Chemical Kinetics and Thermodynamics

Created by C. IppolitoFebruary 2007

Chemical Kinetics and Thermodynamics

Objectives:1. explain the collision theory of reactions2. describe reaction mechanism, rate-determining step,

activated complex, and activation energy 3. explain how rate of reaction is affected by the nature, the

surface area, and the concentration of reactants; by the temperature; and by the presence of a catalyst

4. interpret potential energy and energy distribution diagrams

5. interpret the significance of changes in enthalpy 6. use Hess’s law to calculate heats of reaction and heats

of formation7. determine values of changes in free energy use them to

predict spontaneous reactions


Chemical Kinetics

• concerned with the rates and mechanisms of chemical reactions– reaction rate

• moles of reactant used up over time• moles of product produced over time

– reaction mechanism• rearrangement to form products


Collision Theory

• explains different rates of reaction

• collisions may shift electron positions

• breaking old bonds and forming new ones


Effective Collision

• proper angle (orientation)

• proper amount of energy– rate of reaction depends on:

• more collisions • effectiveness of collisions


Factors Affecting Rate

1. nature of reactants

2. temperature

3. concentration of reactants

4. presence of catalysts


Nature of Reactants

• slight rearrangements occur rapidly– ionic substances in solution

• covalent rearrangements occur more slowly– bonds have to be broken and reformed


Temperature

• increase in temperature increases rate temperature kinetic energy kinetic energy collisions collisions effective collisions


Concentration of Reactants

concentration rate of reaction – Homogeneous Reaction

• all reactants are in same phase

– Heterogeneous Reaction• the reactants are in more than one phase• increase of surface area can increase rate in

heterogeneous reactions


Catalysts

• substance that speeds up a reaction without being changed– usually works by increasing effective collisions

• by “positioning”• or orientation


Activation Energy

• Activated Complex– intermediate particles

• collision at proper angle• collision has enough energy

– short lived and unstable– changes into product

• Activation Energy– minimum energy to form complex


Potential Energy Diagram

• Y-axis– potential energy

• reactants• activated complex• products

• X-axis– reaction coordinate

• progress of reaction– a.k.a time

• see p. 478 in text


Activation Energy and Temperature • lower temperature T1

– fewer molecules reach EACT

• higher temperature T2

– more molecules reach EACT

• see p. 480 in text


Activation Energy and Concentration • Higher concentration

– more particles reach EAct

• Lower concentration– less particles reach EAct

• see p. 481


Activation Energy and Catalysts

• Catalysts “tunnel” EACT

– by “positioning”

• more molecules achieve EACT

• see p. 482


Heat Content (Enthalpy)

• Thermodynamics– study of

• changes in energy • influence of temperature

• Heat Content (a.k.a Enthalpy)– represented by H– all forms of energy

• chemical bond energy• potential energy• kinetic energy


Heat of Reaction

H - heat of reaction – measured in kilojoules (kJ) or kilocalories (kcal)

see Table I in Regents Reference Tables• H = Hproducts – Hreactants

– positive H – energy absorbed - ENDOTHERMIC– negative H – energy released - EXOTHERMIC


Heat of Formation

• special H (heat of reaction)– 1 mole of a compound made from elements

• standard heat of formation symbol – temperature 25°C (298K)– pressure 1 atmosphere (101.3kPa)– measured

• kilojoules/mole• kilocalories/mole

• see table p. 485

fH


Stability of Compounds

• large negative heat of formation– CO2 Hf = -394 kJ/mol

• very stable• decomposition would require large input of energy

• small negative or positive heat of formation– NO2 Hf = +33.1 kJ/mole

• very unstable• decomposition requires little or no energy input to

decompose (explosives)


Hess’s Law of Constant Heat Summation• sum of two or more reactions

– heat of reaction that is the sum of individual heat of reactions (Use Table I)


Hess Law Calculations

• What is the heat of reaction?CuO(s) + H2(g) Cu(s) + H2O(g) H = ? kJ

Look for heat of formation reactions in data table (p.485 or Table I on Regents)

Cu(s) + ½O2(g) CuO(s) H = -155 kJ• but our reaction is the reverse of one in table

CuO(s) Cu(s) + ½O2(g) H = 155 kJ

H2(g) + ½O2(g) H2O(g) H = -242 kJ• check for “same” coefficient, if true no adjustment needed• combine reactions


Hess Law Calculations (cont’d)

CuO(s) Cu(s) + ½O2(g) H = 155 kJ

H2(g) + ½O2(g) H2O(g) H = -242 kJ_______________________________________________

CuO(s) + H2(g) H2O(g) + Cu(s) H = -87 kJ


Entropy

• measures disorder, randomness, or lack of organization– S – entropy

• Si - initial entropy (before change)• Sf –final entropy (after change)

S – change of entropyS = Sf – Si

S = a positive number» entropy increases, disorder increases, decomposition

S = a negative number» entropy decreases, disorder decreases, synthesis


Spontaneous Reactions

• occur without external cause• affected by:

H – change in enthalpy S – change in entropy

• Effect of Signs of H and S H = - (favorable)

• reaction will occur H = + (unfavorable) S = - (unfavorable)

• reaction cannot occur H = - (favorable) S = - (unfavorable)

• reaction can only occur if H > S H = + (unfavorable) S = + (favorable)

• reaction can only occur if S > H


Gibbs Free Energy Equation

G = H - TS– explains relationship of enthalpy and entropy

G – free energyH – change in enthalpy (heat)• T – temperature (in kelvin)S – change in entropy

– spontaneous reactions • G - negative


Free Energy of Formation

• This is the change in free energy when 1 mole of the compound is formed from its constituent elements.

formationofenergyfreerepresentsG f0

Chemical Kinetics and Thermodynamics

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