18.1 Rates of Reaction > 1 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Chapter 18 Reaction Rates and Equilibrium 18.1 Rates of Reaction 18.2 The Progress of Chemical Reactions 18.3 Reversible Reactions and Equilibrium 18.4 Solubility Equilibrium 18.5 Free Energy and Entropy

18.1 Rates of Reaction > 2 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. How can rusting be used to cook a meal? There are products that use the rusting of an iron-magnesium alloy to heat packaged food. These products are known as Meals Ready to Eat, or MREs for short. CHEMISTRY & YOU

18.1 Rates of Reaction > 3 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Describing Reaction Rates * The speed of chemical reactions can vary from very fast to extremely slow. –When you strike a match, it erupts into flame almost instantly and burns quickly. –Millions of years were required for plants buried beneath Earth’s surface to be converted to coal.

18.1 Rates of Reaction > 4 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Describing Reaction Rates A rate is a measure of how much something changes within a specified amount of time. Rates of Change

18.1 Rates of Reaction > 5 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Describing Reaction Rates How is the rate of a chemical reaction expressed?

18.1 Rates of Reaction > 6 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Describing Reaction Rates In chemistry, the rate of a chemical reaction, or the reaction rate, is usually expressed as the change in the amount of reactant or product per unit time.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Describing Reaction Rates The figure below illustrates the progress of a typical reaction. Over time, the amount of reactant decreases and the amount of product increases.

18.1 Rates of Reaction > 8 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Describing Reaction Rates A model called collision theory is used to relate the properties of particles to the rates of chemical reactions. According to the collision theory: atoms, ions, and molecules can react to form products when they collide if the particles have enough kinetic energy. Collision Theory

18.1 Rates of Reaction > 9 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Describing Reaction Rates A model called collision theory is used to relate the properties of particles to the rates of chemical reactions. According to the collision theory: atoms, ions, and molecules can react to form products when they collide if the particles have enough kinetic energy. Particles that do not have enough energy to react bounce apart unchanged when they collide. Collision Theory

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Describing Reaction Rates An effective collision of oxygen and hydrogen molecules produces water molecules.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Describing Reaction Rates An ineffective collision of oxygen and hydrogen molecules produces no reaction; the reactants bounce apart unchanged.

18.1 Rates of Reaction > 12 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Describing Reaction Rates The minimum energy that colliding particles must have in order to react is called the activation energy.

18.1 Rates of Reaction > 13 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Describing Reaction Rates The minimum energy that colliding particles must have in order to react is called the activation energy. You can think of the activation energy for a reaction as a barrier that reactants must cross before products can form.

18.1 Rates of Reaction > 14 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Describing Reaction Rates When two reactant particles collide, they may form an activated complex. An activated complex is an unstable arrangement of atoms that forms for a moment at the peak of the activation-energy barrier.

18.1 Rates of Reaction > 15 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Describing Reaction Rates When two reactant particles collide, they may form an activated complex. An activated complex is an unstable arrangement of atoms that forms for a moment at the peak of the activation-energy barrier. The activated complex forms only if the colliding particles have enough energy and if the atoms are oriented properly.

18.1 Rates of Reaction > 16 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Describing Reaction Rates When two reactant particles collide, they may form an activated complex. The lifetime of an activated complex is typically about seconds.

18.1 Rates of Reaction > 17 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Describing Reaction Rates When two reactant particles collide, they may form an activated complex. The lifetime of an activated complex is typically about seconds. Its brief existence ends with the reformation of the reactants or with the formation of products.

18.1 Rates of Reaction > 18 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Describing Reaction Rates When two reactant particles collide, they may form an activated complex. The lifetime of an activated complex is typically about seconds. Its brief existence ends with the reformation of the reactants or with the formation of products. Thus, the activated complex is sometimes called the transition state.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Remember: An endothermic reaction absorbs heat, and an exothermic reaction releases heat. Interpret Graphs The activation-energy barrier must be crossed before reactants are converted to products.

18.1 Rates of Reaction > 20 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Describing Reaction Rates Collision theory explains why some reactions are extremely slow at room temperature. Carbon and oxygen react when charcoal burns, but the reaction has a high activation energy. The O—O and C—C bonds must be broken to form the activated complex. At room temperature, the collisions of oxygen and carbon molecules are not energetic enough to break the bonds. Thus, the reaction rate of carbon with oxygen at room temperature is essentially zero.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. What factors determines whether a molecular collision results in a reaction?

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The molecules must collide oriented properly and with enough energy in order to react. The minimum amount of energy needed is called the activation energy. What factors determines whether a molecular collision results in a reaction?

18.1 Rates of Reaction > 23 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates By varying conditions, you can modify the rate of almost any reaction.

18.1 Rates of Reaction > 24 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates What four factors influence the rate of a chemical reaction?

18.1 Rates of Reaction > 25 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates Factors that can affect the rate of a chemical reaction are temperature, concentration, particle size, and the use of a catalyst.

18.1 Rates of Reaction > 26 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates Temperature Raising the temperature usually speeds up a reaction. Lowering the temperature usually slows down a reaction.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates Temperature The frequency of collisions increases along with the percentage of particles that have enough kinetic energy to slip over the activation-energy barrier. Thus, an increase in temperature causes products to form faster. At higher temperatures, particles move faster.

18.1 Rates of Reaction > 28 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates Concentration The number of particles in a given volume affects the rate at which reactions occur.

18.1 Rates of Reaction > 29 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates Concentration Cramming more particles into a fixed volume increases the concentration of reactants, and, thus, the frequency of collision. The number of particles in a given volume affects the rate at which reactions occur.

18.1 Rates of Reaction > 30 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates Concentration Cramming more particles into a fixed volume increases the concentration of reactants, and, thus, the frequency of collision. Increased collision frequency leads to a higher reaction rate. The number of particles in a given volume affects the rate at which reactions occur.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates The lighted splint glows in air and soon dies out because air is only 20% oxygen. When the glowing splint is plunged into pure oxygen, it immediately bursts into flame. The increased concentration of oxygen greatly speeds up the combustion reaction.

18.1 Rates of Reaction > 32 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates The total surface area of a solid or liquid reactant affects the rate of a reaction. Particle Size

18.1 Rates of Reaction > 33 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates The smaller the particle size, the greater the surface area is for a given mass of particles. The total surface area of a solid or liquid reactant affects the rate of a reaction. Particle Size

18.1 Rates of Reaction > 34 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates The smaller the particle size, the greater the surface area is for a given mass of particles. The result of an increase in surface area is an increase in the frequency of collisions and the reaction rate. The total surface area of a solid or liquid reactant affects the rate of a reaction. Particle Size

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates Only atoms at the surface of the metal are available for reaction. Dividing the metal into smaller pieces increases the surface area and the number of collisions. When a piece of magnesium is placed in dilute acid, hydrogen ions can collide with magnesium atoms. Mg(s) + 2H + (aq)  Mg 2+ (aq) + H 2 (g)

18.1 Rates of Reaction > 36 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates Another way to increase the surface area of solids is to dissolve them. In a solution, particles are separated and more accessible to other reactants. Particle Size

18.1 Rates of Reaction > 37 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates Small dustlike particles, however, can be dangerous when suspended in air. You can also increase the surface area of a solid by grinding it into a fine powder. Particle Size

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates An explosion destroyed this sugar refinery. Particle Size The tiny size of the reactant particles (sugar dust) caused the reaction of the sugar with oxygen in the air to be explosive.

18.1 Rates of Reaction > 39 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates Catalysts Increasing the temperature is not always the best way to increase the rate of a reaction. A catalyst is often better.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates Catalysts Increasing the temperature is not always the best way to increase the rate of a reaction. A catalyst is often better. Recall that a catalyst is a substance that increases the rate of a reaction without being used up during the reaction.

18.1 Rates of Reaction > 41 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates Catalysts Increasing the temperature is not always the best way to increase the rate of a reaction. A catalyst is often better. Recall that a catalyst is a substance that increases the rate of a reaction without being used up during the reaction. Catalysts permit reactions to proceed along a lower energy path.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. When the barrier is lower, a greater amount of reactants have the energy to form products within a given time. Interpret Graphs The activation-energy barrier for the catalyzed reaction is lower than that of the uncatalyzed reaction.

18.1 Rates of Reaction > 43 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. When salt water is added to the metal alloy in an MRE, the rate of the rusting reaction increases, and heat is produced rapidly. Which factor that can affect reaction rates is being applied in this situation? Salt acts as a catalyst for the reaction between the metal and water, speeding up the reaction without being consumed. CHEMISTRY & YOU

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates Catalysts The rate of reaction of hydrogen and oxygen at room temperature is negligible. But with a small amount of platinum (Pt) as a catalyst, the reaction is rapid. 2H 2 (g) + O 2 (g) 2H 2 O(ℓ) Pt

18.1 Rates of Reaction > 45 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates Catalysts A catalyst is not consumed during a reaction. 2H 2 (g) + O 2 (g) 2H 2 O(ℓ) Pt

18.1 Rates of Reaction > 46 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates Catalysts A catalyst is not consumed during a reaction. Therefore, it does not appear as a reactant in the chemical equation. 2H 2 (g) + O 2 (g) 2H 2 O(ℓ) Pt

18.1 Rates of Reaction > 47 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates Catalysts A catalyst is not consumed during a reaction. Therefore, it does not appear as a reactant in the chemical equation. Instead, the catalyst is often written above the yield arrow, as in the equation above. 2H 2 (g) + O 2 (g) 2H 2 O(ℓ) Pt

18.1 Rates of Reaction > 48 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates At normal body temperature (37  C), reactions in the body would be too slow without catalysts. Catalysts

18.1 Rates of Reaction > 49 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates At normal body temperature (37  C), reactions in the body would be too slow without catalysts. The catalysts that increase the rates of biological reactions are called enzymes. Catalysts

18.1 Rates of Reaction > 50 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates At normal body temperature (37  C), reactions in the body would be too slow without catalysts. The catalysts that increase the rates of biological reactions are called enzymes. When you eat a meal containing protein, enzymes in your digestive tract help break down the protein molecules in a few hours. Catalysts

18.1 Rates of Reaction > 51 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates An inhibitor is a substance that interferes with the action of a catalyst. Inhibitors

18.1 Rates of Reaction > 52 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates Some inhibitors work by reacting with, or “poisoning,” the catalyst itself. An inhibitor is a substance that interferes with the action of a catalyst. Inhibitors

18.1 Rates of Reaction > 53 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates Some inhibitors work by reacting with, or “poisoning,” the catalyst itself. Thus, the inhibitor reduces the amount of catalyst available for a reaction. An inhibitor is a substance that interferes with the action of a catalyst. Inhibitors

18.1 Rates of Reaction > 54 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Factors Affecting Reaction Rates Some inhibitors work by reacting with, or “poisoning,” the catalyst itself. Thus, the inhibitor reduces the amount of catalyst available for a reaction. Reactions slow or even stop when a catalyst is poisoned. An inhibitor is a substance that interferes with the action of a catalyst. Inhibitors

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Which of the following factors could be increased in order to decrease a reaction rate? A.Catalyst concentration B.Concentration C.Temperature D.Particle size

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. A.Catalyst concentration B.Concentration C.Temperature D.Particle size Which of the following factors could be increased in order to decrease a reaction rate?

18.1 Rates of Reaction > 57 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Key Concepts In chemistry, the rate of a chemical reaction, or the reaction rate, is usually expressed as the change in the amount of reactant or product per unit time. Factors that can affect the rate of a chemical reaction are temperature, concentration, particle size, and the use of a catalyst.

18.1 Rates of Reaction > 58 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. END OF 18.1