Exam 4 Outline Numbers 1 25 are from previous chapters but the test builds on these 1 For a general reversible reaction such as a aA bB cC dD b The Law of Mass Action states that if the system is at equilibrium at a given temperature then the following is constant i ii Example 1 N2O4 g 2 NO2 g 2 Keq NO2 2 N2O4 c The larger the equilibrium constant the more it proceeds to completion i If K is large products are favored ii If K is less than 1 then the reactants are favored 1 Kfwd Krev 1 Kfw Krev 2 Equilibrium Constants can be expressed using Kc or Kp a Kc uses the concentration of the reactants and products b Kp uses pressure of the gaseous reactants and products i i c Example i 2 NO g O2 g 2NO2 g Concentrations of the reaction species at equilibrium are found to be NO 0 0542 M O2 0 127 M NO2 15 5 M Calculate the equilibrium constant Kc Kc NO2 2 NO 2 O2 Kc 15 5 2 0 0542 2 0 127 Kc 6 44 x 10 5 1 If you were given pressures you could use Kp d N2O4 g 2NO2 g i In terms of Kc 1 Kc NO2 2 N2O4 ii In terms of Kp 1 Kp PNO2 2 PN2O6 2 Kc does not equal Kp 3 Remember when writing equilibrium constant expressions a The concentrations of the reacting species in the condensed phase are expressed in M In the gaseous phase the concentrations can be expressed in M or in atm b The concentrations of pure solids pure liquids and solvents do not appear in the equilibrium constant expressions c The equilibrium constant is a dimensionless quantity d In quoting a value for the equilibrium constant you must specify the balanced equation and the temperature If the reaction can be expressed as a sum of two or more reactions then the equilibrium constant for the overall reaction is given by the product of the equilibrium constants of the individual reactions e 4 Multiple Equilibria a A reaction can be an individual reaction step or a multistep reaction b If the overall reaction is the sum of two or more reactions the overall reaction of the Equilibrium Constant is the product of the Equilibrium Constants for the steps i ii iii Example iv Important 1 1 If the equation is REVERSED the equilibrium constant is inverted a A 2B 3C b 3C A 2B i Kfwd C 3 A B 2 i Krev A B 2 C 3 5 Reaction Quotient chemical reaction a A function of the concentrations or pressures of the chemical species involved in a b The reaction quotient Q has the same form as the equilibrium constant K c The major difference between Q and K is that the concentrations used in Q are not necessarily equilibrium values d e Q Kc f Q Kc g Q Kc i The system is at equilibrium ii Concentrations of reactants and products stays the same i The reaction proceeds to the right ii Alligator eats to the right iii Generates more products consumes more reactants i The reaction proceeds to the left ii Alligator eats to the left iii Consume more products generate more reactants h Q helps us predict how the equilibrium will respond to an applied stress 6 Predicting the Shift without calculations a CO g 2H2 g CH3OH g b What would happen if methanol CH3OH is added i Equilibrium will be disrupted and the increase in products mean that Q K ii In order to reestablish equilibrium the reaction will progress to the left towards the reactants c What would happen if more reactants were added i Q K because the value for the amount of reactants or the denominator of the Q expression has increased ii Reaction will favor product so reaction will progress to the right iii These demonstrate Le Chatelier s Principle a If we are given the concentrations in a reaction mixture and Kc then we can predict which direction the reaction will proceed AND we can also calculate the concentration of each species when it reaches equilibrium b Steps of the ICE Method 7 Using the ICE Method i Write the balanced equation ii Write the reaction quotient Q iii Convert all amounts into the correct units M or atm iv When reaction diagram is not known compare Q with K v Construct a reaction table 1 Check the sign of x the change in the concentration or pressure vi Substitute the quantities into K equation vii To simplify the math assume that x is negligible viii Solve for x 1 Check that assumption is justified 5 error If not solve quadratic equation for x ix Find the equilibrium quantities c Example i Cis stilbene trans stilbene 1 Check to see that calculated values give the known K The equilibrium constant Kc for this system is 24 0 at 200 degrees C Suppose that initially only cis stilbene is present at a concentration of 0 850 mol L How do we calculate the concentrations of cis and trans stilbene at equilibrium Going to the right x Because initially only the reactant is present so it will proceed to the right X is calculated by K trans cis 24 x 0 850 x X 0 816 M Plug in x for the ICE table and solve 8 Sometimes close enough is good enough a We can assume that A initial x is approximately A initial IF i Kc is relatively small and or A initial is relatively large 9 Le Chatelier s Principle b b a When a chemical system at equilibrium is disturbed it returns to equilibrium by undergoing a net reaction that reduces the effect of the disturbance If the system is at equilibrium we poke it the system is no longer at equilibrium the principle says the system will react to return to equilibrium c d A change in concentration has NO effect on the value of K e Overall to remember f Hanson Method easier to remember i i 10 Acids and Bases a Acid b Base i Substance that dissociates in water to yield H3O i Substance that dissociates in water to yield OH c Bronsted Acid i Able to donate a proton in the form of hydrogen ions protons H ii AH A H iii Characteristics 1 Have a sour taste 2 Change litmus from blue to red 3 React with carbonates to produce CO2 4 React with metals to produce hydrogen gas 5 Aqueous acidic solutions conduct electricity d Bronsted Base i Abe to accept a proton in the form of hydrogen ions protons H ii B H BH iii Characteristics 1 Have a lone pair of electrons 2 Have a bitter taste 3 Change color of litmus from red to blue 4 Feel slippery 5 Aqueous acidic solutions conduct electricity e Determining an acid or base i Count the hydrogens on each substance before and after the reaction If the number of hydrogens has decreased that substance is the acid donates hydrogen …
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