12CHEM - mod 5. equlibrium and acid reactions

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reversible reaction

reactions that can proceed in both the forward and reverse direction. these reactions occur at the same time

irreversible reaction

reactions that proceed in only one direction

non-equilibrium

systems that can never achieve a state of equlibrium. instead, it proceeds in one direction until the reaction is complete

• reaction will favour positive entropy and negative enthalpy

examples of non-equilibrium systems/reactions

combustion and photosynthesis

why is combustion a non-equilbrium system?

• combustion = spontaneous, non-equilbrium system

• forward reaction = exothermic and increases entropy, proceeding to completion

• reverse reaction doesn’t occur because of extremely positive ΔG

why is photosynthesis a non-equilbrium system?

• photosynthesis = non-spontaneous, non-equilbrium system 

• forward reaction - despite positive ΔG, forward reaction occurs through smaller spontaneous reactions using the sun’s energy

• reverse reaction - respiration doesn’t occur in the same system as photosynthesis

equilbrium

the point in a reversible reaction where there’s no change in the concentration of products and reactants, and the rate of the forward and reverse reactions are equal

dynamic vs static equilbrium

• dynamic eqm: both the forward and reverse reactions occur at the same rate

  • no change overall

  • E.g. 3H2+ N2 ⇌ 2NH

• static eqm: the forward and reverse reactions aren’t occuring at all, the rate of the forward and reverse reaction is zero

  • E.g. diamond ⇌ graphite (exists in eqm but Ea for diamond → graphite is so high that the rate of reverse reaction is zero)

Explain why macroscopic properties remain constant at equilibrium

• Macroscopic properties (e.g. temperature, pressure, colour) remain constant at eqm because the system is in dynamic eqm

  • rate of forward reaction = rate of reverse reaction → conc. doesn’t change → macroscopic changes depend on concentration → no net change in observable properties

• Microscopic change occurs continually

Discuss the impact of a closed system on dynamic equilibrium

distinguish between open and closed systems

• open system: a system that exchanges both energy and matter with its surroundings

  • prevents eqm being reached

• closed system: a system that only exchanges energy with its surroundings

  • eqm can only be reached in a closed system 

homogenous vs heterogenous system

• homogenous system: reactants and products in the same state

• heterogenous system: a eactants and products in different states