KINETICS How Fast Does A Reaction Occur?

Energy Diagrams l Reactants always start a reaction so they are on the left side of the diagram. Reactants l Products are on the right. Products

Exothermic Reactions l The exothermic reaction gives off heat because the products are at a lower energy level than the reactants.

Graph of an Exothermic Reaction

l In an exothermic graph, the reactants have greater energy than the products. Reactants The change in energy ( Δ H) is a negative value. Products

Exothermic Reactions l When climbing a ladder, you are more stable near the bottom than at the top. l In an exothermic reaction, products are more stable because they are lower in energy.

Exothermic Reactions l For an exothermic reaction, heat is a product and is found on the right side of the arrow in a chemical reaction. H 2 (g) + Cl 2 (g) → 2HCl(g) + heat

Endothermic Reactions l The endothermic reaction absorbs heat because the products are at a higher energy level than the reactants.

Graph of an Endothermic Reaction

l In an endothermic graph, the products have greater energy than the reactants. Reactants l The change in energy is a positive value. Products

Endothermic Reactions l In an endothermic reaction, reactants are more stable because they have lower energy.

Endothermic Reactions l For an endothermic reaction, heat is a reactant and is found on the left side of the arrow in a chemical reaction. heat + C (s) + H 2 O (g) → CO (g) + H 2 (g)

Exothermic And Endothermic Reactions l Scientists have observed that the energy released in the formation of a compound from its elements is always identical to the energy required to decompose that compound into its elements.

Energy Diagrams l Activation energy is the minimum amount of energy that reacting particles must have to form the activated complex. Activation energy activated complex

Energy Diagrams l The activated complex is a short- lived, unstable arrangement of atoms that may break apart and re- form the reactants or may form products. activated complex

Energy Diagrams l To calculate the activation energy, take the top of the peak and subtract the reactants. Activation energy

Energy Diagrams The enthalpy or heat of reaction ( Δ H) is the amount of heat released or absorbed in the reaction. Enthalpy/heat of reaction ( Δ H)

Energy Diagrams To determine Δ H, take the energy of the products and subtract the energy of the reactants. Enthalpy/heat of reaction ( Δ H)

Problem 1) The heat content of the reactants of the forward reaction is ______ kilojoules. (160)

Problem 2) The heat content of the products of the forward reaction is ______ kilojoules. (80)

Problem 3) The heat content of the activated complex of the forward reaction is about _____ kilojoules. (240)

Problem 4) The activation energy of the forward reaction is _____ kilojoules. (240 – 80 = 160 )

Problem 5) The heat of reaction ( Δ H) of the forward reaction is _____ kilojoules. (160 – 80 = 80 )

Problem 6) The forward reaction is (endothermic or exothermic). (endothermic - products are higher in energy; Δ H is positive)

Problem 7) The heat content of the reactants of the forward reaction is ______ kJ. (40)

Problem 8) The heat content of the products of the forward reaction is about ______ kilojoules. (20)

Problem 9) The heat content of the activated complex of the forward reaction is about _____ kilojoules. (100)

Problem 10) The activation energy of the forward reaction is about _____ kilojoules. (100 – 40 = 60 )

Problem 11) The enthalpy change ( Δ H) of the forward reaction is ____ kilojoules. (20 – 40 = – 20 )

Problem 12) The forward reaction is (endothermic or exothermic). (exothermic - products are lower in energy; Δ H is negative)

Problem 13) The heat content of the reactants of the reverse reaction is ______ kilojoules. (20)

Problem 14) The heat content of the products of the reverse reaction is ______ kilojoules. (40)

Problem 15) The enthalpy change ( Δ H) of the reverse reaction is ____ kilojoules. (40 – 20 = + 20 )

Forward or Reverse Favored? l Remember that enthalpy is the heat of the reaction (products minus reactants). l The enthalpy factor favors the forward reaction (the products) if ΔH is negative because the products are lower in energy and are thus more stable. l The reverse reaction is favored if ΔH is positive.

l Entropy is the degree of randomness or disorder. l The entropy factor favors the side of the reaction more gas molecules because gases have more entropy than liquids and solids. Forward or Reverse Favored?

l The forward reaction (product side) is favored if there are more gas molecules on the right side of the arrow. l The reverse reaction (reactant side) is favored if there are more gas molecules on the left side of the arrow. Forward or Reverse Favored?

Problem 2H 2 O (l) → 2H 2 (g) + O 2 (g) ΔH = +572 kJ 16) Does the enthalpy factor favor the forward or the reverse reaction? Why? (ΔH is positive. Reactants are lower in energy and thus more stable. Reactants are favored, so enthalpy favors the reverse reaction.)

Problem 2H 2 O (l) → 2H 2 (g) + O 2 (g) ΔH = +572 kJ 17) Does the entropy factor favor the forward or the reverse reaction? Why? (There are 0 gas moles on the left. There are 3 gas moles on the right. Entropy favors the side with more gas moles. Therefore, the product side is favored, i.e. the forward reaction.)

Problem CO (g) + NO 2 (g) → CO 2 (g) + NO (g) ΔH = kJ 18) Does the enthalpy factor favor the forward or the reverse reaction? Why? (The forward reaction is favored because ΔH is negative. Products are lower in energy and thus more stable. )

Problem CO (g) + NO 2 (g) → CO 2 (g) + NO (g) ΔH = kJ 19) Does the entropy factor favor the forward or the reverse reaction? Why? (There are 2 gas moles on the left. There are 2 gas moles on the right. Therefore, neither side is favored.)

Problem H 2 (g) + Cl 2 (g) ↔ 2HCl(g) + heat 20) Does the enthalpy factor favor the forward or the reverse reaction? Why? (Heat is a product which means the reaction is exothermic The forward reaction is favored because ΔH is negative. Products are lower in energy and thus more stable. )

Problem H 2 (g) + Cl 2 (g) ↔ 2HCl(g) + heat 21) Does the entropy factor favor the forward or the reverse reaction? Why? (There are 2 gas moles on the left. There are 2 gas moles on the right. Therefore, neither side is favored.)

Energy Diagrams l The activation energy can be lowered by adding a catalyst. effect of the catalyst

Energy Diagrams l The catalyst lowers the activation energy by providing an alternate pathway for the reaction to occur. l Enzymes are biological catalysts.

Expressing Reaction Rates l As you know, some chemical reactions are fast and others are slow; however, fast and slow are inexact, relative terms. l Chemists often need to be more specific.

Expressing Reaction Rates l We generally define the average rate of an action or process to be the change in a given quantity during a specific period of time. l The rate of a reaction can be described by the increase in the concentration of the products with time or the decrease in the concentration of the reactants with time.

Expressing Reaction Rates l Reaction rates cannot be calculated from balanced equations as stoichiometric amounts can. l Reaction rates are determined experimentally by measuring the concentrations of reactants and/or products in an actual chemical reaction.

Collision Theory l According to the collision theory, atoms, ions, and molecules must collide with each other in order to react.

Collision Theory l The following three statements summarize the collision theory.  1. Particles must collide in order to react.

Collision Theory  2. The particles must collide with the correct orientation.

Collision Theory  3. The particles must collide with enough energy to form an unstable activated complex, also called a transition state, which is an intermediate particle made up of the joined reactants.

Collision Theory activated complex

Collision Theory l The minimum amount of energy that colliding particles must have in order to form an activated complex is called the activation energy of the reaction. l Particles that collide with less energy than the activation energy cannot form an activated complex.

Collision Theory l In an exothermic reaction, molecules collide with enough energy to overcome the activation energy barrier, form an activated complex, then release energy and form products at a lower energy level.

Energy Reaction coordinate Reactants Products

Energy Reaction coordinate Reactants Products Activated Complex or Transition State

Energy Reaction coordinate Reactants Products Activation Energy

Energy Reaction coordinate Reactants Products Overall energy change

Collision Theory l In the reverse endothermic reaction, the reactant molecules lying at a low energy level must absorb energy to overcome the activation energy barrier and form high-energy products.

Factors Affecting Reaction Rates l The reaction rate for almost any chemical reaction can be modified by varying the conditions of the reaction.

Factors Affecting Reaction Rates 1)The reactive nature of the reactants affects the rate of a chemical reaction. Some reactions are naturally slow because the bonds involved are strong and unreactive or the electrons are tightly held.

Factors Affecting Reaction Rates 1)cont., The greater the number of bonds being broken and made (like double or triple bonds, the slower the reaction.

Factors Affecting Reaction Rates 2)Another important factor that affects the rate of a chemical reaction is the concentration of the reactants. Reactions speed up when the concentrations of reacting particles are increased. Increasing the number of reactants increases probability of collisions.

Factors Affecting Reaction Rates 2)continued. The rate of gaseous reactions can be increased by pumping more gas into the reaction container.

Factors Affecting Reaction Rates 3)Surface area of the reactants affects the rate of a chemical reaction. Increasing the surface area of reactants provides more opportunity for collisions with other reactants, thereby increasing the reaction rate.

Factors Affecting Reaction Rates 4)Temperature affects the rate of a chemical reaction. Generally, increasing the temperature at which a reaction occurs increases the reaction rate. Raising the temperature raises both the collision frequency and the collision energy.

Factors Affecting Reaction Rates 5)Adding a catalyst affects the rate of a chemical reaction. A catalyst is a substance that increases the rate of a chemical reaction without itself being consumed in the reaction. In fact, catalysts are not included in the chemical equation.

Factors Affecting Reaction Rates A catalyst lowers the activation energy of a reaction by providing an alternate pathway for the reaction to occur (the blue line).

Factors Affecting Reaction Rates 6)Compressing gases affects the rate of a chemical reaction. When two gases react, compressing the gases (increasing the pressure) generally increases the rate of the reaction.