EQUILIBRIUM TIER 3 Deduce the equilibrium constant expression Kc from the equation for a homogeneous reaction Outline the characteristics of chemical and physical systems in a state of equilibrium Deduce the extent of a reaction from the magnitude of the equilibrium constant

The Equilibrium Expression The H2, I2, HI Equilibrium System The rate of the reaction between H2 and I2 vapor in a sealed flask at an elevated temperature can be followed by observing the rate at which the violet color of the iodine vapor diminishes. The color fades to a constant intensity but does not disappear completely because the reaction is reversible. Hydrogen iodide decomposes to re-form hydrogen and iodine. The constant color achieved indicates that equilibrium exists among hydrogen, iodine, and hydrogen iodide.

Rate Comparison for H2(g) + I2(g) 2HI(g)

The Equilibrium Expression, continued The H2, I2, HI Equilibrium System, continued The net chemical equation for the reaction is The following chemical equilibrium expression is The value for K is constant for any system of H2, I2, and HI at equilibrium at a given temperature.

The Equilibrium Expression, continued The H2, I2, HI Equilibrium System, continued At 425°C, the equilibrium constant for this equilibrium reaction system has the average value of 54.34. THIS SHOWS HOW THE CONCENTRATIONS CAN BE DIFFERENT(DIFFERENT EQUILIBRIUM POSITIONS) BUT THE EQUILIBRIUM CONSTANTS ARE THE SAME WITHIN EXPERIMENTAL ERROR.

PROBLEM: WRITE THE EQUILIBRIUM EXPRESSION FOR THE FOLLOWING REACTION; N2 +3H2 2NH3

N2 +3H2 2NH3 ANSWER: K = [NH3]2 [N2] [H2]3

CHARACTERISTIC OF THE EQUILIBRIUM STATE THE REACTION IS DYNAMIC. IT HAS NOT STOPPED BUT BOTH THE FORWARD AND REVERSE REACTION ARE STILL OCCURRING EQUILIBRIUM IS ACHIEVED IN A CLOSED SYSTEM THE CONCENTRATION OF THE REACTANTS AND PRODUCTS REMAIN CONSTANT AT EQUILIBRIUM AT EQUILIBRIUM, THERE IS NO CHANGE IN MACROSCOPIC PROPERTIES EQUILIBRIUM CAN BE REACHED FROM EITHER DIRECTION. THE SAME EQUILIBRIUM MIXTURE WILL RESULT UNDER THE SAME CONDITIONS NO MATTER WHETHER THE REACTION IS STARTED WITH ALL THE REACTANTS, ALL THE PRODUCTS OR A MIXTURE OF BOTH

MAGNITUDE OF K AND EXTENT OF THE REACTION A LARGE K VALUE INDICATES THAT THE REACTION FAVORS THE FORWARD REACTION A SMALL K VALUE INDICATES THAT THE REACTION FAVORS THE REVERSE REACTION A K>>1 MEANS THAT THE REACTION GOES ALMOST TO COMPLETION A K<<1 MEANS THAT THE REACTION HARDLY PROCEEDS AT ALL THIS SHOULD BE OBVIOUS IF YOU LOOK AT THE EQUILIBRIUM EXPRESSION. SINCE THE EQUILIBRIUM EXPRESSION IS A RATIO OF THE PRODUCTS TO THE REACTANTS, A LARGE K FAVORS THE PRODUCTS AND A SMALL K FAVORS THE REACTANTS