8. energetics

• what does enthalpy mean?

  the systems internal energy plus product of its pressure and volume. the total enthalpy of a system cannot be measured directly in joules.
• what is a system?


  the reactants
• what is a surrounding?

  the product/everything around the system.
• what is enthalpy change?


  total energy change between the system and surroundings measured at a constant pressure.
• what is an open system?


  system which can exchange energy and matter with the surrdoundings.
• what is a closed system?

  system which can exchange energy but not matter with the surroundings.
• what is an endothermic reaction?

  • reactions that absorb energy from surroundings
  • enthalpy change is positive and arrows point up
  • enthalpy of reacting system increases

  
• what is an exothermic reaction?

  • reactions that transfer/release energy to the surroundings
  • enthalpy change is negative and arrows point down
  • enthalpy of reacting system decreases

  
• what are standard condition requirements?

  • pressure of 100kpa
  • temperature of 298k (25 degrees)
  • standard states
  • solutions to have concentration of 1 moldm3
• what is the standard enthalpy change of a reaction?

  the enthalpy change when a reaction occurs in the molar quantities under standard conditions.
• what is the standard enthalpy change of formation?

  the enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions.
• what is the standard enthalpy change of combustion?

  the enthalpy change when one mole of a substance is completely burnt in oxygen, under standard conditions.
• what is the standard enthalpy change of neutralisation?

  the enthalpy change measured from the energy given out when acids react with alkalis in a aqueous solution to form a salt and one mole of water.
• what equation is used to measure enthalpy changes?

  Q=mcΔt

  ΔH=-Q/n

  where n is the number of moles = mass/mr or concentration x volume
• what are some sources of error when measuring enthalpy changes?

  • some of the heat energy is transferred to air not water
  • some of the fuel not completely burnt in to form co2 and h2o
  • some of heat is transferred to polystyrene cup and not water
  • conditions arent standard
  • heat capacity of calorimeter not included
  • experiment takes time so not all heat energy transferred is compensated for
• what can you change to reduce error and improve accuracy?

  • for enthalpy of combustion, use a bomb calorimeter which is designed to ensure sample burns completed and energy losses are minimised
  • for enthalpy of neutralisation/reaction, use an insulated polystyrene cup or insulated plastic cup, as it has negligible heat capacity and a good insulator
• what is calorimetry?

  the practical determination of enthalpy changes usually involving heating or cooling of known volumes of water and allows for calculations of combustion, formation and neutralisation.

  • when water or an object increases in temperature, reaction is exothermic
  • when water or an object decreases in temperature, reaction is endothermic
• what is Hess's Law?

  states that the total enthalpy change of a reaction, depends only on the initial and final states of the reactants and products, and is independent of the route taken.
• what goes on the line below of hess's cycle in combustion?

  CO2 + H2O

  arrows point down
• what goes on the line below of hess's cycle in formation?

  the singular elements to form the compound

  arrows point up
• what is bond enthalpy?

  the energy needed to break one mole of a particular covalent bond in a gaseous molecule.
• what is mean bond enthalpy?

  the enthalpy change needed to break the covalent bonds into gaseous atoms averaged over different molecules.

  always positive- breaking bonds
• what is bond breaking?

  • to break a bond, energy needs to be absorbed
  • bonds are broken in the reactants and this is an endothermic process so ΔH is positive.
• what is bond making?

  • to form a bond energy is released/transferred
  • bonds are made when products are being produced and this is an exothermic process so ΔH is negative.
• what is the formula for mean bond enthalpy?

  ΔH= total bonds broken - total bonds made
• describe a calorimetry experiment? steps 1-4?

  • washes the equipment with solutions to be used
  • dry the cup after washing
  • put polystyrene cup in a beaker for insulation and support
  • Measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup
• describe a calorimetry experiment? steps 5-6?

  • clamp thermometer into place making sure the thermometer bulb is immersed in solution
  • measure the initial temperatures of the solution or both solutions if 2 are used. Do this every minute for 2-3 minutes
• describe a calorimetry experiment? steps 6-8?

  • At minute 3 transfer second reagent to cup. If a solid reagent is used then add the solution to the cup first and then add the solid weighed out on a balance.  If using a solid reagent then use ‘before and after’ weighing method.
  • stirs mixture (ensures that all of the solution is at the same temperature) Record temperature every minute after addition for several minutes
• explain the calorimetry experiment?

  If the reaction is slow then the exact temperature rise can be difficult to obtain as cooling occurs simultaneously with the reaction.

  To counteract, take readings at regular time intervals and extrapolate the temperature curve/line back to the time the reactants were added together.

  Take the temperature of the reactants for a few minutes before they are added together to get a better average temperature.

  If the two reactants are solutions then the temperature of both solutions need to be measured before addition and an average temperature is used.