Reaction Rates and Equilibrium

1. Reaction Rates and Equilibrium - Rate – a measure or the speed of any change that occurs within any interval of time. Reaction rate (rate of a chemical change) is usually expressed as the amount of reactant changing per unit of time. Rates of chemical reactions are related to the properties of atoms, ions, and molecules through a model called the collision theory.

2. Collision Theory – According to this theory atoms, ions, and molecules can react to form products when they collide with one another, providing the colliding particles have enough kinetic energy. • Particles lacking sufficient KE to react will bounce apart unchanged when they collide. • Activation energy – the minimum amount of energy that colliding particles need in order to react.

3. When two reactant particles with the necessary activation en collide, a new entity is formed called the activated complex. • Activated Complex – an unstable arrangement of atoms that forms momentarily at the peak of the activation-energy barrier. • An activated complex forms only if the colliding particles have sufficient en and the atoms are oriented properly.

4. The activated complex has a brief existence and ends with the re-formation of the reactants or with the formation of the products. Thus, the activated complex is sometimes called the transition state. Factors Affecting Reaction Rates - The rate of a chemical reaction depends upon temperature, concentration, particle size, and the use of a catalyst.

5. 1) Temperature – usually raising the temperature speeds up reactions, while lowering temperature slows down reactions. • Increasing the temperature increases both the frequency of collisions and the number of collisions. This in turn, increases the number of particles which have enough KE to slip over the activation-en barrier to become products. • An increase in temperature causes products to form faster.

6. 2) Concentration - When you increase the concentration of the reactants you increase the frequency of collisions. Increased collision frequency leads to a higher reaction rate. • 3) Particle size - The total surface area of a solid or liquid reactant affects the reaction rate. The smaller the particle size, the larger is the surface area for a given mass of particles. An increase in surface area increases the amount of the reactant exposed for reaction, which further increases the collision frequent and the reaction rate.

7. Ways to Increase the Surface Area Include: • Dissolve them – in a solution, the particles are separated and more accessible to other reactants. • Grind them into a fine powder. 4) Catalysts – a substance that increases the rate of a reaction without being used up during the reaction. A catalyst lowers the activation-energy barrier and allows more reactants to have enough energy to form products within a given time.

8. Enzymes are biological catalysts that increase the rate of biological reactions that occur inside your body. Without enzymes the digestion process would taken many years instead of a few hours. • Inhibitors – a substance that interfers with the action of a catalyst. • Reversible Reactions – a reaction in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously.

9. Chemical Equilibrium – When the rates of the forward and reverse reactions are equal. • At chemical equilibrium, no net change occurs in the actual amounts of the components of the system. • Equilibrium position – indicates whether the reactants or products are favored in a reversible reaction.

10. Factors Affecting Equilibrium: Le Chatelier’s Principle • When the equilibrium of a system is disturbed, the system makes adjustments to restore the equilibrium. • Le Chatelier’s Principle – If a stress is applied to a system in dynamic equilibrium, the system changes in a way that relieves the stress. • Stresses that upset equilibrium include: changes in concentration, pressure, and temperature.

11. Concentration • A + B --- C + D --- • Add A • (shifts right) • Add B • (shifts right) • Add C • (shifts left) • Add D • (shifts left) • Remove A • (shifts left) • Remove B • (shifts left) • Remove C • (shifts right) • Remove D • (shifts right)

12. Temperature • A + B --- heat + C --- • Add heat • (shift left) • Remove heat (shift to right) • A + heat --- B + C • Add heat • (shift right) • Remove heat • (shift left)

13. Pressure • A change in pressure of a system only affects gaseous equilibria that have an unequal number of moles of reactants and products. • When the pressure is increased it will shift to the side with the least number of moles of gas. • When the pressure is decreased it will shift to the side with the higher number of moles of gas.

14. N 2 (g) + 3H 2 (g) --- 2NH 3 (g) --- • An increase in pressure • (shifts to the right) • A decrease in pressure • (shifts to left)

15. Equilibrium Constants • Chemists express the position of the equilibrium as a number value. • The equilibrium constant (Keq) is the ratio of product concentration at equilibrium, with each concentration raised to a power equal to the number of moles of that substance in the balanced chemical equation.

16. aA + bB --- cC + dD--- • Keq = [C]c[D]d ------------- [A]a[B]b • The exponents in the equilibrium-constant expression are the coefficients in the balanced equation. • The brackets indicate the concentrations of the substances in moles per liter (mol/l).

17. The value of Keq depends on the temperature of the reaction. • If the temperature changes, so does the value of Keq. • The size of the equilibrium constant show whether products or reactants are favored at equilibrium. • A value of Keq > 1 (products are favored at equilibrium). • A value of Keq < 1 (reactants are favored at equilibrium. • Keq has no units!