exam

• Thermodynamics tells us if a reaction will occur based on the energy differences between starting materials and products.
• The enthalpy change is the heat of reaction, and can be thought of as the difference between the strength of the bonds formed and the bonds broken during a reaction. It is measured in kJ mol-1
• Exothermic reactions release energy to the surroundings (ΔH is –ve)
• endothermic reactions take in energy from their surroundings (ΔH is +ve).
• Entropy changes are a measure of the change in disorder (randomness) in processes. It is measured in J mol-1 K-1
• The Gibbs free energy change for a reaction represents a combination of heat and disorder, and tells us about the spontaneity of a process. It is measured in kJ mol-1 .
• ∆𝐺 = ∆𝐻 − 𝑇∆𝑆 (measured in kJ/mol).
• when ∆𝐺 = -ve, the reaction wil proceed spontaneously
• When ∆𝐺 = 0, the system will be at equilibrium.
• When ∆𝐺 = +ve, the reaction will not occur spontaneously.
• A negative value of ∆G means that the products have lower energy than the reactants so the reaction can happen spontaneously without any input of energy.
• 2ND Law of Thermodynamics: The net entropy of the universe tends to increase
• ∆𝑆 = +VE, products are more disordered than reactants
• ∆𝑆 = -VE, reactants more disordered than products
• ∆𝑆 = 0, processes are at equilibrium
• Solids = High order, low disorder, low entropy
• Gases = low order, high disorder, high entropy
• many processes lead to local decreases in randomness, eg crystallization where molecules become more organised in their structure. Local decreases in entropy must occur with overall increases in the entropy of the surroundings
• temperature impacts ΔG and thus spontanity.
• If ΔH<0 and ΔS>0

  ΔG=-VE for all T, reactions occur spontaneously in this condition at all T.

  Increasing T makes the "-TΔS" term more -ve and thus ΔG more -ve
• If ΔH<0 and ΔS<0

  In ΔG = ΔH - TΔS, with both ΔH and ΔS being -ve, the "-TΔS" term becomes positive.

  We need the overall equation to be -ve, so the "-TΔS" term must be as small as possible so that it doesnt overcome the -ve ΔH term and make ΔG +ve and means reactions will be non-spontaneous.

  This is achieved at low T, reactions are spontaneous at low T If ΔH<0 and ΔS<0
• If ΔH>0 and ΔS>0

  In ΔG = ΔH - TΔS, with both ΔH and ΔS being +ve, the "-TΔS) term becomes negative.

  We need the overall equation to be -ve, so the "-TΔS" term must be large enough to overcome the +ve ΔH term to make ΔG -ve and allow for spontaneous reactions.

  This is achieved at high T, reactions are spontaneous at high T, if ΔH>0 and ΔS>0
• If ΔH>0 and ΔS<0

  In ΔG = ΔH - TΔS, if ΔH is +ve and ΔS is -ve, the "-TΔS" term will be positive for all T.

  In this condition, both terms result as a +ve for all T, thus ΔG can never be -ve and the reaction will be non-spontaneous for all T
• ΔG = -RTln[Keq] or [Keq] = e^(-ΔG/RT)
• ΔG < 0 , Keq > 1
• ΔG > 0, Keq < 1
• ΔG = 0 , Keq = 1
• If Keq > 1, it means tht at equilibrium the concentration of products is large, meaning the reaction favours the formation of products.
• If Keq < 1, it means the concentration of reactants at equilibrium is large. This means the reaction favours the formation of reactants
• If Keq = 1 there is significant amounts of both reactants and products
• Dynamic equilibrium is where reactants and products are formed at such a rate that the concentration of neither changes
• Dynamic equilibrium is when the rate of the foreward and reverse reactions are equal, the ratio of products to reactants is constant but no necessarily (and generally not) equal
• Reactions with a Keq equal or greater than 10^4 tend to go to completion.

  With this Keq, there is a chemically insignificant amount of reactants present at the equilibrium position because it is so far to the right in favour of products.
• Factors affecting position of equilibrium.

  Temperature

  Concentrations (of products or reactants)

  Pressure

  Catalysts ( dont effect position of equilibrium but allows equilibrium to be achieved faster or at lower T)
• reversible reactions have a Keq between 10^-3 and 10^3