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Chemical Equilibrium

Definition. Occurs when opposing reactions are occurring at the same timeWe've had physical equilibria beforedissolvingvapor pressureWe will use partial pressure for gases We will use molarity for solutes. Why partial pressure?. A(g) ? B(g)A(g) ? B(g) rate = kf[A]A(g) ? B(g)

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Chemical Equilibrium

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    1. Chemical Equilibrium Chapter 15 in Brown & LeMay

    2. Definition Occurs when opposing reactions are occurring at the same time We’ve had physical equilibria before dissolving vapor pressure We will use partial pressure for gases We will use molarity for solutes

    3. Why partial pressure? A(g) ? B(g) A(g) ? B(g) rate = kf[A] A(g) ? B(g) rate = kr[B] For ideal gas equation, what is the concentration of the gas? n/v = P/RT

    4. At Equilibrium: Rate of forward = rate of reverse kfPA/RT = krPB/RT R and T are constant Kf/kr = PB/PA This is called the equilibrium constant

    5. The constant Use this as an example: N2(g) + 3H2(g) ? 2NH3(g) This is called the Haber Process. Why? Equilibrium conditions can be reached from either direction Equilibrium expression uses coefficients as exponents, products over reactants Do it for above reaction

    6. Good Website http://jersey.uoregon.edu/vlab/Thermodynamics/

    7. Constant cont’ It is the value specific for that reaction under those conditions, no matter what you start with animation Keq greater than 1 – products favored Keq less than 1 – reactants favored Temperature dependent – defined for one temperature Keq for a reaction is 1/Keq for the reverse If reaction is multiplied by 2, Keq is squared, etc. Dimensionless Try some problems

    8. Heterogeneous Equilibria Not just gases or solutions Solids, liquids included in reaction CaCO3(s) ? CaO (s) + CO2(g) Pure solids, liquids, and solvents are not included in the equilibrium expression because their concentration is usually in excess and does not change significantly in the reaction.

    9. Calculating Keq Easy when all concentrations are known – just substitute Try PE

    10. RICE Problems RICE is a table approach to solving equilibria problems Reaction, Initial conc,Change in conc, Equilibrium conc Used when there are unknowns in a problem Try one together – bottom of page 589 PE

    11. Q Problems Used when concentrations of reactants and products are given (or calculated by RICE) and Keq is given Question asked is “Has equilibrium been reached?” Called Q or “reaction quotient” Calculated using Keq expression If Q < Keq, reaction must go to right If Q > Keq, reaction must go to left

    12. “X” problems Used when amounts of reactants or products at equilibrium are unknown You may or may not need to use RICE Use “x” for unknown amounts We will not have to use the quadratic equation Try PE

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