Chemical Reactions and Collision Theory

What are Chemical Reactions? A chemical reaction is a process in which at least one new substance is produced as a result of a chemical change.

Chemical Reaction Terminology In chemical reactions there are reactants and products. Reactants are the chemicals that react together to form a product. Reactants are on the left side of a chemical equation. A + B  AB

Chemical Reaction Terminology Products are the chemicals formed in a chemical reaction. Products are on the right side of the chemical reaction equation. A + B  AB

Chemical Reaction Terminology So, what’s the arrow in the middle of the equation? A + B  AB The arrow represents what happens when A runs into B. It is the chemical reaction.

Chemical Reaction Terminology Making cookies is a lot like a chemical reaction. When you make cookies what are the reactants? What are the products? And what is the reaction, how do you make them? (i.e. what does the arrow represent?)

Chemical Reaction Terminology Reactants: flour, sugar, butter, chocolate chips, etc. Products: cookies. The reaction (the arrow): mixing, and stirring, and baking. Flour + Sugar + Butter + Chocolate Chips Cookies Mixing Stirring Baking

Collision Theory and Chemical Reactions Picture a container with two different reactants, A and B, where the chemical equation is +  B B A A B A A B B A B A

Collision Theory and Chemical Reactions Reactants must interact (that is, collide) with one another before any reaction can occur. In other words, must hit before can form. B A B A B A B B A B A A

Collision Theory and Chemical Reactions If reactants must collide to react, then the frequency of collisions (the number of collisons per amount of time) would affect how quickly the reactants form the product. How could you increase the frequency (how often) the reactants (molecules and )collide? B A A B B A B A A B

Collision Theory and Chemical Reactions You could increase the temperature, increase the size of the reactants, or increase the concentration of the reactants. How would each of these increase the frequency of collisions and thereby increase the rate of the reaction? Predict and record your answers in your notebook before proceeding. B A A B B A B A

Collision Theory and Chemical Reactions - Temperature Increasing the temperature would increase how quickly molecules A and B are moving in the container and would thereby increase the chances of the two reactants colliding. If the reactants collide more frequently then they form product more frequently, and the reaction rate (how quickly product is formed) is increased. B A A B B A B A

Collision Theory and Chemical Reactions – Size Increasing the size of the reactants would increase the likelihood of collision, and the therefore the frequency of collision would increase and so would the reaction rate. A B A B B B A A

Collision Theory and Chemical Reactions - Concentration Increasing the concentration of the reactants would increase the number of particles that could collide and form product. With more reactants colliding and forming product, the reaction rate increases. B B B B A A B A A B B A B A A A

Collision Theory and Chemical Reactions – Activation Energy Colliding particles must possess a certain minimum amount of energy, called the activation energy, if a collision is to be effective (that is, result in a reaction). When two reactant particles collide they do not always result in the formation of product. Sometimes, they rebound unchanged. The collision theory states that in order for a reaction to occur, particles must collide with a certain minimum energy. That is the kinetic energies of the colliding particles must add to a certain minimum value. The activation energy is the minimum combined kinetic energy that reactant particles must possess in order for their collision to result in a reaction.

Collision Theory and Chemical Reactions – Activation Energy Increasing the temperature of the reactants has two effects: 1) It increases the rate of movement (kinetic energy) of the reactants making collision more likely, and 2) It increases the kinetic energy of the reactants making collisions more likely to be effective (more likely to form product) by overcoming the activation energy. In the case of the above pictured reaction, increasing the temperature increases the speed of movement and the kinetic energy of A and B. Think of it as having an increased kinetic energy allows A and B to overcome the energy barrier (the activation energy) that is required for A and B to combine and form product. B A A B B A B A

Collision Theory and Chemical Reactions – Activation Energy Here’s a drawing to help. This drawing shows that when reactants don’t have enough energy they can’t collide with enough energy to produce product. The kinetic energy increases with temperature and allows the reactants to overcome the activation energy and collide with enough energy to form product. reaction B A Increased kinetic Energy from Increased temperature Activation Energy B A

The Effects of Catalysts on Reaction Rates A catalyst is a substance that increases reaction rate without being consumed in the reaction. Catalysts increase reaction rates by providing alternate reaction pathways that have lower activation energies than the original, uncatalyzed pathway. In biological systems, enzymes are catalysts. Enzymes catalyze (speed up) reactions in animals that would not otherwise occur at appreciable rates at normal animal body temperature because the temperature is not enough to overcome the activation energy.

The Effects of Catalysts on Reaction Rates Here’s a drawing to show how catalysts increase reaction rates by lowering the activation energy. Compare this drawing to the previous drawing where temperature was increased to increase the kinetic energy. Here, it’s the activation energy that decreases. The kinetic energy of the reactants doesn’t increase. With a lower activation energy, collisions are more likely to contain sufficient energy to lead to product formation. Catalysts decrease The activation energy Allowing the reaction To proceed at a lower Energy level. B A reaction B A Activation Energy

Experiment on Reaction Rates For background information: H2O2 (hydrogen peroxide) + catalase (yeast)  O2(gas)+ catalase (yeast) The above reaction involves the production of oxygen from hydrogen peroxide when the enzyme catalase (found in yeast) is added. Remember, catalase is found on both the reactant and product side because it is a catalyst and is not consumed in the reaction.

Experiment on Reaction Rates Materials needed: Hydrogen peroxide from the local store Regular baking yeast from the store You decide what else you need

Experiment on Reaction Rates Design an experiment using hydrogen peroxide and yeast and any other equipment you need that will answer the following questions? Is oxygen produced from hydrogen peroxide without adding yeast? Why? What component of yeast catalyzes the formation of oxygen from hydrogen peroxide? How does a catalyst affect the energy of activation? How does the temperature of the hydrogen peroxide affect the rate of oxygen production? How could you test to know that the gas produced is oxygen? Record your experimental design and the answers to these questions in your notebook.

Now proceed back to the assignment page to complete part 2 on dynamic equilibrium and Le Chatelier’s principle.