RATE EXPRESSION AND REACTION
MECHANISM
RATE LAW
• CONCENTRATION OF REACTANTS AND RATE OF REACTION – AS THE CONCENTRATION OF ANY OF
THE REACTANTS CHANGE, THE RATE OF THE REACTION CHANGES.
• THE RATE LAW FOR A REACTION DESCRIBES HOW THE RATE OF THE REACTION DEPENDS ON THE
CONCENTRATION OF THE REACTANTS.
• FOR THE REACTION
• WHERE, k IS CALLED THE RATE CONSTANT AT A PARTICULAR TEMPERATURE. THE EXPONENTS, x AND
y, AND THE RATE CONSTANT BEAR NO NECESSARY RELATIONSHIP TO THE COEFFICIENTS OF THE
BALANCED CHEMICAL EQUATION; THEY MUST BE DETERMINED EXPERIMENTALLY.
RATE LAW
• THE EXPONENTS ARE USUALLY POSITIVE INTEGERS, OR ZERO, BUT THEY CAN OCCASIONALLY BE
A FRACTION OR A NEGATIVE NUMBER.
• FOR THE ABOVE RATE LAW,
REMEMBER
UNITS OF THE RATE CONSTANT (k)
• THE UNITS OF THE RATE CONSTANT DEPEND ON THE OVERALL ORDER OF THE REACTION.
GRAPHICAL REPRESENTATIONS OF REACTION KINETICS
• RATE VS. CONCENTRATION GRAPHS SHOW THE DIFFERENCE BETWEEN ZERO ORDER, FIRST ORDER,
AND SECOND ORDER REACTIONS.
ZERO-ORDER REACTIONS
• IN ZERO ORDER REACTIONS, THE CONCENTRATION OF THE REACTANTS DOES NOT AFFECT THE
RATE OF THE REACTION, SO THE RATE-CONCENTRATION GRAPH IS A HORIZONTAL LINE
FIRST-ORDER REACTIONS
• IN FIRST ORDER REACTIONS, THE RATE IS DIRECTLY PROPORTIONAL TO THE CONCENTRATION
OF A. THE RATE-CONCENTRATION GRAPH IS A STRAIGHT LINE THAT PASSES THROUGH THE
ORIGIN.
SECOND-ORDER REACTIONS
• IN A SECOND ORDER REACTION, THE RATE IS PROPORTIONAL TO THE SQUARE OF THE
CONCENTRATION OF A. /SO, THE RATE-CONCENTRATION GRAPH IS A PARABOLA.
FIRST ORDER REACTIONS HAVE A CONSTANT HALF-LIFE
• CONCENTRATION VS. TIME GRAPHS FOR FIRST ORDER REACTIONS SHOW THAT THE TIME IT
TAKES FOR THE CONCENTRATION OF THE REACTANT TO DECREASE BY HALF OF ITS ORIGINAL
VALUE IS INDEPENDENT OF THE STARTING CONCENTRATION. THE TIME IT TAKES FOR HALF THE
SAMPLE TO REACT IS KNOWN AS THE HALF-LIFE AND IT IS CONSTANT FOR FIRST ORDER
REACTIONS.
DETERMINING THE ORDER OF THE REACTION
• THE ORDER OF THE REACTION CAN BE DETERMINED BY THE INITIAL RATES METHOD.
• THIS METHOD INVOLVES CARRYING OUT SEPARATE EXPERIMENTS WHERE THE
CONCENTRATION OF ONE OF THE REACTANTS IS KEPT CONSTANT WHILE THE
CONCENTRATION OF THE OTHER REACTANT IS CHANGED.
• THE PROCESS IS THEN REPEATED CHANGING THE CONCENTRATION OF THE OTHER REACTANT
WHILE KEEPING THE CONCENTRATION OF THE FIRST REACTANT CONSTANT.
REACTION MECHANISMS
• A REACTION MECHANISM IS THE STEP-BY STEP PATHWAY BY WHICH A REACTION OCCURS.
• SOME REACTIONS TAKE PLACE IN A SINGLE STEP, HOWEVER, MOST REACTIONS OCCUR IN A
SERIES OF ELEMENTARY STEPS.
• EVIDENCE CAN SUPPORT A REACTION MECHANISM, BUT IT CANNOT PROVE IT TO BE CORRECTONLY THAT IT IS CONSISTENT WITH THE OBSERVED DATA.
REACTION MECHANISMS
• FOR EACH OF THESE ELEMENTARY STEPS:
1.
THE ORDER FOR THE REACTANTS IN ANY SINGLE ELEMENTARY STEP IS EQUAL TO ITS
COEFFICIENT.
NOTE: THE RATE LAW EXPONENTS DO NOT NECESSARILY MATCH THOSE OF THE COEFFICIENTS OF
THE OVERALL BALANCED EQUATION.
2. THERE IS AN ELEMENTARY STEP THAT IS MUCH SLOWER THAN THE OTHERS. THIS SLOW STEP IS
CALLED THE RATE-DETERMINING STEP. THE SPEED AT WHICH THE RATE-DETERMINING STEP
OCCURS LIMITS THE RATE AT WHICH THE OVERALL REACTION TAKES PLACE. FAST STEPS
OCCURRING AFTER THE RATE DETERMINING STEP HAVE NO EFFECT ON THE RATE LAW.
REACTION MECHANISMS
3.
THE SUM OF ALL THE ELEMENTARY STEPS IS EQUAL TO THE BALANCED EQUATION FOR THE
OVERALL REACTION.
4.
SPECIES THAT ARE FORMED AND THAT ARE CONSUMED ARE CALLED INTERMEDIATES.
INTERMEDIATES ON BOTH SIDES CANCEL OUT.
REACTION MECHANISMS
5.
THE TERM THAT DESCRIBES THE NUMBER OF MOLECULES REACTING IS CALLED THE
MOLECULARITY OF A REACTION. MOST ELEMENTARY STEPS ARE UNIMOLECULAR (ONLY ONE
REACTANT PARTICIPATES) OR BIMOLECULAR (TWO REACTANTS PARTICIPATE). THERE ARE VERY
FEW TERMOLECULAR REACTIONS.
REACTION MECHANISMS
• THE RATE EXPRESSION FOR AN OVERALL REACTION IS DETERMINED BY THE REACTION
MECHANISM.
• IF THE RATE-DETERMINING STEP IS THE FIRST STEP IN THE MECHANISM, OR THE ONLY STEP,
THEN ITS RATE LAW IS THE SAME AS THE RATE EXPRESSION FOR THE OVERALL REACTION.
REACTION MECHANISMS
• WHEN THE RATE-DETERMINING STEP IS NOT THE FIRST STEP IN THE MECHANISM, IT IS
NECESSARY TO TAKE INTO ACCOUNT THE EARLIER STEPS.