4. Energetics

• Exothermic


  Energy is transferred to the surroundings, making a bond e.g. combustion

  
• Endothermic


  Energy is taken from the surroundings, breaking a bond e.g. photosynthesis

  
• Enthalpy Change (ΔH)


  The heat energy change measured under conditions of constant pressureenthalpy of products - enthalpy of reactantsOne arrow down, one arrow up
• Standard enthalpy changes refer to standard conditions, what are the standard conditions


  100kPa, deltaH298
• Enthalpy of combustion (∆Hc)


  Enthalpy change when 1 mole of a substance burned completely in oxygen with all reactants and products in their standard state under standard conditions. Both arrows down

  
• Enthalpy of formation

  Enthalpy change when 1 mole of a substance is formed from its constituent elements, with both reactants and products being in their standard states under standard conditions - always zero for elementsBoth arrows up

  
• Enthalpy of reaction

  Enthalpy change when molar quantities of reactants as stated in the equation react together under standard conditions

  
• Calorimetry


  An accurate way of deciding if a reaction is endo or exothermic
• q = m c ΔT

  q = enthalpy change (J)m = mass of substance being heated (g)c = specific heat capacity (4.18)ΔT = temperature change (celsius/kelvin)
• 0.25g of sodium reacted with 100cm3 of water producing a temperature rise of 6.2K. Find the enthalpy change per mole of sodium (SHC = 4.18)

  Moles Na = mass/Mr = 0.25/23 = 0.0109 molq = mcΔT = 100 x 4.18 x 6.2 = 2592JScale up to 1 mole:ΔH = q/moles = 2592/0.0109 = 237798Convert J--->kJ = 237798/1000 = 238kJ= - 238 kJ/mol-1
• Required Practical 2


  e.g. magnesium + hydrochloric acid ---> magnesium chloride + hydrogen1) measure 25cm3 of 1.0mol/dm3 HCl into an insulated coffee cup with a pipette2) record the temperature at the start and every minute for 3 minutes3) on the 4th minute, add 1g Mg and stir4) Record the temperature every minute up to 10 minutes5) repeat the experiment 3 times to find anomalies and calculate an average6) plot a graph to show the temperature change and work out if endo or exothermic
• What does Hess's Law state?


  The overall enthalpy change for a reaction is independent of the route the reaction takes
• Using Hess's Law to find enthalpy of formation


  1) write out the elemental form of the equation2) add in arrows pointing upwards3) reverse the arrows necessary to make a circuit and change the corresponding enthalpy signs4) substitute the values from the question
• Using Hess's Law to find enthalpy of combustion


  1) write out the products of complete combustion2) add in arrows pointing downwards3) reverse the arrows necessary to make a circuit and change the corresponding enthalpy signs4) substitute the values from the question
• Mean bond enthalpy


  The average bond dissociation enthalpy for a particular bond in a range of different compoundsEnergy required to break 1 mole of covalent bond in a gaseous stateArrows go down
• Why do values from mean bond enthalpy calculations differ from those determined using Hess's law?

  Because bond enthalpy is an estimate/average of many different compounds
• Write an equation to show the enthalpy of combustion of methane

  CH4(g) + 2O2(g) --> CO2(g) + 2H2O(l)
• Write an equation to show the enthalpy of combustion of potassium


  K + 1/4O2 --> 1/2K2O
• Explain why values quoted in the table are not mean bond enthalpy values

  Bonds only occur in these molecules
• Suggest why the value obtained is different from the quoted value

  Heat lost to the surroundingsNot specific to molecules in the reactionIncomplete combustionMean bond enthalpies are not exact
• Working out bond enthalpy


  Products - reactants
• When working out enthalpy of formation and standard enthalpy change, is it products - reactants or reactants - products


  products - reactants
• When working out enthalpy of combustion, is it products - reactants or reactants - products


  reactants - products
• delta h = q / moles


  delta h is the enthalpy change per moleq is heat energy
• What is the standard state for elements

  Zero
• For an alkane with n carbon atoms, enthalpy combustion = -(496n + 202) kJ mol-1Enthalpy of combustion of an alkane was -6650 kJ mol-1Deduce the formula of the alkane


  -6550 = - (496n + 202)496n = 6650 - 202496n = 6448n = 6448/496 = 13C13H28
• When isooctane is burned, enthalpy change is -47.8 kJg-1It has a density of 0.692 g cm-3Calculate heat energy released in kJ when 1dm3 of it burns in excess oxygen


  Mass = density x volume = 0.692 x 1000 = 692gq = deltaH x mass (because enthalpy change is in g not mol) = 47.8 x 692 = 33100 kJ
• The data book value for combustion of butanone is -2430 and refers to the formation of carbon dioxide gas and water in a gaseous state. How would this value differ if it referred to the formation of water in the liquid state?

  More negativeBecause heat is released when water vapour condenses
• Write equations for titanium oxide being converted into titanium chloride, and titanium chloride into titanium

  TiO2 + 2Cl2 + 2C --> TiCl4 + 2COTiCl4 + 4Na --> Ti + 4NaCl
• Suggest one reason may the value cannot be measured directly

  Hard to react the exact amount of water
• The bond dissociation enthalpy for chlorine is +242 and fluorine is +158. Standard enthalpy of formation of ClF is -56. Calculate bond enthalpy of Cl-F bond. Then calculate enthalpy of formation of ClF3

  79 + 121 - (-56)= 256 kJmol-1reactants - products(0.5Cl-Cl) + (1.5F-F) - (3Cl-F)121 + 237 - 768= -410 kJmol-1
• Suggest why a value for the Na-Cl bond enthalpy is not found in any data book

  NaCl is ionic
• Enthalpy change how to do the questions

  Draw the arrows in whatever direction the enthalpy values are given. E.g. if given enthalpy of formation values but need to find enthalpy of combustion, draw the cycle with the arrows pointing upwards and put it all the values, and then make a route by changing direction of arrows and remember to change +/- of the value if the arrow changes direction
• Equation to represent the reaction that has a standard enthalpy change equal to the standard enthalpy of formation for barium chloride

  Ba(s) + Cl2(g) --> BaCl2(s)
• 50cm3 of 0.5mol dm-3 aqueous HCl are reacted with 50cm3 with 0.5mol dm-3 aqueous NaOHdeltaH = -57.1kJmol-1The initial temperature of each solution is 18.5CCalculate the maximum final temperature of the reaction mixtureSHC = 4.18Density = 1gcm-3Suggest how, without changing apparatus, percentage uncertainty could be reduced


  q = mcdeltatdeltat = q/mc = (1.4275x1000)/(100x4.18) = 3.4q = 57.1 x 0.025 = 1.4275kJ18.5 + 3.4 = 21.9Increase concentration of solutions
• Which reaction has an enthalpy change equal to the standard enthalpy of formation of silver iodideAg(g) + 0.5I2(g) --> AgI(s)Ag(s) + 0.5I2(s) --> AgI(s)Ag+(g) + I-(g) --> AgI(s)Ag+(aq) + I-(aq) --> AgI(s)


  Ag(s) + 0.5I2(s) --> AgI(s)