Exam chapters 4 10 14 not 5 6 Chemical equilibrium Chemical reactions are reversible State of equilibrium is eventually achieved between products and reactants Outside forces can effect equilibrium Main points Arrow conventions All reactant molecules are converts to product molecules at the end Not all reactants corn vert to products H2 I2 2HI All interdependent Relative amounts of H2 I2 and HI Describing reaction at equilibrium Equilibrium constant For general equation aA bB cC dD reaction dynamics concentration changes Forward reaction reactants products When reaction starts reactants are consumed and products are made Eventually products can react to reform some of the reactants Processes that proceed in both forward and reverse direction are reversible Reverse reaction Products reactants Relationship between rates of forward and reverse reactions As forward reaction slows and the reverse reaction accelerates eventually they reach same rate Dynamic equilibrium is condition wherein rates of forward and reverse reactions are equal Once the reaction reaches equilibrium the concentrations of all the chemicals remain constant Equilibrium does not mean equal concentrations equal rates of appearance vs disappearance Some reactions reach equilibrium only after almost all reactant molecules are consumed we say the position of equilibrium favors the products Other reactions reach equilibrium when only small percentage of reactant molecules are consumed position of equilibrium favors reactants Writing equilibrium constant expressionism Interpreting value of Keq When value of K is much larger than 1 we know that when the reaction reaches equilibrium there will be many more product molecules present than reactant molecules When K is much less than 1 when reaction reaches eq there will be many more reactant molecules present than product molecules Position of eq favors reactants Position of eq favors products Example write equilibrium constant expressions K K is very large so favors products Favors reactants Relationships between K and chemical reactions When reactions written backwards equilibrium constant is inverted When coef cients of equation are multiplied by factor eq constant is raised to that factor When add equations to get new equation the eq constant of new equation is product of eq constants of old equations Example what is eq constant at 25 degrees C for reaction Pure solids and pure liquids do not change concentration during reaction Not included in K expression Heterogenous equilibria Example write equilibrium constant expressions and predict position of equilibrium for following Equilibrium calculation types Calculating eq constant using concentrations at equilibrium Calculating concentrations at equilibrium using eq constant Calculating eq constant using initial concentrations Signi cance of reaction stoichiometry Solving quadratic equations Approximation conventions calculating eq constantsamts from measured eq concentrations Value of eq constant will always be same A long as temp constant Value of eq constant is independent of initial amounts of reactants and products See text book for initial concentration and eq concentration and eq constant Calculating eq concentrations Stoichiometry can be used to determine eq concentrations of all reactants and products if you know initial concentrations and one eq concentration Use balanced chem equation Construct eq table Example 1 mol of SO2 and 1 mol of O2 are placed in 1L ask at 1000 C At eq the concentration of SO3 is 0 925 mol Calculate Kc for reaction at this temp example nd value of Kc for reaction at 1700 deg C is initial CH4 0 115 M and eq C2H2 0 035 M The reaction quotient If reaction mixture containing both reactants and products is not at equilibrium compare current concentration rations to eq constant Concentration ratio of the products is called reaction quotient Q Equation is same as K Predicting direction of change Products decrease reactants increase Products increase reactants decrease If Q K reaction will proceed in reverse direction If Q K reaction will proceed in forward direction If Q K reaction is at equilibrium If reaction mixture contains just reactants Q 0 and reaction will proceed in forward direction If reaction mixture contains just products Q in nity and reaction will proceed in reverse direction Example for reaction Kc 3 59 at 900 deg C Finding eq concentrations when given the eq constant and initial concentration Compare Q to K Decide which direction the reaction will proceed De ne changes of all materials in terms of x Solve for x Use coef cient from chem equation as coef cient of x Quadratic formula Example for reaction the value of Kc 376 x 10 5 at 1000 K If 1mole of I2 is placed into 2 00 L ask and heated what will be the equilibrium concentrations of I2 and I Approximations to simplify math When eq constant is very small position of eq favors reactants For relatively large initial concentrations of reactants the reactant convenient ration will not change signi cantly when it reaches equilibrium Checking the approximation and re ning as necessary Compare approximate value of x to initial concentration If approx value of x is less than 5 of initial concentration the approximation is valid Disturbing and restoring equilibrium If conditions are changed concentrations of all chemicals will change u til equilibrium is rested New concentrations will be different but the K constant as long as temp constant Le Chateliers principle If a system at equilibrium is disturbed the position of eq will shift to minimize disturbance Helps predict effect of various changes in conditions have on position of eq Disturbing equilibrium adding reactants Reaction proceeds to the right until equilibrium is restored At new eq position the concentrations of reactants and products will be such that the value of the equilibrium constant is the same Disturbing equilibrium removing reactants Reaction proceeds to left until eq is restored At new eq position the concentrations of reactants and products will be such that the value of the eq constant is the same Effect of volume change on eq for reactions that involve gases volume concentration and pressure are interrelated increasing volume decreases concentration decreases pressure Eq should shift to the side where concentration is lowered Effect of temp change on eq position Exothermic reactions release energy and endothermic reactions absorb energy Increase in temp shifts