Chapter 17reaction rates

Chemical kineticsThe study of reaction rates (speed)

Enthalpy & Entropy Only tell us if a reaction will occur but not how long it will take.

KineticsMeasures the time required for a reaction to occur.

2H2(g) + O2(g) 2H2O + EnergyJust because something has the potential to react doesnt mean it will do so immediately.H2(g) + O2(g) may stay together for lifetime without reacting to form water.

Speed at which reactant is used up.Speed at which product forms.Oxidation: Paper burning Paper turning yellowOxidation: Nails rusting

Fast: Slow:REACTION RATE IS DEFINED AS:

Fast:Slow:Slower:

Reaction ratesChange in concentration of a reactant or product per unit of time (mol/Ls)Brackets around formulas denote molar concentrations---[NO2] is read concentration of NO2 in mol/LAverage rate = concentration t

Calculation of rate Experimental data for H2 + Cl2 2HClTime(s) [H2] [Cl2] [HCl]0.00 0.030 0.050 0.0004.00 0.020 0.040 0.020

Calculate the reaction rate for H2

Rate = concentration t

Show the DE duringa reaction.Potential Energy diagrams-HActivation energy EactEnergyA temporary state where bonds arereforming.ActivatedComplex

POTENTIAL ENERGY DIAGRAMSshow relationship between potential energy and reaction path

Activation energy---minimum energy needed to reach activated complex

Lower the energy the faster the reaction

Collision theoryStates that atoms, molecules, ions must collide in order to reactMust collide with: a. correct orientation b. enough energy to form activated complex or transition state

A. Correct orientation

B. Enough energy to break bonds and form activated complex

activatedcomplex

A reaction wont happen if: Molecules are not aligned correctly.Insufficient energy to break bonds. N2 O2 N2 O2

Activation EnergyThe activation energy EactIs the minimum energy needed for a reaction to take place upon proper collision of reactants. activated complex

Factors that affect reaction ratesNature of the reactantsConcentrationSurface areaTemperatureCatalyst

Nature of the reactants

reactant structure(polar vs. nonpolar)physical state of reactants ---ionic compounds dissolve faster than covalent compounds in water (salt dissolves faster than sugar)more active elements will react more vigorously. Cs is more reactive than Na so it will react faster and more vigorously than Na when placed in water.stronger acids and bases will react faster than weaker acids and basesdissolving substances makes reactions go faster

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reactant concentrationincreasesthen# of collisions increasessoreaction rate increases

Concentration More Reactants:

Surface areamore surface area more collisions faster reactionPowdered sugar reacts faster than cube sugar

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TemperatureHigher Temperature:Faster molecules More collisionsMore Energy More collisionsReacting molecules move faster, providing colliding molecules with greater activation energy

Catalyst Adding a CatalystLower Eact faster reaction

Lower Eact faster reaction

Catalysts do not change the product

Catalysts are not used up

Enzymes are biological catalysts

Catalytic Converter

Catalytic Converter

GasesWhy does rate increase as pressure increases for gases?

Which would have a higher rate of reaction, a gas at 1.0 atm or 0.5 atm?

Reaction mechanismsA complex reaction consists of 2 or more elementary steps (Hesss Law)Complete sequence is referred to as a reaction mechanismIntermediates are products in one step that are consumed in anotherThe slowest step in the reaction mechanism determines the rateA catalyst goes through the reaction unchanged

Rate lawequation that expresses the mathematical relationship between the rate of reaction and the concentration of the reactantsExpressed as moles of product made per unit of timeRate = k[A]a[B]b ---[ ] refer to concentrations in moles/liter ---k is a proportionality constant and is found by substitution --- a and b are exponents that tell us how much an increase in concentration affects the rate

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