Topic 13 Reaction Rates 2. Factor Affecting Reaction rates Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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

Factors Affecting Reaction Rates What four factors influence the rate of a chemical reaction? Factors that can affect the rate of a chemical reaction are: Temperature Concentration Particle size The use of a catalyst. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Factors Affecting Reaction Rates Temperature Usually, raising the temperature speeds up a reaction. Lowering the temperature usually slows down a reaction. At higher temperatures, particles move faster. The frequency of collisions increases along with the percentage of particles that have enough kinetic energy to slip over the activation-energy barrier. 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. 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. 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. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Factors Affecting Reaction Rates Particle Size The total surface area of a solid or liquid reactant affects the rate of a reaction. The smaller the particle size, the greater the surface area is for a given mass of particles Mg(s) + 2H+(aq)  Mg2+(aq) + H2(g) Dividing the metal into smaller pieces increases the surface area and the number of collisions. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Factors Affecting Reaction Rates Particle Size An explosion destroyed this sugar refinery. The tiny size of the reactant particles (sugar dust) caused the reaction of the sugar with oxygen in the air to be explosive. 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. 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.

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

CHEMISTRY & YOU 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. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Factors Affecting Reaction Rates Catalysts 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. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

A. Catalyst concentration B. Concentration C. Temperature 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.