Chemical Equilibrium

• Equilibrium

  • the term refers to what we might call a "balance of "forces"
  • Chemical equilibrium occurs when a reaction and its reverse reaction proceed at the same rate.
  • A Chemical reaction is in equilibrium when there is no tendency for the quantities of reactants and products to change.
  • The direction in which we write a chemical reaction is arbitrary.
• The Concept of Equilibrium

  • As a system approaches equilibrium, both the forward and reverse reactions are occurring.
  • At equilibrium, the forward and reverse reactions are proceeding at the same rate.
• A System at Equilibrium 

  Once equilibrium is achieved, the amount of each reactant and product remains constant. 
• Depicting Equilibrium

  In a system at equilibrium, both the forward and reverse reactions are being carried out; as a result, we write its equation with a double arrow. 
• Chemical Equilibrium

  • is the state in which the chemical activities ro concentrations of the reactants and products have no net change over time.
  • Usually, this would be the state that results when the forward chemical process proceeds at the same rate as their reverse reaction.
  • The reaction rates of the forward and reverse reactions are generally not zero but, being equal, there are no net changes in any of the reactant or product concentrations.
  • This process is called dynamic equilibrium.
• Law of Mass Action

  • stem from the research by Cato Guldberg and Peter Waage (1864-1879)
  • 2 aspects of the Law of Mass Action
  • the equilibrium aspect, concerning the composition of a reaction mixture at equilibrium
  • the kinetic aspect concerning the rate equations for elementary reactions
  • Chemical equilibrium is a dynamic process in which rates of reaction for the forward and backward reactions must be equal.
  • gives an expression for the equilibrium constant, quantity characterizing chemical equilibrium.
• • Gas-phase reactions that use units of partial pressure, Kp
  • Dissociation of water: dissociation constant of water, Kw
  • Dissociation of acids: acid dissociation constant, Ka
  • Reaction of bases with water. base dissociation constant, Kb
  • Solubility of precipitates: solubility product Ksp
• What Does the Value of KMean?

  • If K >> 1 the reaction is product-favored; product predominates at equilibrium.
  • If K << 1, the reaction is reactant-favored; reactant predominates at equilibrium.
• Reaction Quotient (Q)

  • is a quantitative measure of the extent of reaction, the relative proportion of products and reactants present in the reaction mixture at some instant of time.
  • To calculate Q, one substitutes the initial concentrations of reactants and expression equilibrium.
  • Q gives the same ratio the equilibrium expression gives, but for a system that is not at equilibrium
• • If Q = K, the system is at equilibrium.
  • If Q > K, there is too much product and the equilibrium shifts to the left.
  • If Q < K, there is too much reactant, and the equilibrium shifts to the right.
• Le Châtelier’s Principle - if a system at equilibrium is disturbed by a change in temperature, pressure, or the concentration of one of the components, the system will shift its equilibrium position so as to counteract the effect of the disturbance.
• Effect of Concentration

  • When the concentration of a product is increased, the reaction proceeds in reverse to decrease the concentration of the products.
  • When the concentration of a reactant is increased, the reaction proceeds forward to decrease the concentration of reactants. 
• Effect of Pressure

  • In reactions where gasses are produced, an increase in pressure will force the reaction to move to the left (in reverse). 
  • If pressure is decreased, the reaction will proceed forward or increase pressure. of reactants.
• Effect of Temperature

  • The effect of a change of temperature on a reaction will depend on whether the reaction is exothermic or endothermic.
  • When the temperature increases, Le Chatelier's principle says the reaction will proceed in such a way as to counteract this change, i.e. lower the temperature.
  • Therefore, endothermic reactions will move forward, and exothermic reactions will move backwards (thus becoming endothermic.
• Effect of Catalyst on Equilibrium

  • A catalyst does not affect either Kc or the position  of equilibrium, it only affects the rate of reaction. As the rate of forward reaction and reverse reaction is affected equally then the equilibrium cannot be affected. 
• Catalysts increase the rate of both the forward and reverse reactions. 
• Equilibrium is achieved faster, but the equilibrium composition remains unaltered. 
• Acids and Bases

  Properties of Acids and Bases

  • Acids: pH <7, sour, tum litmus red, clear in phenolphthalein indicator.
  • Bases: pH > 7, bitter, tum litmus blue, pink in phenolphthalein, slippery.
• Arrhenius

  • Acid : Substance that, when dissolved in water, increases the concentration of hydrogen ions.
  • Base : Substance that, when dissolved in water, Increases the concentration of hydroxide ions.
• Brensted-Lowry

  • Acid: Proton donor; must have a removable (acidic) proton
  • Base: Proton acceptor; must have a pair of nonbonding electrons
• What happens when an Acid dissolves in water?

  • Water acts as a Bnensted-Lowry base and abstracts a proton (H+) from the acid.
  • As a result , the conjugate base of the acid and hydronium ions are formed.
• Conjugate Acids and Bases:

  • From the Latin word conjugare, meaning “to join together”
  • Reactions between acids and bases always yield their conjugate bases and acids.
• Autoionization of Water

  • As we have seen, water is amphoteric.
  • In pure water, a few molecules act as bases and few act as acids.
  • This is referred to as autoionization. 
• What is "pH"?

  • The term "pH' refers to the measurement of the strength of an acid or base.
  • "pH" comes to us from "portenz of hydrogen" portenz means strength and the hydrogen's - chemical symbol is H. (p + H= pH)'
  • pH is measured using the pH scale which runs numerically from 0 to 14.
  • 0 - 7 is the region of acids (acidic solutions)
  • 7 is perfectly neutral (as pure water would be) 
  • 7-14 is the region of bases (alkaline solutions)
• The pH Scale

  • The pH scale ranges from 0-14. A pH of 7 is perfectly neutral like pure water.
  • As you go away from seven and count down toward zero - you are getting more and more acidic.
  • As you count up from seven toward fourteen - you are getting more and more basic.
• • pH measures the amount of hydrogen ions released by the acid when placed in a solution.
  • When an acid is placed in a solution, it releases the hydrogen (H+) ions and these ions attach themselves to the water molecules (H2O) to make the hydronium ion (H2O+). The more hydronium ions that are made - the more Ho ions that must have been released - the stronger the acid.
• • The pH measures the strength of an acid but it does not do the same for a base. 
  • A base is a hydrogen ion acceptor so when you put it in water - it doesn’t form the hydronium ion - it forms the hydroxide ion (OH-) because it takes an H+ from the water. 
• Measuring pH

  • The Hydrangea macrophylla blossoms in pink or blue, depending on soil pH. In acidic soils, the flowers are blue; in alkaline soils, the flowers are pink.
• How Do We Measure pH?

  • For less accurate measurements, one can use
  • Litmus paper
  • "Red" paper turns blue above 
  • (pH = 8)
  • "Blue" paper tums red below 
  • (pH = 5)
  • An indicator
• Indicators

  • An indicator is used to measure the pH of a substance.
  • Is a chemical detector for hydronium ions (H3O+) or Hydrogen ions (H+)
  • The indicator causes the color of the solution to change depending on the pH.
  • Common indicators are congo red, phenolphthalein, methyl orange, phenol red, bromothymol blue, bromocresol green and bromocresol purple.
• pH Meter - electronic instrument used to measure the pH. A typical pH meter consists of a special measuring probe (a glass electrode) connected to an electronic meter that measures and displays the pH reading
• Indicator Paper - pH measurement with indicator paper
• Strong Acid

  • Dissociates more or less completely when it dissolves in water, in other words, it is a very good H+ donor.
  • Reactions involving the dissociation of strong acids can be written with a regular forward to arrow, because the equilibrium lies very far to the right, i.e. favoring the products (the dissociated ions).
• Weak Acid

  • Dissociates only slightly when it dissolves in water. In other words, it is a poor donor.
  • Reactions involving the dissociation of weak acids are written with a forward/backward arrow to indicate that the products are not heavily favored.
• Acid Dissociation Constant Ka

  • The acid-dissociation equilibrium constant is the mathematical product of the equilibrium concentrations of the products of this reaction divided by the equilibrium concentration of the original acid, In other words, K, is the product of the concentrations of the products over the concentration of the reactants (not including water): 
  • Water is not included in the acid-dissociation equilibrium expression because the [HO] has no effect on the equilibrium.
• Acid Dissociation Constant

  • As the Ka value of an acid increases, so does the strength of the acid. By definition:
  • strong acid: Ka >1
  • weak acid: Ka <1
• Acid Dissociation Constant

  • When a strong acid reacts with water, water acts as a strong base. The strong acid and the strong base react to form a weaker acid and a weaker base.
  • When a weak acid reacts with water, water acts as a weak base. The reaction attempts to convert the weak acid and the weak base into a stronger acid and a stronger base, but the reaction cannot naturally proceed in this direction.
  • This is why strong acids dissociate nearly completely whereas weak acids dissociate only slightly.
• Strong Bases

  • Strong bases are the soluble hydroxides, which are the alkali metal and heavier alkaline earth metal hydroxides (Ca2+, Sr2+, and Ba2+).
  • Again, these substances dissociate completely in aqueous solution.
• Weak Bases

  • Bases react with water to produce hydroxide ions.
• Base Dissociation Constant Kb

  • Base equilibrium problems can be solved much like acid equilibrium problems
  • Base-ionization equilibrium constant (Kb) - A measure of the relative strength of a base. For the generic base dissociation reaction with water,