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13 - Chemical Kinetics Rates of Reactions

# 13 - Chemical Kinetics Rates of Reactions - Chapter 13...

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Chapter 13: Chemical Kinetics: Rates of Reactions Chemical kinetics : the study of the speeds of reactions and the nanoscale pathways or rearrangements by which atoms and molecules are transformed from reactants to products Knowing kinetics allows us to control many kinds of reactions (i.e., combustion) 13.1Reaction Rate For a reaction to occur, reactant molecules must come together so that atoms can be exchanged or rearranged; atoms can move more in gas phase or solution Homogeneous reaction –one in which reactants and products are all in the same phase (gas or solution) For homogeneous reaction, four factors affect speed of reaction: o Properties of reactants and products – molecular structure & bonding o Concentrations of reactants and sometimes products o Temperature at which reaction occurs o Presence of a catalyst and if present, its concentration Catalyst speed up reactions but undergo no net chemical change itself Heterogeneous reaction – take place at a surface that is interface between two different; increasing surface at which reaction occurs increases reaction rate; i.e., spray powder into air and combust vs. pile of powder Rate is the speed of any process and is the change in some measurable quantity per unit of time Reaction rate is the change in concentration of a reactant or product per unit time (independent of volume of reaction mixture) Reaction Rates and Stoichiometry o aA + bB cC + dD Rate = - Δ[A]/a Δt = - Δ[B]/b Δt = Δ[C]/c Δt = Δ[D]/d Δt o Rate of change in concentration of any of the reactants or products is multiplied by the reciprocal of the stoichiometric coefficient to find the rate of reaction o If the stoichiometric coefficients of the balanced equation are changed, the definition of reaction rate also changes Average Rate and Instantaneous Rate o

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