Structure and Reactivity Lecture-3 10th August 2015

Kinetics and Thermodynamics of Simple Chemical Processes The simplest chemical reaction may be described as equilibration of two distinct species A reaction can be said to be under thermodynamic control if it produces more stable products than the reactants. A reactions can be said to be under kinetic control if the products obtained is the one that forms faster.

Equilibrium is governed by the thermodynamics of a chemical reaction In all reversible chemical reactions, reactants convert to products to various degrees. When the concentration of reactant and product no longer change then the reaction is said to have achieved equilibrium. Equilibrium is described by Equilibrium constant K The equilibrium constant can be directly related to Gibbs standard free energy change ΔG°

R is a gas constant, its value is 8. 314 JK-1 mol-1, 1 R is a gas constant, its value is 8.314 JK-1 mol-1, 1.986 cal K-1 mol-1

The Free Energy Change ΔG° is related to changes in Bond Strength and Energy Dispersal in the system ΔH° is the enthalpy change of a reaction at constant pressure, enthalpy changes in a reaction mainly related to difference in the strengths of bond in products compared to starting materials. DH° is Bond Dissociation Energy

Exothermic and Endothermic Processes If the bonds formed are stronger than bonds broken then the reaction will be exothermic (release of energy) If the bonds broken is stronger than bonds formed then the reaction will be endothermic (absorption of energy)

Entropy Change ΔS° ΔS° is described as changes in energy dispersal Increase in disorder correlates to increase in S°. The value of S° increases with increasing dispersal of energy content among the constituents of a system. A positive value of ΔS° makes negative contribution to ΔG°. In other words, going from less energy dispersal to more energy dispersal is thermodynamically favorable In many reactions entropy change is small, therefore ΔH° is just sufficient to predict the possibility of a reaction

Activity

Activity

Rate of a chemical reaction depends on the activation energy Thermodynamic factors can only predict the feasibility of a reaction and does not tell us anything about the rate of a reaction. For example combustion of methane is an exothermic process, but this reaction is very slow at normal temperature, methane does not undergo spontaneous ignition. We need to trigger the reaction by a spark or a flame.

Why this highly favorable process so slow by nature

Collisions supply the energy to get past the activation energy At room temperature molecules have only 0.6 kcal/mol this energy is far less than the activation energy Boltzmann Distribution Curve At higher temperature the average kinetic energy of molecule increases

Concentration of the reactions can affect reaction rates A reaction in which rate of the reaction depends on the concentration of two substrates is called as second order reaction.

Activity

Activity

Arrhenius equation describes how rate of a reaction is affected by Temperature Kinetic energy of a molecule increases when they are heated, which means a larger fraction of them have sufficient energy to overcome activation energy barrier Ea. Normally an increase of 10°C leads to a rate acceleration of 2 to 3 times Swedish chemist Arrhenius noticed the reaction rate k dependence of Temperature T R = Gas Constant A = Reaction Constant

Activity If A = 1014 and Ea = 58.4 kcal mol-1. Using Arrhenius equation, calculate k at 500 °C for this reaction. R = 1.986 cal K-1 mol-1 (Caution: R is in cal K-1 mol-1 )

Acids and bases; Electrophiles and Nucleophiles; using curved “Electron-Pushing” Arrows Bonds consists of electrons. Chemical change is defined as a process in which bonds are broken and/or formed. Therefore , when Chemistry takes place electrons move. Electrons being negatively charged, are attracted to centers of positive charge or centers electron deficiency. A curved arrow will show the flow of electron from a point of origin There are four types of reaction:

Activity Complete the following reaction and Identify the category of the following reactions Hint: Write all the Lewis structure by adding the missing lone pairs

Acid and Base Strengths are measured by equilibrium constants Bronsted and Lowry definition of acids and bases: Acid is a proton donor and base is a proton acceptor.

Acid and Base Strengths are measured by equilibrium constants Bronsted and Lowry definition of acids and bases: Acid is a proton donor and base is a proton acceptor.

Acid and Base Strengths are measured by equilibrium constants The acidity of a general acid HA is expressed by following general equation together with associated equilibrium constant In dilute solutions [H2O] is a constant 55 mol/L pKa = - log Ka

Relative Acidities of Common compounds at 25°C

Conjugate base of a strong acid is a weak base Conjugate base of a weak acid is a strong base

Activity

Activity

Activity

We can estimate relative acid and base strength from its molecular structure 1. The increase of electronegativity of A increases acid strength 2. The increase in size of A increases acid strength

We can estimate relative acid and base strength from its molecular structure 3. The resonance in A− that allows delocalization of charge over several atoms

The resonance effect is more pronounced in Sulfuric Acid

Activity

Activity

Lewis Acids and Bases A more generalized description of acid-base interaction in terms of electron sharing was introduced by Lewis Lewis Acid is a species that contains an atom that is at least two electron short of closed outer shell Lewis Base is a species that has at least one lone pair of electron in its outer shell

A Lewis Base shares its lone pair with a Lewis acid to form a new covalent bond.

Electrophiles and nucleophiles also interact through movement of an electron pair Many Processes in Organic Chemistry exhibit characteristic of acid-base reactions Electrophile means electron friendly: Example: CH3Br (carbon center is electron deficient) Nucleophile means nucleus friendly: Example: HO− (Oxygen is electron rich)

Activity: Show the curved arrows for the above reaction

Assignments will be posted shortly

Thank You