Chemical Equilibrium Reference: Chapter 9 Reactions Rates and Equilibrium

Introduction  Two factors that are important in many areas of Chemistry, in particular Industrial Chemistry are:  how quickly a reaction proceeds, the rate  how much product is formed, the yield.

Introduction  Reaction rates deal with the ‘speed’ of a chemical reaction.  Equilibrium reactions are chemical reactions that do not go to completion, and that produce final reaction mixtures with certain quantities of both reactants and products present.

How does a chemical reaction occur?  All chemical reactions involve the rearrangement of atoms that are already present. For such an arrangement to occur, existing (or old) bonds need to be broken and ‘new’ bonds allowed to form.

How does a chemical reaction occur?  This means that there is always an energy requirement before a chemical reaction can take place; this corresponds to breaking the ‘old’ bonds. This energy required to break these bonds is called the activation energy and comes from either or both:  thermal energy (heating)  the kinetic energy of colliding particles.

Enthalpy  The total energy stored in a substance in called the enthalpy, or heat content.  It is given the symbol H.  The change in enthalpy can be measured when a chemical reaction occurs.  It is give the symbol ΔH

Energy Profile Diagrams  Exothermic Reactions

Energy Profile Diagrams  In an exothermic reaction:  The energy of the products is less than the energy of the reactants.  The activation energy is less than the energy released when new bonds are formed.  There is therefore a net release of energy to the surroundings.

Energy Profile Diagrams  Endothermic Reactions

Energy Profile Diagrams  In an endothermic reaction:  The energy of the products is greater than the energy of the reactants.  The activation energy is greater than the energy released when new bonds are formed.  There is therefore a net absorption of energy from the surroundings.

Collision Theory  A chemical reaction involves particles that are moving around and sometimes colliding with each other. The greater the number of successful collisions there are the faster the rate.  Not all collisions will result in a reaction; the particles may simply bounce off each other.

Collision Theory  In summary, in order for a reaction to take place the reactants must:  collide  have the correct orientation for bond breaking to occur  have sufficient energy for the reaction to occur. The reactant particles must contain enough energy to overcome the activation energy barrier, allowing the particles present to come apart and then to rearrange themselves into the products.

Measuring Reactions Rates  The following methods can be used.  the volume of a gas evolved  the mass of a solid formed  the decrease in mass due to gas evolving  the intensity of colour of a solution  the formation of a precipitate  the change in pH  the change in temperature

Factors that effect the rate of a reaction - Concentration  Increasing the concentration of the reactants will cause an increase in the reaction rate.  More reactants means more collisions and hence an increase in reaction rate  Refer to Table 9.1 pg for an example.

Factors that effect the rate of a reaction - Temperature  Most chemical reactions proceed quicker as the temperature increases.  An increase in temperature will cause the particles to move quicker and hence have more collisions.  It will also give particles more energy to overcome the activation energy.

Factors that effect the rate of a reaction - Catalysts  Catalysts increase the rate of a chemical reaction without themselves being consumed.  Sometimes they can be used to slow down a reaction. In this case they are called negative catalysts or inhibitors.

Energy Profile Diagram for a Catalyst.  The catalyst has caused a decrease in the activation energy and hence the reaction rate will increase.

Catalytic Converters  Catalytic Converters are used in motor vehicles to reduce the amount of harmful gases leaving the exhaust.  They work by providing a surface, usually composed of Rhodium or Platinum, by which the gases are forced to pass over slowly.

The Result of a Catalytic Converters

Factors that effect the rate of a reaction – Surface Area  This is an important factor in heterogeneous reactions — reactions where the reactants are in different phases, such as a solid and a liquid.  Its effect is due simply to the fact that, by increasing the surface area, more of a substance is brought into contact with other substances with which it might react.  Wood will burn much faster if it is cut into smaller pieces as more of its surface will be in contact with oxygen.

Questions  Complete Q1 pg 201, Q2 pg 204

Chemical Equilibrium  Most of the reactions you have studied in the past have ‘gone to completion.’  This means that all of the reactants have been converted into products or either one reactant has been in excess.  In either case, you have been able to use stoichiometry to calculate the amount of product made.

Chemical Equilibrium  Whilst many reactions go to completion, some do not.  These reactions are called equilibrium reactions.

Reaction Type 1 The Decomposition of Hydrogen Bromide

Reaction Type 2 The Decomposition of Hydrogen Iodide

Chemical Equilibrium  As can be seen in Reaction Type 1, all of the reactant is eventually consumed.  However, in Reaction Type 2, some of the reactant is not consumed and remains present in the mixture.  REACTION TYPE 2 IS AN EXAMPLE OF AN EQUILIBRIUM REACTION.

The Equilibrium Law  Consider the following table that contains data for the reaction between hydrogen gas and iodine gas.

The Equilibrium Law  Consider the following table that contains data for the reaction between hydrogen gas and nitrogen gas.

The Equilibrium Law  For a general equation,  Then the equilibrium constant K, is defined as

The Equilibrium Law  K is known as the equilibrium constant as its value is defined as occurring when a system is at equilibrium.  Another value Q, can be obtained by calculating the value at any other time other than at equilibrium.

The Dynamic Nature of Equilibrium  When a system is at equilibrium, it does not mean that no reactions are occurring.  It simply means that the forward and reverse reactions are occurring at the same time.

Questions  Complete Q3 -5 pg 210

Making Changes to Equilibrium Mixtures  An important consequence of the equilibrium law is that it is possible for every equilibrium mixture to be different.

Le Chatelier’s Principle  Le Chatelier’s Principle states that: ‘Any change that affects the position of an equilibrium will cause that equilibrium to shift, if possible, in such a way as to partially oppose the effect of that change.’

Adding or Removing a substance involved in the reaction.  If more of a reactant is added, the system will adjust by trying to use up more of that reactant.  This means the forward reaction will temporarily be faster than the reverse reaction.  The equilibrium is said to “shift to the right.”

Adding or Removing a substance involved in the reaction.  If more of a product is added, the system will adjust by trying to produce more reactants.  This means the reverse reaction will temporarily be faster than the forward reaction.  The equilibrium is said to “shift to the left.”

Changing Volume  If the volume is decreased, then the total concentration of the substances will increase.  To counter this, the system will react in the direction that will produce the smallest number of particles.  If the volume is increased, then the total concentration of the substance will decrease.  The counter this, the system will react in the direction that will produce the largest number of particles

Changing Volume  If the number of moles of gas particles on either side of the equation is equal, then changing the volume will have no effect on the position of the equilibrium.

Changing Temperature  This is the only method that will alter the value of the equilibrium constant.  If a reaction is exothermic, an increase in temperature will decrease the equilibrium constant.  If a reaction is endothermic, an increase in temperature increases the equilibrium constant.

Questions  Complete Q6 - 8 pg 210

The Yield of a Chemical Reaction  The yield of a chemical reaction is how much of a substance you actually obtained divided by what you should have theoretically obtained represented as a percentage.

Questions  Complete Q pg 217