Kinetics of Chemical Reactions
Learn about the basis of chemical reaction kinetics, including rate equations, single and multiple reactions, elementary and nonelementary reactions, and the representation of reaction mechanisms. Discover how temperature, composition, and energy influence reaction rates, and delve into stoichiometry, rate constants, and collision theory.
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Lecture 1.1 Basis of Kinetics The Rate Equation Suppose a single-phase reaction The most useful measure of reaction rate for reactant Ais The rates of reaction of all molecules are relatedby Temperature Experience shows that the rate of reaction is influenced by the composition and energy of the material. 2
Lecture 1.1 Basis of Kinetics The Rate Equation Suppose a single-phase reaction The most useful measure of reaction rate for reactant Ais The rates of reaction of all molecules are relatedby Temperature Experience shows that the rate of reaction is influenced by the composition and energy of the material. 3
Lecture 1.1 Basis of Kinetics Single and MultipleReactions When a single stoichiometric equation and single rate equation are chosen to represent the progress of the reaction, we have a single reaction. When more than one stoichiometric equation is chosen to represent the observed changes, then more than one kinetic expression is needed to follow the changing composition of all the reaction components, and we have multiple reactions. Seriesreactions, more complicated, Parallelreactions, 4
Lecture 1.1 Basis of Kinetics Elementary and NonelementaryReactions aA+ bB cC+dD The rate-controlling mechanisminvolves the collision or interaction of a A molecules with b B molecules The number of collisions is proportional to the concentration of reactants in the mixture (T constant) The number of collisions of molecules A with B isproportional to the rate of reaction Such reactions are called elementary reactions. Otherwise, the ones are called nonelementary reactions. Mass interactionlaw -rA = kCA CB a b For an elementaryreaction: 5
Lecture 1.1 Basis of Kinetics Representation of an ElementaryReaction any measure equiva- lent to concentration the order unchanged, but k different However, I AMBIGUITY: correct -r expression? k1 refers to ? II 1)write the stoichiometric equation followed by the complete rate expression. 2)give the units of the rate constant 6
Lecture 1.1 Basis of Kinetics Representation of a NonelementaryReaction Stoichiometry: Rate: Develop a multistep reaction model to explain the kinetics Br2 2Br Br + H2 HBr + H H + Br2 HBr + Br Br and H unobserved intermediates Suggested by chemistry of the materials Determined by experiments 7
Lecture 1.1 Basis of Kinetics Molecularity and Order ofReaction Must be integer The molecularity of an elementary reaction (must be an elementary reaction) is the number of molecules taking part in the reaction. This has been found to have the values of one, two, or occasionally three. bth order with respect to B nth order overall ath order with respect toA k: rate constant, (time)-1(concentration)1-n For non-elementary reaction: a, b, . . . , d are not necessarily related tothe stoichiometric coefficients. We call the powers to which the concentrations are raised the order of the Reaction. A fractionalvalue is allowable 8
Lecture 1.1 Basis of Kinetics Temperature Dependency from Arrhenius'Law Activationenergy J/mol Arrhenius Law Frequencyfactor Sameconcentration Actually, Mask pre- exponentialterm sensitive Collision andtransition state theories 9
Lecture 1.1 Basis of Kinetics Activation Energy and TemperatureDependency 1, Reactions with high activation energies are very temperature- sensitive. 2, Reactions are much more temperature-sensitive in low temperature range than in a hightemperature range. Fig 1.1 Sketch showing temperature dependency of the reactionrate 10
