpies of Formation

  Used with Hess's law to calculate an enthalpy change
• Enthalpies of Combustion


  Used with Hess's law to calculate an enthalpy change
• Catalyst


  Lowers the activation energy (the minimum energy required for a reaction to occur) of a reaction by providing an alternative reaction route
• Heterogeneous Catalysts

  • Catalysts that are in a different phase or state to the species in the reaction
  • A solid heterogeneous catalyst works by adsorbing molecules onto an active site on the surface of the catalyst, which increases the proximity of molecules and weakens the covalent bonds in the molecules, allowing reactions to occur more easily
• Impurities in a reaction mixture may bind to a heterogeneous catalyst's surface and block reactants from being adsorbed, which is known as catalyst poisoning and reduces the activity of the catalyst
• Cracking

  A process used to convert long-chain hydrocarbons into shorter, more useful hydrocarbons by breaking the carbon-carbon bonds
• Types of Cracking


  • Thermal cracking (uses high temperatures and pressures)
  • Catalytic cracking (produces aromatic compounds with carbon rings, uses lower temperatures around 720 K and normal pressure, but a catalyst is also used)
• Combustion of Fuels


  Fuels are combusted to release energy, but this also releases pollutants into the atmosphere