C4 energetics

• in a chemical reaction, bonds are broken and bonds are made
• chemical reactions involve an energy change as energy is taken in to break bonds, and energy is given out when bonds are made
• if overall in the reaction energy is given out, it is exothermic

  if overall in the reaction energy is taken in, it is endothermic
• exothermic reactions have a positive enthalpy change
• endothermic reactions have a negative enthalpy change
• an enthalpy change is a heat change under constant pressure
• enthalpy is measured in kJmol^-1
• the symbol for enthalpy is $H$, the symbol for enthalpy change is $\Delta H$
• the standard conditions for measuring enthalpy changes are pressure of 100 kPa, and temperature of 298 K
• the symbol for an enthalpy change under standard conditions is $\Delta H^\degree$
• overall enthalpy change can be calculated with: $\Delta H$= energy to break bonds + energy to make bonds
• enthalpy level diagrams are used to represent enthalpy changes, they show the relative enthalpy levels of the reactants and products
• in an enthalpy level diagram, the y-axis represents enthalpy and the x-axis represents extent of reaction, usually without units
• in an enthalpy level diagram, if the reaction is exothermic, the line for products is below the line for reactants, and if the reaction is endothermic, the line for products is above the line for reactants
• standard enthalpy of formation is the enthalpy change when one mole of substance is formed from its constituent elements under standard conditions
• standard enthalpy of formation is represented by $\Delta H_f^\degree$
• standard enthalpy of combustion is the enthalpy change when one mole of substance is completely burnt in oxygen under standard conditions
• standard enthalpy of combustion is represented by $\Delta H_c^\degree$
• temperature is related to the average kinetic energy of the particles in a system, so is independent of the number of particles present

  as particles move faster their kinetic energy increases so their temperature increases
• heat is a measure of the total energy of all the particles present in an amount of substance, so is dependent on the number of particles present
• specific heat capacity is the amount of heat needed to raise the temperature of 1g of substance by 1K

  it is represented by $c$and is measured in Jg^-1K^-1
• the equation to calculate enthalpy change is: q = mcT

  q is enthalpy change in kJmol^-1

  m is mass in g

  c is specific heat capacity in Jg^-1K^-1

  T is temperature change in K
• a calorimeter can be used to find an enthalpy change, it involves burning a fuel to heat a known mass of water and recording the temperature change of the water

  you assume that all the heat produced goes into heating the water, to improve the accuracy, you need to reduce heat loss by adding insulation
• hess’ law states that the enthalpy change for a chemical reaction is the same, no matter what route is taken to turn reactants into products
• hess' law is based on the idea that energy cannot be created nor destroyed
• hess’ law can be used to find the enthalpy change of a reaction which cannot be easily measured using calorimetry or similar

  to do this, create a hess’ cycle with the reaction as well as an intermediate product, with arrows between all the stages in the correct direction

  the enthalpy change of the reaction will be equal to any other way round the cycle you go that starts from the reactants and ends at the products
• a hess’ cycle using enthalpy of formation has the intermediate stage of all the constituent elements in standard states, shown with state symbols

  the arrows go up if the constituent elements are at the bottom
• a hess’ cycle using enthalpy of combustion has the intermediate stage of the carbon dioxide and water which would be formed when the reactants or products were combusted

  the arrows go down if the combustion products are at the bottom
• enthalpy diagrams can be used instead of thermochemical cycles to represent enthalpy changes in chemical reactions
• the enthalpies of all elements in their standard states are taken as zero
• enthalpy diagrams do not need to be to scale
• bond dissociation enthalpy is the enthalpy change required to break a covalent bond with all species in the gaseous state
• the same bond in different molecules will have slightly different bond dissociation enthalpy, so you use the average value
• mean bond enthalpy is the average enthalpy change required to break a covalent bond with all species in the gaseous state
• the difference between the energy put in to break the bonds and the energy given out to form bonds is the approximate enthalpy change of the reaction