2. Classifying based on energy transfer

Collision Theory of Reactions A chemical reaction occurs when collisions between molecules have sufficient energy to break the bonds in the reactants. - bonds between atoms of the reactants (N2 and O3) are broken and new bonds (NO) can form

Activation Energy - is the minimum energy needed for a reaction to take place. Activated complex When a collision provides energy equal to or greater than the activation energy, product can form.

Reactions are classified by looking at whether energy is required or released during a chemical reaction: Exothermic Reaction water Sodium Peroxide Zinc Sodium Oxide + Zinc Oxide + heat 2Na2O2 + 2Zn 2Na2O + 2ZnO + heat EXOTHERMIC (exo= outside)- energy is released during a chemical reaction (i.e. is a product), given off as heat 2. ENDOTHERMIC (endo= inside)- energy is required during a chemical reaction (i.e. is a reactant), and heat is absorbed Ammonium thiocyanate + barium hydroxide + heat  barium thiocyanate + Ammonia 2NH4SCN + Ba(OH)2 + heat  BaSCN + 2NH3 reaction takes in heat as a result, the bottom of the beaker becomes cold enough to freeze the water and stick to the wood.

Exothermic & Endothermic Reactions In an exothermic reaction, heat is released the energy of the products is less than the energy of the reactants heat is a product C(s) + 2 H2(g) CH4(g) + 18 kcal In an endothermic reaction Heat is absorbed The energy of the products is greater than the energy of the reactants Heat is a reactant (added) N2(g) + O2 (g) + 43.3 kcal 2NO(g)

Factors Affecting Reaction Rate “Things that speed up or slow down a reaction.” is the speed at which reactant is used up. is the speed at which product forms.

Factors that Affect Chemical Reaction Rates Collision Theory The collision theory says that when more collisions occur between the reactants, there is a better chance that products will be made. So, more collisions = products form faster. This would be a fast reaction rate. Factors that Affect Chemical Reaction Rates Concentration Temperature Catalyst Surface Area

Concentration - how much solute (element or compound) is dissolved in solution. Adding more packets of KoolAid into the same glass of water will make the resulting solution more concentrated Increasing Concentration  If there are more reactants available, there is a greater chance that the molecules will collide. Therefore, there is a greater chance of products forming more quickly. Increased concentration = increased rate of the reaction.

Temperature - is a measure of the average energy of molecules. Changing the temperature, changes the energy molecules have Higher Temperature = Faster Moving Molecules = More Collisions = FASTER Rx Lower Temperature = Slower Moving Molecules = Less Collisions = Slower Rx

Surface Area - how much of an object is exposed to it’s surroundings Greater Surface Area = Faster the reaction Smaller surface area = Slower the reaction

Catalysts - a chemical substance that is added to a reaction to speed it up! Another name for a catalyst is an enzyme (in biological systems) The catalyst affects the reaction rate, but it does not react as a reactant How? They lower the energy needed to make the reaction go. So now… the reaction can occur more easily, and faster

the Rate of the Reaction? So, How Can I Decrease So, How Can I Increase the Rate of the Reaction? So, How Can I Decrease the Rate of the Reaction? Add a catalyst to speed up the reaction Increase the temperature Increase the surface area (break solid into smaller pieces) Increase the concentration of the liquid (put more solute in the water) Decrease the temperature where the reaction occurs. Decrease the concentration of reactants. Decrease the amount of surface area that reacts. Remove a catalyst or add an inhibitor.