2046chapter14

2046chapter14 - Chapter 14 Chemical Kinetics Learning goals...

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1 Chapter 14 Chemical Kinetics Learning goals and key skills: h Understand the factors that affect the rate of chemical reactions h Determine the rate of reaction given time and concentration h Relate the rate of formation of products and the rate of disappearance of reactants given the balanced chemical equation for the reaction. h Understand the form and meaning of a rate law including the ideas of reaction order and rate constant. h Determine the rate law and rate constant for a reaction from a series of experiments given the measured rates for various concentrations of reactants. h Use the integrated form of a rate law to determine the concentration of a reactant at a given time. h Explain how the activation energy affects a rate and be able to use the Arrhenius Equation. h Predict a rate law for a reaction having multistep mechanism given the individual steps in the mechanism. h Explain how a catalyst works. C (diamond) C (graphite) Δ G ° rxn = -2.84 kJ spontaneous! C (graphite) + O 2 (g) CO 2 (g) Δ G ° rxn = -394.4 kJ spontaneous! • A reaction may be thermodynamically favored but not kinetically favored. • Kinetics studies the rates of a chemical process. • The study of kinetics gives insights into the reaction mechanism (i.e., how a reaction occurs). Chemical Kinetics
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2 Reaction Rates Rates of reactions can be determined by monitoring the change in concentration of either reactants or products as a function of time. •reaction rate: number of atoms or molecules that react in a given time •for a reaction to proceed, –contact is necessary between reactants –contact must lead to breaking of bonds (need sufficient energy) –the reactants must have proper orientation Collision model theory Measured from the change in concentration of reactants or products per unit time. Reaction rates are affected by • concentration of the reactants • physical state of the reactants, i.e. , surface area/particle size • temperature • catalysts (and inhibitors) Reaction rates
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3 Reaction Rates • Note that the average rate decreases as the reaction proceeds. • This is because as the reaction goes forward, there are fewer collisions between reactant molecules. C 4 H 9 Cl ( aq ) + H 2 O ( l ) C 4 H 9 OH ( aq ) + HCl ( aq ) Reaction Rates • A plot of concentration versus time for this reaction yields a curve like this. • The slope of a line tangent to the curve at any point is the instantaneous rate at that time. C 4 H 9 Cl ( aq ) + H 2 O ( l ) C 4 H 9 OH ( aq ) + HCl ( aq ) Reaction rates rate = change in conc. / change in time aA + bB cC + dD The rate can be related to the concentration of the reactants or products. Reactants will have a negative sign. Don’t forget to use the stoichiometric coefficients.
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4 Example Ammonia reacts with oxygen to form nitrogen
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2046chapter14 - Chapter 14 Chemical Kinetics Learning goals...

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