Energetics I

• enthalpy change - the amount of heat energy taken in or given out per mole during any change in a system at a constant pressure.
• exothermic process - energy is transferred from the system to the surroundings, giving the products less energy than the reactants. Thus enthalpy change is negative.

  
• endothermic process - energy is transferred from the surroundings to the system, giving the products more energy than the reactants. Thus enthalpy change is positive.

  
• standard conditions

  • 298K
  • 100KPa
  • concentration of 1 moldm^3
• standard enthalpy of combustion - enthalpy change under standard conditions when one mole of a substance is completely combusted in excess oxygen to make carbon dioxide and water.
• standard enthalpy of formation - enthalpy change under standard conditions when one mole of a compound is made from its elements in their standard states.

  e.g. Na(s) +1/2 N2 (g) + 1/2 O2 (g) = NaNO3

  there has to be 1 mol of NaNO3 by definition
• standard enthalpy of neutralisation - enthalpy change under standard conditions when one mole of water is made from the reaction between an acid and alkali.
• Q of a reaction = -Q of water
• bond enthalpy - enthalpy change required to break one mole of gaseous covalent bonds into gaseous atoms.
• mean bond enthalpy - enthalpy change required to break one mole of gaseous covalent bonds into gaseous atoms, but averaged over a range of different molecules.
• mean bond enthalpies are used because every bond in a compound has a slightly different enthalpy.
• bond breaking is endothermic
• bond making is exothermic
• enthalpy = bonds broken - bonds formed
• Hess' Law - total enthalpy change for a reaction is independant of the route by which the chemical change takes place/ its the same no matter which route is taken.
• enthalpy change of formation for elements is 0. This is because no energy is required to make an element in its standard state from the element in its standard state.
• equation for enthalpy of combustion of C6H4(OH)2

  e.g. C6H4(OH)2 (l) + 6.5 O2 (g) = 6 CO2 (g) + 3 H2O (l)

  
  

  there is 1 mol of C6H4(OH)2 by definition
• equation for enthalpy of formation of CH3CH2OH (l)

  e.g. CH3CH2OH = 2 C (s) + 3 H2 (g) + 1/2 O2 (g)

  there is 1 mol of CH3CH2OH by definition.
• the enthalpy of combustion of carbon is the same as the enthalpy of formation of carbon dioxide.

  e.g. C(s) + O2 (g) = CO2
• the enthalpy of combustion of hydrogen is the same as the enthalpy of formation of water.

  e.g. H2 (g) + 1/2 O2 (g) = H2O (l)
• enthalpy of formation hess diagram

  
• enthalpy of combustion hess diagram

  
• CORE PRACTICAL 8 - to determine the enthalpy change of a reaction using Hess' Law (part 1)

  • Calorimetry is a technique used to measure changes in enthalpy of chemical reactions
  • A calorimeter can be made up of a polystyrene drinking cup, a vacuum flask or metal can
• 
  

  
  

  CORE PRACTICAL 8 - to determine the enthalpy change of a reaction using Hess' Law (part 1)

  • Calorimetry is a technique used to measure changes in enthalpy of chemical reactions
  • A calorimeter can be made up of a polystyrene drinking cup, a vacuum flask or metal can
  • The energy needed to raise the temperature of 1 g of a substance by 1 K is called the specific heat capacity (c) of the liquid
  • The specific heat capacity of water is 4.18 J g-1 K-1
  • The energy transferred as heat can be calculated by:
  • remember q=mc∆t, q = heat energy change, m = the mass of water

  enthalpy change = Q/N where N = moles of the fuel

  
• CORE PRACTICAL 8 - to determine the enthalpy change of a reaction using Hess' Law (part 2)

  1. Using a measuring cylinder place 25 cm3 of the 1.0 mol dm-3 copper(II) sulphate solution into the polystyrene cup
  2. Weigh about 6 g of zinc powder - as this is an excess there is no need to be very accurate
  3. Put a thermometer or temperature probe in the cup, stir, and record the temperature every half minute for 2.5 minutes.
  4. At precisely 3 minutes, add the zinc powder to the cup
  5. Continue stirring and record the temperature for an additional 6 minutes
• CORE PRACTICAL 8 - to determine the enthalpy change of a reaction using Hess' Law (part 3)

  IMPORTANT!!!!!

  assumptions that can be made during the experiment:

  • The specific heat capacity of the solution is the same as pure water, i.e. 4.18 J g-1 K-1
  • That the density of the solution is the same as pure water, i.e. 1 g cm-3
  • The specific heat capacity of the container is ignored
  • The reaction is complete
  • There are negligible heat losses
• CORE PRACTICAL 8 (part 4)

  Temperature correction curve:

  • During a non-instantaneous reaction, there may be a delay before the max temp is reached, and in that time, the substances themselves may be losing heat to the surroundings, so that the true maximum temperature is never actually reached

  Solution:

  1. Add the second reactant and continue recording the temperature and time, after the first reactant is added.
  2. Plot the graph and extrapolate the cooling part of the graph until you intersect the time at which the second reactant was added.

  
• Working out the enthalpy change of formation from enthalpy changes of combustion (oxygen cant have an enthalpy change of combustion value, since it can't burn itself):

  
• spirit burner enthalpy change experiment:

  set up a simple calorimeter, where the burning alcohol in the spirit burner transfers heat to a known mass of water, allowing you to calculate the heat released based on the measured temperature change of the water, and then determine the enthalpy of combustion per mole of alcohol by accounting for the mass of alcohol burned.
• enthalpy of atomisation - The enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state under standard conditions:

  
  

  *it's half the value for a bond enthalpy

  
  

  e.g:

  $NH_3(g)\ \rightarrow\ 1\ N\ (g)\ +$$\ 3\ H\ (g)$

  $\frac{1}{2}H_2\ (g)\ \rightarrow\ 1\ H\ (g)$
• when doing a calorimetry reaction with two solutions, your mass in the q=mc∆t calculation is the total mass of both solutions together.
• Allery chemistry hess cycle further example:

  *imagine there's a roadblock on your enthalpy of formation/reaction route. Hence you have to take the alternative route using your enthalpy of combustion data.

  
• Hess' Law Calculation

  
• when using the $ΔH\ =$$Q\ \div N$ the moles depends on the reaction you're investigating:

  e.g. when finding the enthalpy change of combustion (measured per 1 mol of fuel thats combusted) you use the moles of your fuel.

  when finding the enthalpy of neutralisation (per mol of water produced in the reaction) you use the moles of water.
• in the equation $Q\ =$$\ m\times c\timesΔT$

  the specific heat capacity you're given corresponds to the substance who's mass you need to use.
• since water has a density of 1, its mass in grams is the same as its volume.