CHEMICAL CHANGE AND YOUR BODY YOUR BODY DOES A LOT OF CHEMISTRY. IT IS ESTIMATED THAT THERE ARE 6 X REACTIONS/CELL/SEC IN THE AVERAGE HUMAN. SKIN IS COMPLETELY REPLACED EACH MONTH. STOMACH LINING IS REPLACED IN 5 DAYS. YOUR LIVER IS REPLACED IN 6 WEEKS. YOUR SKELETON IS REPLACED N 3 MONTHS. AT THE END OF A YEAR, 98% OF THE ATOMS IN YOUR BODY HAVE BEEN REPLACED.

NOT ALL CHEMICAL REACTIONS TAKE PLACE AT THE SAME RATE. SOME ARE VERY FAST, AND SOME ARE VERY SLOW. WE ARE CONCERNED ABOUT RATES OF REACTIONS FOR MANY REASONS: -HOW COULD WE SLOW DOWN HOW FAST STEEL RUSTS -HOW CAN WE CONTROL HOW FAST ADHESIVE SETS -HOW CAN WE CONTROL THE RATE AT WHICH ROCKET FUEL BURNS -HOW DOES OUR BODY CONTROL HOW FAST IT BURNS CARBOHYDRATES WHAT FACTORS CONTROL THE SPEEDS OF CHEMICAL REACTIONS?

WE KNOW THAT FOR A CHEMICAL REACTION TO TAKE PLACE, CHEMICAL BONDS HAVE TO BE BROKEN AND REFORMED. SO, ONE MAJOR FACTOR IN CHEMICAL REACTIONS WILL BE THE NATURE OF THE CHEMICAL REACTANTS. HOWEVER, THERE ARE OTHER FACTORS: 1. THE PHYSICAL STATE OF THE REACTANTS – MOLECULES OF THE REACTANTS HAVE TO COME TOGETHER FOR A REACTION TO OCCUR. ANYTHING WE CAN DO TO INCREASE THE EASE OF CONTACT WILL SPEED UP THE REACTION. REACTIONS MIGHT GO FASTER IN THE GASEOUS STATE THAN IN THE LIQUID STATE. IF YOU ARE REACTING A SOLID WITH A LIQUID, IF YOU INCREASE THE SURFACE AREA OF A SOLID, YOU INCREASE THE EASE OF CONTACT.

2. CONCENTRATION – AS YOU INCREASE THE CONCENTRATION, YOU INCREASE THE RATE OF CONTACT OF THE MOLECULES (MORE COLLISIONS PER UNIT TIME), AND YOU INCREASE THE RATE OF THE REACTION. 3. TEMPERATURE – AS THE TEMPERATURE INCREASES, THE KINETIC ENERGY INCREASES (SPEED OF MOLECULES), SO YOU HAVE MORE COLLISIONS PER UNIT TIME. THE REACTION RATE INCREASES WITH TEMPERATURE. 4. PRESENCE OF A CATALYST – A CATALYST IS A SUBSTANCE THAT INCREASES THE RATE OF A CHEMICAL REACTION WITHOUT ITSELF BEING CONSUMED. A CATALYST WORKS BY ORIENTING THE REACTANTS IN THE PROPER ARRANGEMENT FOR A REACTION TO OCCUR. CATALYSTS ARE ESPECIALLY IMPORTANT IN BIOLOGICAL REACTIONS (ENZYMES).

COLLISION THEORY OF REACTIONS THE COLLISION THEORY CAN BE THOUGHT OF AS AN EXTENSION OF THE KINETIC MOLECULAR THEORY. IT SAYS: 1) FOR A REACTION TO OCCUR, THE REACTANT MOLECULES OR ATOMS MUST MAKE CONTACT (COLLIDE). 2) THE COLLIDING REACTANT PARTICLES MUST BE ORIENTATED IN SUCH A WAY THAT REACTION CAN OCCUR. 3) THE COLLIDING REACTANT PARTICLES MUST COLLIDE WITH ENOUGH ENERGY FOR REACTION TO OCCUR.

LET’S TAKE A LOOK AT THE FIRST POINT: 1) FOR A REACTION TO OCCUR, THE REACTANT MOLECULES OR ATOMS MUST MAKE CONTACT (COLLIDE). WE KNOW FROM LOOKING AT EXPRESSIONS FOR RATES OF A REACTION THAT AS CONCENTRATIONS OF REACTANTS INCREASE, THE RATE OF A REACTION INCREASES. THE COLLISION THEORY TELLS US THAT FOR A REACTION TO OCCUR, A COLLISION MUST OCCUR. AS THE CONCENTRATION INCREASES, THE NUMBER OF COLLISIONS PER UNIT TIME WOULD INCREASE BECAUSE WE HAVE MORE REACTANT PARTICLES.

LET’S TAKE A LOOK AT #2: 2) THE COLLIDING REACTANT PARTICLES MUST BE ORIENTATED IN SUCH A WAY THAT REACTION CAN OCCUR. NOT ALL COLLISIONS ARE EFFECTIVE. THE MORE COMPLEX THE REACTING MOLECULES, THE LESS EFFECTIVE THE COLLISIONS BECOME. IN BIOLOGICAL SYSTEMS, THERE ARE MOLECULES CALLED ENZYMES, COMPLEX PROTEINS, THAT FOLD IN A CERTAIN WAY SO AS TO PROVIDE ADSORPTION SITES FOR REACTING MOLECULES. ONCE THE REACTANTS ARE ADSORBED INTO THE SLOTS IN THE MOLECULES, REACTION CAN TAKE PLACE EASILY.

LET’S TAKE A LOOK AT #3: 3) THE COLLIDING REACTANT PARTICLES MUST COLLIDE WITH ENOUGH ENERGY FOR REACTION TO OCCUR. THERE IS A MINIMUM AMOUNT OF ENERGY NECESSARY FOR A REACTION TO OCCUR. THIS IS CALLED THE ACTIVATION ENERGY. THINK OF THIS ENERGY AS THE ENERGY NECESSARY TO EITHER BREAK BONDS, BEND BONDS, OR FORM AN ACTIVATED COMPLEX SO THAT A REACTION COULD PROCEED. AN ENERGY DIAGRAM FOR AN EXOTHERMIC REACTION IS GIVEN ON THE NEXT SLIDE.

THE ACTIVATION ENERGY IS THE MINIMUM ENERGY NECESSARY FOR A REACTION TO OCCUR. THIS DIAGRAM REPRESENTS THE POTENTIAL ENERGY OF THE CHEMICAL BONDS. IN THIS CASE, THE POTENTIAL ENERGY OF THE PRODUCTS IS LESS THAT THAT OF THE REACTANTS, SO ENERGY WOULD BE TRANSFERRED TO THE SURROUNDINGS IN THE REACTION, AND IT WOULD BE EXOTHERMIC.

THIS IS AN ENERGY DIAGRAM FOR AN ENDOTHERMIC REACTION. NOTE THAT THE PRODUCTS ARE AT A HIGHER POTENTIAL ENERGY LEVEL THAN THE REACTANTS, SO ENERGY WOULD HAVE TO BE PUT INTO THE SYSTEM.