Section 1 - What Affects the Rate of a Reaction?

2. Section 1 - What Affects the Rate of a Reaction? A rate indicates how fast something changes with time. Therefore the rate of a chemical reaction is how fast the reaction move from reactants to products. Factors Affecting Rate For a reaction to occur the chemical involved in the reaction must come into contact with each other. Changing factors that increase the chance of chemical coming into contact with each other will increase the rate of reaction. Also chemicals must stick each other at the proper angle for the reaction to occur.

3. A reaction will not occur if the collision occurs too gently, as in a, or with the wrong orientation as in b. An effective collision, as in c, must deliver sufficient energy and bring together the atoms that bond in the products.

4. Temperature: Increasing temperature greatly affects the rate of reaction. The higher the Temperature the faster the particle are moving. This increase the number of collision s and the greater chance of the reaction taking place. Surface Area: If a solid substance is to be used in a chemical reaction surface are greatly affects the rate of reaction. Firewood that is split into smaller pieces will burn much faster that one large block of wood. This is because the smaller reactants are have more surface area to collide with other reactants. Powders react much faster than large pieces of reactants. Concentration: The higher the concentration of reactants the greater the chance that reactants will come into contact with each other. Low concentration puts more space between the particles decreasing the chances that they will react. Pressure: Changes in pressure will have little affect with reactants that are solids or liquids because these particles are already located very close to each other. due to the large distance between particles of gas substances, increasing pressure will greatly affect the rate of reaction for reactions involving gases. Increasing pressure will decrease the spacing between particles and increase the chance of collisions with other reactants.

5. Catalysts Catalysts are substances that changes the rate of a chemical reaction without being consumed or changed significantly. Catalysts affect the forward rate and reverse rate the same. Nature of Reactants Some chemicals naturally react faster than other. Covalent molecules tend To react much slower than ionic.

6. Section 2 - How Can Reaction Rates BeExplained? Potential Energy diagrams are used to explain reaction rates. These graphs illustrate energy changes during a chemical reaction. Terms used on potential energy diagrams: Activation energy: energy needed to break chemical bonds and reach the activated complex. Activated complex: location on the potential energy diagram where all reactants have broken their bonds and are prepared to form new bonds Heat of Reaction, H: Energy that is gained or lost after the reaction has completed. H is positive if endothermic and negative if exothermic.

7. Building a Potential Energy Diagram Location of Activated complex Potential Energy Location of reactants of forward reaction Location of products of reverse reaction Location of products of forward reaction Location of reactants of reverse reaction Reaction Coordinate Left to right for forward reaction Right to left for reverse reaction

8. Building a Potential Energy Diagram Potential Energy of Activated Complex Potential Energy Potential energy of reactants (forward reaction) Potential energy of products (reverse reaction) Potential energy of products (forward reaction) Potential energy of reactants (reverse reaction) Reaction Coordinate Left to right for forward reaction Right to left for reverse reaction

9. Building a Potential Energy Diagram Activation energy of the forward reaction Activation energy of the reverse reaction Potential Energy Heat of reaction, H of both forward or reverse reaction. Remember: It always takes energy to break bonds and reach activated complex, and energy is always released when bonds form. Reaction Coordinate Left to right for forward reaction Right to left for reverse reaction

10. Building a Potential Energy Diagram Potential Energy Head Tail Quick Read. If the tail of the graph is lower than the head the PE diagram is demonstrating an exothermic, -H, forward reaction Reaction Coordinate Left to right for forward reaction Right to left for reverse reaction

11. Building a Potential Energy Diagram Tail Potential Energy Head Quick Read. If the tail of the graph is higher than the head the PE diagram is demonstrating an endothermic, H, forward reaction Reaction Coordinate Left to right for forward reaction Right to left for reverse reaction

12. Building a Potential Energy Diagram Potential Energy Catalyst pathway A catalyst speed up chemical reactions, both forward and reverse, By lowering the reaction pathway. The PE of the activated complex is lowered. Reaction Coordinate Left to right for forward reaction Right to left for reverse reaction