Equilibrium Reversible Reactions Many reactions we have studied

Equilibrium

Reversible Reactions • • Many reactions we have studied this year are “forward reactions”, once the reactant has changed into products it stays that way. In reversible reactions, a reaction that can take place backwards or forwards, once a product is formed it can turn back into a reactant and a continuous cycle occurs. Reversible reactions are denoted by a double arrow ↔ How can we predict when these reactions are “finished”– have reached equilibrium?

Work with your table on questions 1 -7

Equilibrium • • Reversible reactions never come to an end: they just reach equilibrium. • Forward and reverse reactions continue to happen, however the moles of any species in the reaction that are produced or used are equivalent, so the overall quantities of all reactants and products remain constant. Equilibrium: the point where there is no further change in concentration of any species in the reaction. Talk with a partner: When equilibrium is reached, how do the moles of reactant lost and product gained in a short period of time compare?

Work with your table… • Complete the data table. • Respond to the 3 questions following the table.

Reaction Rates & Le Chatelier’s Principle 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. • 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.

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.

Collision Theory A model called collision theory is used to relate the properties of particles to the rates of chemical reactions. • According to 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.

Describing Reaction Rates An effective collision of oxygen and hydrogen molecules produces water molecules. An ineffective collision of oxygen and hydrogen molecules produces no reaction; the reactants bounce apart unchanged.

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. • Label on your diagram.

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 activationenergy barrier and is usually short lived. Label on your diagram.

• The activation- energy barrier must be crossed before reactants are converted to products.

• Remember: An endothermic reaction absorbs heat, and an exothermic reaction releases heat.

Talk with your table… • What factor determines whether a molecular collision results in a reaction? Take notes to share with the class.

Le Chatelier’s principle: Factors Affecting Reaction Rates • Le Chatelier’s principle: if a stress is applied to a system in dynamic equilibrium the system changes in a way that relieves this stress. • Factors that can affect the rate of a chemical reaction are: • temperature • concentration • particle size • use of a catalyst

Factors Affecting Reaction Rates: Temperature • Usually, raising the temperature speeds up a reaction. • Lowering the temperature usually slows down a reaction. • Talk with your partner, why do you think this is true? Take notes to share with the class. • At higher temperatures, particles move faster. • The frequency of collisions increases, therefore more particles have enough kinetic energy to slip over the activation-energy barrier. • Thus, an increase in temperature causes products to form faster.

Factors Affecting Reaction Rates: Concentration • The number of particles in a given volume affects the rate at which reactions occur. • Talk with your partner, why do you think this is true? Take notes to share with the class. • 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.

Other 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. • The result of an increase in surface area is an increase in the frequency of collisions and the reaction rate.

Other Factors Affecting Reaction Rates: Catalyst • 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. • The activation-energy barrier for the catalyzed reaction is lower than that of the uncatalyzed reaction. • On your diagram draw a reaction with a catalyst.

Question Time!!!! • 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

Question Time!!!! • What would happen to the reaction rate if additional reactants were added to the system? A. Increase B. Decrease C. No Change

Question Time!!!! • What would happen to the reaction rate if the system was cooled? A. Increase B. Decrease C. No Change

Question Time!!!! • What always results in a reaction occurring? A. Effective Collision B. Heating the system C. Adding a catalyst D. Decreasing the particle size