developing fuels

• enthalpy definition: the heat content of a substance (H), kJmol(^-1)
• system (sys): the molecules reacting
• surroundings (surr): atmosphere and solvent in a reaction
• exothermic: energy is released from the system to the surroundings

  increase in surr temperature

  negative ΔH value
• endothermic: energy absorbed from surr to the sys

  decrease in surr temperature

  positive ΔH value
• standard conditions:

  • 1 atm
  • 298K
  • reactants and products in standard states
  • 1.0 moldm(^-3) solutions
• enthalpy change of reaction

  • ΔrH
  • the enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions
• enthalpy change of combustion

  • ΔcH
  • the enthalpy change when one mole of a substance is burnt completely in excess oxygen under standard conditions (all in standard states)
• enthalpy change of formation

  • ΔfH
  • the enthalpy change when one mole of a compound in its standard state is formed from its constituent elements in their standard states under standard conditions
• enthalpy change of neutralisation

  • ΔneutH
  • the change in enthalpy when one mole of H+ ions react with one mole of OH- ions to form one mole of water under standard conditions
• calculating ΔcH experiment method

  • calculating energy change per mole of fuel burnt

  1. measure 100 cm^3 water into metal calorimeter
  2. measure initial temp of water
  3. weigh and record initial mass of spirit burner
  4. set up apparatus (metal calorimeter containing thermometer in clamp over spirit burner containing liquid fuel, all guarded by draught shield), heat water while stirring until temp stops changing
  5. extinguish fuel by placing lid over flame, reweigh spirit burner and calculate mass of fuel burnt
• calculating ΔcH experiment calculations

  • calculating energy change per mole of fuel burnt

  1. q (J) = mass of water x s.h.c x temperature change (mcΔT)
  2. n(fuel) = mass / Mr
  3. ΔcH = -q (kJ) / n (mol)

  • units = kJmol^-1
  • add plus or minus
• calculating ΔcH experiment safety

  • keep spirit burner away from flames
  • don't blow out your burning fuel, use lid to extinguish the flame
• % error = theory - experiment

  ------------------- x 100

  theory
• % uncertainty = (uncertainty / actual value) x 100
• uncertainty (precision) of apparatus

  • take smallest division of apparatus
  • half the smallest division (unless a digital apparatus)
  • if using a difference of two readings from that apparatus, double the precision error
• specific heat capacity: the amount of energy needed to raise the temperature of 1kg of a substance by 1 degree
• Hess's Law: the overall enthalpy change of a chemical reaction is independent of the route taken
• Hess cycle for calculating ΔfH

  1. write out ΔfH equation
  2. write out box of combustion products underneath
  3. (remember state symbols)
  4. draw and label arrows from each molecule in top equation to the box of combustion products, ignoring oxygen
  5. ΔfH = + (ΔcH of reactants) - (ΔcH of products)
  6. units for ΔfH = kJmol^-1
• hess cycle for calculating ΔrH

  1. write out chemical equation
  2. draw box containing constituent elements underneath
  3. (remember state symbols)
  4. draw and label arrows from the box of elements to each molecule in the top equation, ignoring elements already in standard state as ΔfH will = 0
  5. ΔrH = - (ΔfH of reactants) + (ΔfH of products)
  6. units for ΔrH is kJmol^-1
• enthalpy changes may not be able to be worked out directly as reaction might be

  • reversible
  • very slow
  • impossible or only one of many reactions that might occur
• catalysts:

  • increase the rate of reaction but are chemically unchanged at the end of reaction
  • provide an alternative pathway of lower activation energy
• heterogeneous catalysts: reacts and catalysts are in different states
• mechanism of heterogeneous catalysis

  1. DIFFUSE - reactants diffuse towards catalyst surface
  2. ADSORB - reactants adsorb onto catalyst surface, weakens the bonds and lowers activation energy
  3. BREAK - weaker bonds in the molecules break
  4. FORM - new bonds in the molecules form, new products form on catalyst surface
  5. DESORB - products desorb from the catalyst surface
• catalyst poisoning:

  • catalysts poisoned by a chemical binding to surface
  • very strong adsorption of poison to catalyst surface
  • e.g. sulphur and lead
• cracking:

  • thermal decomposition
  • decomposing hydrocarbons to produce smaller, more useful molecules
• catalytic cracking

  • alkane is heated and the vapour is passed over a hot zeolite catalyst
  • reaction occurs at around 500 degrees C
  • makes more branched products, better fuel
• thermal (steam) cracking

  • alkane vapour is mixed with steam and heated to very high temp (800 degrees C)
  • very high pressure
  • very high yield for short alkenes
  • makes straight chain fuels
• the ideal gas law:

  • when under standard conditions, gases and volatile liquids follow certain trends
  • pressure is proportional to temp
  • volume is proportional to temp
  • pressure and volume are inversely proportional
• the ideal gas equation

  p V = n R T

  pressure x volume = moles x ideal gas constant x temperature
• the ideal gas equation units

  pressure = pascals

  volume = meters cubed

  temperature = kelving

  n = moles

  mass = grams
• the ideal gas equation unit conversion

  pressure

  • atm -- x 1.01 x 10 ^ 5 --> Pa
  • kPa -- x 1000 --> Pa

  volume

  • cm cubed -- x 10^-6 --> meters cubed
  • dm cubed -- x 10^-3 --> meters cubed

  temperature

  • c -- + 273 --> K
• CO pollutant:

  • formed by the incomplete combustion of hydrocarbon fuels
  • very toxic to humans and is oxidised to CO2 in the atmosphere
• NOx pollutant:

  • formed mainly from components in the air
  • NO - when N and O react in high temps of the engine
  • NO2 - some of the NO reacts with more O
• SOx pollutant

  • S - comes from sulfur compounds in the fuel
  • Ox - from the air, combines with S in the heat of the engine
• particulate pollutants

  • very small carbon particles
  • produced by incomplete combustion of hydrocarbons in diesel
• primary pollutant: pollutant released directly into the atmosphere
• secondary pollutant: not released directly into the atmosphere
• particulates

  • from: volcanoes, burning fuels, burning coal
  • effects: penetrate deep into the body causing heart attacks and cancer
• volatile organic compounds

  • from: plants, unburnt fuel from petrol engines
  • effects: photochemical smog