energetics

• what energy change is breaking bonds associated with?

  energy is taken in to break bonds- endothermic reaction
• what energy charge is making bonds associated with

  energy is released to make bonds- exothermic reaction
• what are some uses of thermochemistry

  measuring comparing the energy values of fuels

  calculating the energy requirements for industrial processes working out the theoretical amount of energy released/ taken in a reaction

  predicting if a reaction will take place or not
• what is an endothermic reaction

  one with overall positive enthalpy change (delta H+)- energy is breaking bonds > energy out making bonds
• what is an exothermic reaction

  one with an overall negative enthalpy change(- delta H)- energy is breaking bonds < energy out making bonds
• if a reversible reaction is endothermic one way what type of reaction is the other way

  exothermic
• give 2 examples of exothermic reactions

  combustions of fuels

  neutralisation
• give an example of an endothermic reaction

  thermal decomposition
• define enthalpy change what symbol is used to represent it

  energy change of system at a constant pressure

  represented by delta H
• what are the standard conditions

  100kPA/ 1atm pressure

  298K/25 c temperature
• what does 'in standard state' mean

  the state an element or compound exists at in standard conditions
• draw an enthalpy change diagram for an endothermic reaction and one for an exothermic reaction 


  https://docs.google.com/document/d/16kPXIfEyHwGSQUFJXvCj3veRcybJ7Kbb2ob5TkkCciQ/edit
• define standard enthalpy of formation

  the enthalpy change when one mole of a compound is formed from it constituent elements in standard conditions ( 100kPA ,298K) with reactants and produces in their standard states
• give an example of an equation which represents standard enthalpy formation

  eg.

  H2(g)+1/2 O2(g)->h20(l)
• define standard enthalpy of combustion

  the enthalpy change when one mole of a substance is burnt completely in oxygen in standard conditions (100kPA, 298K) with reactants and products in their standard states
• give an example of an equation which represents standard enthalpy of combustion 


  eg. c(s) +02(g)->co2(g)
• what is the difference between heat and temperature

  heat-sum of all particles energy so its affected by the amount of substance

  temperature- is related to the mean kinetic energy of the particles in a system so is independent of the number of particles present
• how can you calculate enthalpy change from experimental data

  use the equation Q=mc delta T where m is the mass of the substance being heated(usually water) c is the specific heat capacity of that substance and the delta T is the change in temperature
• draw a simple calorimeter

  https://docs.google.com/document/d/16kPXIfEyHwGSQUFJXvCj3veRcybJ7Kbb2ob5TkkCciQ/edit
• how could this calorimeter be made more accurate

  add fraught screens at the sides

  add a lid on top of the beaker

  add mineral wool around the beaker

  all to insulate and reduce heat lost to the surroundings
• what is a flame calorimeter how does it differ to a simple calorimeter

  reduces heat lost to the surroundings

  gives a more accurate result

  has spiral chimney made of copper

  an enclosed flame and the fuel is burnt in pure oxygen

  no air
• how would you measure the enthalpy change for a reaction occuring in (aq)

  use an expanded polystyrene cup as a calorimeter( good insulator -> reduces heat loss)

  heat is generated in the solution measure this temperature change

  take heat capacity of solution to be 4.18 and density of solution = 1gmc -3
• what can you use to make experimental determination of enthalpy change of reaction more accurate

  cooling curves
• what is hess's law ?

  states that the enthalpy change for a reaction is the same regardless of the route taken
• what is the enthalpy of an element 


  the enthalpy of all elements in their standard states ( the states in which they exist at 100kPA and 298K) is defined as 0
• define bond dissociation enthalpy

  the enthalpy change required to break a covalent bond with all species in the gaseous state differs for the same bond type in different molecules
• define mean bond enthalpy

  average value( across different chemical environments) for the bond dissociation enthalpy of a given bond
• why may experimental methods for enthalpy determine not very accurate

  heat is lost to the surroundings

  not in standard conditions

  reaction may not go to completion
• why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation?

  bond enthalpies are a mean for the same bond across different molecules standard enthalpy of combustion and formation apply just to that molecule therefore they are more accurate