Sec. 16.1: A Model for Reaction Rates Ch.16: Reaction Rates Sec. 16.1: A Model for Reaction Rates

Objectives Calculate average rates of chemical reactions from experimental data. Relate reaction rates to collisions between reacting particles.

Reaction Rate Reactions take place at different rates or speeds. Some products form quickly, some form slowly. Reaction rate is defined as the amount of change in the concentration of a reactant or product per unit of time. How fast a reactant disappears or how fast a product appears is what is measured. Reaction rates are very important in many industries - the faster the rate, the more product that can be made in a given amount of time.

Expressing Reaction Rates

Expressing Reaction Rates (cont.) SECTION16.1 Expressing Reaction Rates (cont.) When reaction rates are determined experimentally, usually the changing amount of a reactant is measured. Since the amount of a reactant would normally decrease, a negative sign is used so that the reaction rate is a positive number.

Reaction Rate For any reaction to take place, reactant particles have to collide in order to react. This is called Collision Theory. The reacting particles have to collide with a certain amount of force for a reaction to start. The activation energy is the amount of energy reactant particles must have to form the activated complex and lead to a reaction.

Collision Theory (cont.) SECTION16.1 Collision Theory (cont.) An activated complex is a temporary, unstable arrangement of atoms in which old bonds are breaking and new bonds are forming.

SECTION16.1 If a reaction has a high activation energy, only a small number of collisions will have the required energy to form the activated complex. The reaction rate, therefore, will be slow.

Reaction Rate In an exothermic reaction, the energy of the reactants is greater than the products because energy is released. The reaction STILL requires some input of energy to start it.

Reaction Rate In endothermic reactions, the reactants have less energy than the products because the reaction absorbs energy. More energy is required than is stored in the products because some of the energy is used as activation energy.

SECTION16.1

16.2: Factors Affecting Reaction Rates Objectives Identify factors that affect the rates of chemical reactions. Explain the role of a catalyst.

Reaction Rate Five factors affect reaction rate: Nature of Reactants Concentration of reactants Surface Area of reactants Temperature of system Presence of catalyst/inhibitor

Factors That Influence Reaction Rate 1. The Nature of Reactants or “reactivity” Some substances react more readily than others. This is simply due to their structure. Sodium, for example, is more reactive than magnesium because it has to only give up 1 electron while magnesium must lose 2.

Factors That Influence Reaction Rate 2- Concentration: Increasing the concentration of a reactant will speed up a reaction. This is because, with more reactant particles, more collisions between them are likely. (Concentration is defined as the amount of a substance present in a given volume.) What will decreasing concentration do?

Factors That Influence Reaction Rate Keep in mind that, if there is more than one reactant in a reaction, it will not matter what the concentrations of the other reactants are if one of the reactants is used up. If I want to make s’mores, for example, it will not matter how many marshmallows I have if I only have one piece of chocolate. Chocolate is the limiting reactant.

Factors That Influence Reaction Rate 3 – Surface Area Increasing the surface area of the reactants will increase the rate of a reaction. This is because increasing the surface area of a reactant would allow more collisions to occur. Which will dissolve faster – a sugar cube or sugar granules?

Factors That Influence Reaction Rate 4- Temperature: If the temperature is increased, most reactions go faster. This is because increasing the temperature will increase the speed & force of collisions between reactant particles. What happens if the temperature is decreased?

High-energy collisions are more frequent at a higher temperature. SECTION16.2 High-energy collisions are more frequent at a higher temperature.

Factors That Influence Reaction Rate 5- Catalysts: Adding or removing a catalyst will change a reaction rate. A catalyst is a substance that speeds up the rate of a reaction without being permanently changed or used up itself. Biological catalysts are called enzymes.

Catalysts Catalysts lower activation energy. A lower activation energy means more of the collisions between particles will have enough energy to bring about a reaction.

SECTION16.2 Catalysts A heterogeneous catalyst exists in a physical state different than that of the reaction it catalyzes. A homogeneous catalyst exists in the same physical state as the reaction it catalyzes.

Inhibitors An inhibitor is a substance that slows down or prevents a reaction. These substances are used to slow down spoilage of food and in medications to increase their shelf life. Many medicines will decompose under normal circumstances, limiting their effectiveness.

Chapter 17: Chemical Equilibrium Sec. 17.1: A State of Dynamic Balance

Objectives List the characteristics of chemical equilibrium Write equations for systems at equilibrium Calculate equilibrium constants from concentration data.

Reversible Reactions Some reactions “go to completion” because one of the reactants is completely used up (called the limiting reactant) and the reaction stops. However, many reactions can change direction. They are called reversible reactions.

Reversible Reactions When a reaction takes place, reactants combine to form products. If the products formed begin to react with each other to reform the reactants, the reaction is reversible. The reactants are never used up! To indicate that the reaction is reversible, we use a double arrow in the chemical equation: A + B C + D

Reversible Reactions In reversible reactions, two separate reactions are actually taking place. A forward reaction: A + B C + D A reverse reaction: A + B C + D or C + D A + B

Reversible Reactions At first, the forward reaction occurs (reactants produce products) but no the reverse reaction occurs. This is because the amount of products is not high enough to start a reverse reaction. Soon, though, when the amount of products is high enough, the products will react to reform the reactants.

Equilibrium Eventually, the amount of products formed no longer changes. The amount of each product remains constant. The amount of reactants re-formed also no longer changes. The amount of each reactant remains constant. The “system” has reached equilibrium.

Equilibrium If you were to graph data concerning the changes in concentration of reactants and products over time, a graph like this would result. Note that eventually, the concentrations of both level off and remain constant. This is when equilibrium is reached. Reactants Equilibrium Products Forward: N2(g) + 3H2(g)  2NH3(g) Reverse: N2(g) + 3H2(g)  2NH3(g)

Equilibrium A system is in equilibrium when no net change occurs in the amounts of reactants and products. The forward and reverse reactions are still occurring, but reactants and products are forming at the same rate. It is often called a dynamic equilibrium because reactions are still taking place.

Reversible Reactions CaCO3 CaO + CO2 PCl5 PCl3 + Cl2 Each reaction has reached equilibrium - the amounts of reactants and products don’t change. This does not mean the actual amounts of reactants and products are equal. It means the RATES at which they form are equal.

Reversible Reactions PCl5 PCl3 + Cl2 Actual measurements in the lab show that in this reaction, at equilibrium, there is more PCl5 than PCl3 and Cl2. Even in reversible reactions at equilibrium, stability is favored and PCl5 is more stable. There is a balance that favors stability.

Equilibrium Expressions and Constants A majority of reactions reach a state of equilibrium. By definition, in these reactions, not all reactants are consumed and not all of the product predicted by the equation will be produced. How then can chemists predict the yield of a reaction?

Equilibrium Expressions and Constants The law of chemical equilibrium states that at a given temperature, a system will reach a state in which a ratio of reactant and product concentrations will have a constant value. For the reaction aA + bB  cC + dD, Keq = [C]c[D]d [A]a[B]b

Equilibrium Expressions and Constants The ratio is called the equilibrium constant expression. Brackets indicate concentrations in mol/L. Keq is called the equilibrium constant. It is constant at a specified temperature. The Keq will always be the same, regardless of initial concentrations of reactants and products, at a given temperature. See Table 1, p. 604.

Equilibrium Expressions and Constants To interpret the meaning of Keq values, recall that fractions with large numerators are larger than fractions with large denominators. Therefore Keq > 1 means the equilibrium mixture contains more products than reactants. A Keq < 1 means the equilibrium mixture contains more reactants than products.

Homogeneous equilibria In a homogeneous equilibrium, all the reactants and products are in the same physical state. For H2 (g) + I2 (g)  2HI (g), Keq = [HI]2 [H2][I2] At 731 K, Keq = 49.7 (Note: there are no units). This equilibrium favors . . .?

Heterogeneous equilibria When the reactants and products are present in more than one physical state, it is a heterogeneous equilibrium. Liquids and solids are pure substances whose density (concentration) at a given temperature does not change. Therefore, their concentration is constant and is combined with the K value.

Heterogeneous equilibria Examples: I2 (s)  I2 (g) Keq = [I2(g)] C2H5OH (l)  C2H5OH (g) Keq = [C2H5OH (g)]

Practice Problems Calculate Keq for N2 (g) + 3H2 (g)  2NH3 (g) if [NH3] = 0.933 mol/L, [N2] = 0.533 mol/L, and [H2] = 1.600 mol/L. Calculate Keq for N2O4 (g)  2NO2 (g) if [N2O4] = 0.0185 mol/L and [NO2] = 0.0627 mol/L.

17.2: Factors Affecting Chemical Equilibrium Objectives Describe how various factors affect chemical equilibrium. Explain how Le Châtelier’s Principle applies to equilibrium systems.

Le Chatelier’s Principle Once an equilibrium was established, Le Chatelier proposed that it was possible to disturb the equilibrium. He believed that a disruption to the equilibrium would make the system readjust in a way that would reduce the disturbance and regain equilibrium. In other words, a system under “stress” will act so as to relieve that stress. A stress is any change that upsets an equilibrium.

An Analogy: Le Chatelier’s Principle Before drinking, the water level in the bowl is at equilibrium. Once the dog drinks, the water level is “stressed”. As a result, water will come from the bottle into the bowl to reestablish equilibrium.

Le Chatelier’s Principle Industrial chemists use this principle to increase the amount of product that will form in a reaction. There are 3 ways to stress a chemical reaction. To change the concentration of a reactant or product To add or remove energy (by changing temperature) To change the pressure (or volume) of gases that are in a reaction

Changing the concentration of a reactant or product 1A- Adding more of a reactant will cause the forward reaction to speed up to get rid of the “excess” reactant. We say the reaction “shifts” to the right. 3C + 2Al2O3 4Al + 3CO2 As a result, the concentration of products increases and the concentration of other reactants decreases.

Changing the concentration of a reactant or product 1B- Removing a reactant will cause the reverse reaction to speed up to replace the “lost” reactant. We say the reaction “shifts” to the left. 3C + 2Al2O3 4Al + 3CO2 As a result, the concentration of products decreases and the concentration of other reactants increases.

Changing the concentration of a reactant or product 1C- Adding more of a product will cause the reverse reaction to speed up to get rid of the “excess” product. We say the reaction “shifts” to the left. 3C + 2Al2O3 4Al + 3CO2 As a result, the concentration of reactants increases and the concentration of other products decreases.

Changing the concentration of a reactant or product 1D- Removing a product will cause the forward reaction to speed up to replace the “lost” product. We say the reaction “shifts” to the right. 3C + 2Al2O3 4Al + 3CO2 As a result, the concentration of reactants decreases and the concentration of other products increases.

Changing the concentration of a reactant or product Note: Products that are gases can be easily removed from the reaction. A product that is insoluble (does not dissolve in a liquid) is also easily removed. It will form a precipitate and will remove itself from the reaction.

Adding or Removing Energy Energy is part of every reaction. It can be thought of as a reactant or product. Endothermic reactions require energy so energy is a reactant. A + B + energy C + D Exothermic reactions release energy so energy is a product. A + B C + D + energy

Adding or Removing Energy 3C + 2Al2O3 + energy 4Al + 3CO2 If energy is added to an endothermic reaction (by increasing the temperature), the reaction shifts to the right. If energy is removed from the reaction (by decreasing the temperature), the reaction shifts to the left.

Adding or Removing Energy N2 + 3H2 2NH3 + energy If energy is added to an exothermic reaction (by increasing the temperature), the reaction shifts to the left. If energy is removed from the reaction (by decreasing the temperature), the reaction shifts to right.

Changes in volume or pressure If you decrease the space a gas occupies (decrease its volume), the pressure of the gas increases. If you increase the space a gas occupies (increase its volume), the pressure of the gas decreases. A change in volume or pressure is a stress on a reaction at equilibrium.

Changes in volume or pressure At a given volume, the number of particles in a sample of gas also affects the pressure. If the number of particles increases, the pressure increases. If the number of particles decreases, the pressure decreases.

Changes in volume or pressure CO (g) + 3H2 (g)  CH4 (g) + H2O (g) In this reaction, in the reaction chamber, there will be 4 molecules of reactants for every 2 molecules of products. If the volume on the reaction chamber is decreased, the pressure in the reaction chamber increases and the equilibrium is stressed. To relieve the stress of increased pressure, a lower number of particles is favored, so the equilibrium shifts to the right. If the volume is increased???

Changes in volume or pressure Changes in volume or pressure of a system at equilibrium will only shift the equilibrium if there are gaseous reactants or products in the reaction, AND the number of particles of gaseous reactant is different from the number of particles of gaseous product.

Assume all reactants & products are gases. Practice Problems 2CO + O2  2CO2 3H2 + N2  2NH3 Assume all reactants & products are gases. How would decreasing the volume affect each equilibrium? How would decreasing the pressure affect each equilibrium?