Developing Fuels

• Enthalpy change is when energy is transferred to and from the surroundings. Total Enthalpy Change = the sum of enthalpy change of products - the sum of enthalpy change of reactants.

  Exothermic reactions release energy, therefore enthalpy change is negative (loss of energy), temperature increases.

  Endothermic reactions absorb energy, therefore enthalpy change is positive (gain in energy), temperature decreases.

  Standard conditions = 298K, 100kPa, 1mol dm-3
• Standard Enthalpy Change of Reaction (Hr) = The enthalpy change when molar quantities of reactants stated in the equation react together under standard conditions.

  When calculating the value of enthalpy change, use the idea of moles and ratios.
• Standard Enthalpy Change of Combustion (Hc) = The enthalpy change that occurs when one mole of a substance is completely burnt in oxygen under standard conditions.

  The value of enthalpy change of combustion is always negative since combustion reactions are exothermic.

  Use enthalpy cycles to measure enthalpy change.
• Standard Enthalpy Change of Formation (Hf) = The enthalpy change that occurs when one mole of a compound is formed from its elements under standard conditions in their standard states.

  The value of enthalpy change of formation can be either positive or negative. The E.C.F of a pure element in its standard state is zero because they require no energy to form.

  Use enthalpy cycles to measure enthalpy change.
• Strandard Enthalpy of Neutralisation (Hneut) = The enthalpy change when 1 mole of H+ ions react with 1 mole of OH- ions to form 1 mole of water under standard conditions and in solutions containing 1mol dm-3.

  Vitually, this value is always the same. (-58kJmol-1)
• Measuring Enthalpy Change

  E=mcT, the energy transferred to and from the surroundings is equal to the mass of the liquid/ water multiplied by 4.18Jg-1K-1 and multiplied to the temperature change which is measured in celcius [1 Kelvin = 1 change in celcius]. Convert E to KJ.

  H=E/mol, the enthalpy change is equal to the energy calculted previously divided by the moles of the limiting substance. The units for enthalpy change is KJmol-1
• Hess‘ Law States: The total enthalpy change for the indirect route is the same as the enthalpy change via the direct route, given that the start and end points are the same.

  Hess Cycle Described: H1 is the enthalpy change of the direct route. H2 is the enthalpy change of the reactants. H3 is the enthalpy change of the products. Via the indirect route are the elements in their standard states unless otherwise stated.
• Enthalpy Change of Combustion and Enthalpy Cycles

  Equation: sum of reactants - sum of products.

  Always make sure the value of enthalpy change is negative becasue combustion reactions are exothermic.

  The indirect route for combustion enthalpy cycle will always be CO2 and H2O, and will be balanced to one side of the equation.
• Enthalpy Change of Formation and Enthalpy Cycles

  Equation = sum of products - sum of reactants.

  The indirect route will be the elements in their standard states, this includes state symbols regardless of the equation.
• Hydrocarbons = molecules that contain only carbon and hydrogen. Examples include: Alkanes - saturated hydrocarbons and Alkenes - unsaturated hydrocarbons.

  Saturated = molecule has the max number of hydrogen atoms.

  Aliphatic Compounds = doesn’t contain benzene ring

  Aromatic Compounds = does contain benzene ring

  Alkanes + Arenes relatively unreactive, adding functional group (modifier) changes this.

  Alkenes are much more reactive due to the functional group: double carbon-carbon bond.
• Name of molecule: 2-methylpropane

  What are the type of “diagrams” called:

  1 - full structural formula

  2 - shortened structural formula

  3 - molecular formula

  4 - skeletal formula

  [Side note: you only need to be able to recognise these and draw them correctly for a molecule]

  
• Structural Isomerism = same molecular formula, different structural formula (seen in-depth in DF9)

  Alkyl Groups = alkanes that miss out 1 hydrogen, so it can attach to compounds, helps form structural isomers (suffix = -yl)

  Cycloalkanes = “closed-chain alkanes”, it has 2 fewer hydrogens than the regular alkane. General Formula = CnH2n (preffix = cyclo-)
• Naming Branched Alkanes:

  1 - Identify longest carbon chain, link it to preffix.

  2 - Identify side chains and the smallest carbon number it‘s attatched to. Place this at the front of organic name in alphabetical order.

  Ensure that: hyphens between number and letter, commas between numbers

  See Example Below:

  
• Bond Enthalpy = energy needed to break a specific type of bond between atoms in their gaseous state.

  BENDO MEXO

  The making of bonds is exothermic, it releases energy. The products are where new bonds made.

  The breaking of bonds is endothermic, it absorbs energy. The reactants old bonds break.
• As bond length decreases, bond enthalpy increases due to stronger attraction between closer atoms which require more energy to overcome.

  The equilibrium bond length is reached when the attraction between nuclei and electrons is equal to the repulsion between nuclei.

  The higher the number of bonds, the higher the bond enthalpy.
• 1 = Exothermic

  2 = Endothermic

  For an exothermic reaction, the energy absorbed for breaking bonds is less than the energy released from the making of bonds

  For an endothermic reaction, the energy absorbed for breaking bonds is greater than the energy released from the making of bonds.

  
• Bond enthalpies are measured indirectly via enthalpy cycles.

  The equation for bond enthalpy is: sum of bonds broken - sum of bonds made.

  H2 = breaking bonds

  H3 = making bonds

  Main reasons for the calculated value being different from the actual value: not all elements involved in their standard states (all gaseous); bond enthalpy values are averages from several compounds - essentially it’s an assumption.