enphathy

• • pH = -log[H+] for a strong acid

  • Ka = [H+][A-] [HA]

  Or Ka = [H+]2 [HA]

  for a weak acid

  K = [H+][OH-] = 1x10-14 w

  For a strong base
• When chemical reactions happen, there is usually a change in heat energy, which is known as the enthalpy change of a reaction.
• Enthalpy change (ΔH) is defined as the amount of heat energy transferred during a chemical reaction, at a constant pressure.
• The units of enthalpy change are kJ mol-1.
• Enthalpy change is represented by the symbol ΔH.
• Reactions involve breaking old bonds and forming new bonds.
• You need energy to break bonds, which is absorbed from the environment.
• Making bonds releases energy into the environment.
• The enthalpy change of a reaction is the overall effect of these two changes, which are endothermic and exothermic reactions.
• A reaction is exothermic when the energy required to break the bonds is less than the energy released when forming new bonds. 

  oThere is an overall release of energy into the surroundings.

  oTherefore, the enthalpy change is negative (- ΔH ) 

  o The products have less energy than the reactants- the energy of the system has decreased.

  •The release of heat energy into the surroundings can be measured using a thermometer- the temperature rises. 

  •Examples of exothermic reactions include:

  oCombustion

  oOxidation 

  oNeutralisation
• A reaction is endothermic when the energy required to break the bonds is more than the energy released when forming new bonds. 

  oThere is an overall absorption of energy from the surroundings so, the ΔH  is positive. 

  o The products have more energy than the reactants- the energy of the system has increased (decrease in temperature).

  •Examples of endothermic reactions:

  oThermal decomposition

  oElectrolysis

  oThe first stage of photosynthesis (sunlight supplies the energy)
• Enthalpy change of reaction (ΔHrꝋ): the enthalpy change associated with a stated stoichiometric equation.
• Enthalpy of formation (ΔHfꝋ): the enthalpy change when 1 mole of a compound is formed from its elements under standard conditions.
• Enthalpy of combustion (ΔHcꝋ): the enthalpy change for the complete combustion of one mole of a substance (in excess oxygen) under standard conditions.
• Enthalpy of neutralisation (ΔHnꝋ): the enthalpy change when one mole of water is formed from reacting an acid and alkali under standard conditions.
• Thermodynamic measurements are carried out under standard conditions.
• To show that a reaction has been carried out under standard conditions, the symbol Ꝋ is used.
• Each substance involved in the reaction is in its standard physical state (solid, liquid or gas).
• Standard conditions:

  100kPa or 1atm

  298k or 25oc
• Enthalpy changes can be calculated directly from appropriate experimental results

  •In these experiments, a fuel in a spirit burner (e.g., propane) is burnt beneath a calorimeter.

  oA simple calorimeter can be made from a polystyrene drinking cup, a vacuum flask or metal can with a lid.

  • You then record the change in temperature of the water.

  oThe energy needed to increase the tempera
• Q = mcAT
• AH = Q / moles
• In a calorimetry experiment, the data collected may differ from enthalpy values found in a data book for the following reasons:

  oNon-standard conditions used

  oHeat lost to the surroundings

  oHeat absorbed by the apparatus

  oIncomplete combustion

  •To make calorimetry experimental data more accurate, you should try to minimise heat loss by using a lid or insulated beaker (cotton wool could be used to insulate).
• Bond enthalpies are defined as “the energy required to break one mole of a specific covalent bond in the gas phase”

  •Bond energies are affected by other atoms in the molecule (the environment)

  •Therefore, an average of several of the same type of bond but in different environments is calculated

  •This bond energy is known as the average bond energy.

  •Recap: breaking bonds is endothermic, while making bonds is exothermic (think: BENDO MEXO)
• ENTHALPY CHANGE (AH) = ENERGY TO BREAK BONDS - ENERGY RELLASED MAKING BONDS
• Hess’s law states that “the total enthalpy change of a reaction is the same, regardless of the route taken”. It is useful for calculating enthalpy changes that can’t be determined experimentally.
• The standard enthalpy change of formation (Δf°H) is the enthalpy change when one mole of substance is formed from its elements in their standard state at 298 K.
• Enthalpy change of solution (Δsoln°H): The enthalpy change when one mole of solute dissolves completely in sufficient solvent to form one dm3 of saturated solution at 298 K.
• Enthalpy change of neutralisation (Δn°H): The enthalpy change when one mole of water is produced from one mole of acid and one mole of alkali in their standard states at 298 K.
• Enthalpy change of combustion (Δc°H): The enthalpy change when one mole of gaseous products is formed from one mole of reactants in their standard states at 298 K.
• Enthalpy change of fusion (Δfus°H): The enthalpy change when one mole of liquid is formed from one mole of solid at its melting point.