Untitled Flashcards Set

Week 3: Chemical kinetic and Equilibrium Reaction

RATE OF REACTION

The rate of chemical reaction is the amount of reactant used up or amount of  product formed per unit time or  the Rate of Reaction refers to the Speed of the Conversion of Reactants into Products in a Chemical Reaction or the rate of reaction is the speed at which a chemical reaction takes place

FACTORS AFFECTING RATE OF REACTION

1. Temperature

2. Concentration

3. Pressure: Increasing the pressure of  gases increases reaction rate.

4. Surface Area: The larger the surface area the faster the rate of reaction. For instance  zinc granule with react slowly with HCl while powdered zinc will react faster with HCl.

5. Light: some reactions are faster when exposed to sunlight. This is called photocatalytic reaction. Example photosynthesis production of starch by plants, chlorination of methane, decomposition of hydrogen peroxide.

6. Catalyst: The catalyst lowers the activation energy for the reaction to proceed faster without changing the average kinetic energy of the molecules.

ENERGY PROFILE FOR CATALYZED AND UNCATALYZED REACTION

Example of catalytic reactions

           production of ammonia in the Haber process,

           N_2(g) + 3H_{2 (g)}  2NH3 _(g)

           Decomposition of potassium trioxochlorate(v)

           2KClO_3(s)    2KCl_(s) + 3O_{2 (g)}

           Production of sulphur (vi) oxide

           2SO_2(g)   +  O_2(g)    2SO_{3 (g)}

           Decomposition of hydrogen perioxide

           2H₂O_2(aq)    2H₂O _(l) + O_{2 (g)}

COLLISION THEORY

Collision theory states that for a chemical reaction to occur, the reacting particles must collide with one another.  The collision must be strong enough to overcome the energy barrier or activated complex for effective collision leading to the formation of products.

The rate of the reaction depends on the frequency of collisions. The theory also tells us that reacting particles often collide without reacting. For collisions to be successful, reacting particles must (1) collide with (2) sufficient energy, and (3) with the proper orientation.

ACTIVATION ENERGY, E_A

This is the minimum amount of energy reacting species or reactants particles must possess before a chemical reaction occurs or takes place.

ACTIVATED COMPLEX

This is the highest energy state that a reaction goes through in order to change reactants to products. Activation energy is what is required to overcome this complex.

EFFECTIVE COLLISION

This is the collision that leads to the formation of a product.

EQUILIBRIUM SYSTEM

Equilibrium is the state of a system where there is no observable change in the properties of the system with respect to time.

Static equilibrium refers to a condition where the reaction occurring in a system is completely halted, and there exists no movement between the reactants and the products corresponding to the chemical reaction.

Dynamic equilibrium is a state of reversible reaction when the concentration of reactants and products becomes constant. It means that the rate of the forward reaction becomes equal to the rate of the reverse reaction at this stage.

A reversible reaction is the one that proceeds in both directions under suitable conditions.

A reversible reaction is endothermic in one direction, and exothermic in the other. The same amount of energy is transferred each time. In a closed system, a reversible reaction reaches a state of dynamic equilibrium, where the forward and back reactions take place at the same rate. So there is no overall change.

CHARACTERISTICS OF A SYSTEM IN CHEMICAL EQUILIBRIUM

1. Forward and reverse reactions have the same  rate in chemical equililbrium. i.e a chemical equilibrium is dynamic in nature because the forward and reverse reactions never cease.

2. At equilibrium, all of the reactants and products are present.

3. At equilibrium, the concentrations of the reactants and products are constant at constant temperature.

4. The free energy change,

   

   , of an equilibrium system is equal to zero.

5. A catalyst has no effect on the equilibrium position. However, it alter the time required to reach equilibrium.

6. Every chemical equilibrium system has a unique equilibrium constant, K_eq , at constant temperature.

FACTORS AFFECTING EQUILIBRIUM REACTION

Temperature: An increase in temperature will shift the equilibrium position to the right favoring products side for endothermic reaction and reactants side for exothermic reaction.  Also, decrease in temperature will shift the equilibrium position to the right favoring products side for exothermic reaction and reactants side for endothermic reaction.

Concentration: When the concentration of one or more reactant increases it favor products side and vice versa. However, a decrease in amount of reactants shift equilibrium  position to the reactants side and vice versa.

Pressure: When there is an increase in pressure, the equilibrium will shift towards the side of the reaction with fewer moles of gas. When there is a decrease in pressure, the equilibrium will shift towards the side of the reaction with more moles of gas.

Catalyst: Catalyst does not affect the position of equilibrium and hence it does not have any effect on the value of equilibrium constant. only change in temperature can change the equilibrium constant. catalyst only affects the forward and reverse reaction equally.

EQUILIBRIUM CONSTANT

                       The equilibrium constant, denoted as K, is a value that expresses the ratio of the concentrations of products to reactants at equilibrium for a reversible chemical reaction. It provides insight into the extent of the reaction and whether reactants or products are favored at equilibrium.

For a general reaction:

The equilibrium constant K is given by: K=

where [A], [B], [C], and [D] are the molar concentrations of the reactants and products at equilibrium, and a, b, c, and d are the stoichiometric coefficients from the balanced equation.

·         If K > 1, products are favored at equilibrium.

·         If K <  1, reactants are favored.

·         If K = 1, neither side is favored.

Example: Write the Kc, of the following reaction:

      1. N_2(g) + 3H_2 (g) 2NH3 _(g)

      2. CaCO_3(s)  CaO_(s) + CO_2(g)

           Solution

1.        2.

LE CHATELIER’S PRINCIPLE

The principle states that if a chemical system is in equilibrium and one of the factors involved in the equilibrium is altered, the equilibrium will shift so as to neutralize the effect of the change.

INDUSTRIAL APPLICATION OF CHEMICAL EQUILIBRIUIM

1. THE HABER PROCESS

N_2(g) + 3H_{2 (g)} 2NH_{3 (g)}

Conditions favouring the yield of NH₃ are:

- Low temperature of about 450⁰C                                 

- A high pressure of about 200atm

- Catalyst – finely divided iron; it speeds up the rate of higher yield of ammonia at about 250⁰C.

Sources of hydrogen (methane) and nitrogen (air).

2. THE CONTACT PROCESS (Production of H₂SO₄)

2SO_2(g)   +  O_2(g)    SO_{3 (g)}

The conditions required for the high yield of SO3 are:

- Low temperature of about 450 – 500⁰C

- High pressure atmospheric pressure of 1 atm gives a high yield of SO₃ (98%)

- Catalyst – vanadium (V) oxide, V₂O₅.

Sources of the sulfur dioxide (burning sulfur or roasting sulfide ores) and oxygen (air)