Energetics

• What are energy and reactions?

  Chemical reactions that involve the release/supply of energy
• Define an exothermic reaction

  Gives out (transfers) energy to the surroundings

  the products have less energy than the reactants

  
• Define an endothermic reaction

  
Takes in heat energy from the surroundings the products have more energy than the reactants

  
• How is the energy demonstrated during the reactions?

  Joules, J
• Define activation energy

  minimum amount of energy the reactant particles need in order to collide with each and react
• what does the symbol ^H signifies ?

  The change of heat (energy)
• what would be the change of heat energy in exothermic reactions?

  negative change
• what would be the change of heat in an endothermic reaction?

  positive change
• explain the making and breaking of bonds

  energy must be supplied/release to break/form bonds/ions

  Breaking of bonds is through endothermic reactions. Different bonds have different strengths and so their are different amounts of energy need to break.

  Forming of bonds is through exothermic reactions. The energy must be conserved and cannot va is, therefore, it is released.
• how do we call the energy needed to separate/form bonds?

  bond energies
• why do some exo or endo thermic reactions do not occur immediately?

  Activation energy
• What are the standard conditions for endo/exothermic reactions to occur?

  298 K

  100 kPa

  1 mol/dm-3 (concentration when liquid is inolved)

  The element must be in its most stable form
• Define the standard enthalpy change of formation

  standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its elements under standard conditions and with standard states
• Define the standard enthalpy change of combustion

  The standard enthalpy change of combustion is the enthalpy change when one mole of a compound is burned completely in oxygen understand standard conditions and it’s standard states.
• Define the standard enthalpy change of neutralisation

  The standard enthalpy change of neutralisation is the enthalpy change when solutions of acids and alkalis produce one mole of water under standard conditions
• Define the standard enthalpy change of reaction

  The standard enthalpy change of reaction is the enthalpy change for a specific reaction when the reaction quantities react under standard conditions
• what does Q = mc^T stand for ?

  Q = energy transferred (Kj or Kj/mol)

  m = mass of water heated or total volume of solution

  c = specific heat capacity of water (4.18)

  ^T = change in temperature
• what does ^H stand for ?

  Q/mol
• Why could the experimental value be so different to the actual value?

  Due to heat loss to the surroundings and to the apparatus

  Not fully dissolving in small amounts of water

  The heat not being completed
• what are the steps to calculating the amount of energy absorbed followed by the enthalpy change ?

  Finding the moles

  calculating Q

  calculating enthalpy change using Q because enthalpy change = Q/n
• Define Hess’ Law

  Hess’ Law is when one mole of a compound is formed from its elements under standard conditions with all of the chemicals in their standard states
• Define the enthalpy change in Hess’ Law 


  enthalpy change of a chemical reaction is independent of the path taken
• explain the calculations present for enthalpy change

  Enthalpy change of reaction

  
• Describe the arrows direction for standard enthalpy change of formation and combustion

  formation = point upwards

  combustion = point downwards

  others = one down then one up
• Define bond enthalpy

  Amount of energy required to break a covalent bond and separate atoms.

  The energy vale when the reactants and products are at the same state (gaseous)
• Differences in bond energies

  Not all bonds are equal when the rest of the molecule is taken into account.

  E.g a C-H bond in a molecule may have a different bond energy to further C-H bonds in the same molecule after the first bond is broken.

  Or a C-H bond in ethanol may have a different bond energy to a C-H bond in benzene.
• How are bond energies in polyatomic molecules demonstrated?

  quoted as averages
• Define bond length

  difference between the two nuclei involved in the bond
• what affects bond length?

  greater the size of the atoms, the greater the length

  shorter the length, the greater the bond energy

  double/triple bonds are shorter and stronger
• Explain the link between energy and length in bond energies

  Both nuclei have a strong attraction to the bonding pair/s, therefore, the bond is strong and short
• Explain the differences in bond energies between bonds being broken and made

  Endothermic reactions has a positive bond enthalpy as the energy is going into the system

  Exothermic reactions have a negative bond enthalpy as the energy is being released from the system
• Explain the calculations done that shows whether the reaction is endothermic or exothermic

  Bond enthalpy calculation

  
• write an equation example for a formation reaction(C + H)

  C(s) + 2H^2 (g) -> CH4 (g)
• write an equation example for a combustion reaction (methane + oxygen)

  CH4 (g) + 2O^2(g) -> CO2(g) + 2H2O
• write an equation example for a 'reaction' reaction(hydrogen + sulfur S8)

  8H2(g) + S8(g) -> 8H2S(g)
• write an example equation for a neutralisation reaction (NaOH + HCL)

  NaOH (aq) + HCL(aq) -> NaCL(aq) + H2O(l)
• when energy is released in an exothermic reaction, the temperature of the reaction mixture increases.
• when energy is absorbed in an endothermic reaction, the temperature decreases.