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 collisions per amount of time) would affect how quickly the reactants form the product. How could you change 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 change the temperature, change the concentration of the reactants, change the surface area of particles, or use a catalyst or an inhibitor. How would each of these change the frequency of collisions and thereby 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 total surface area of the reactants would increase the likelihood of collision, and the therefore the frequency of collision would increase and so would the reaction rate.

Collision Theory and Chemical Reactions - Concentration Changing the concentration of the reactants would change 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.

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 Inhibitors on Reaction Rates Inhibitors are substances that prevent a chemical reaction from occurring. Inhibitors interfere with reactants to either increase the activation energy required for the reaction to take place or change the configuration of the reactants which prevent the reaction.