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Chapter 17 reaction rates

Chapter 17 reaction rates. Chemical kinetics. The study of reaction rates (speed) Enthalpy & Entropy Only tell us if a reaction will occur but not how long it will take. Kinetics Measures the time required for a reaction to occur. 2H 2(g) + O 2(g)  2H 2 O + Energy.

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Chapter 17 reaction rates

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  1. Chapter 17reaction rates

  2. Chemical kinetics • The study of reaction rates (speed) • Enthalpy & Entropy • Only tell us if a reaction will occur but not how long it will take. • Kinetics • Measures the time required for a reaction to occur.

  3. 2H2(g) + O2(g) 2H2O + Energy H2(g)+ O2(g)may stay together for lifetime without reacting to form water. Just because something has the potential to react doesn’t mean it will do so immediately.

  4. REACTION RATE IS DEFINED AS: Speed at which reactant is used up. Speed at which product forms. Fast: Oxidation: Paper burning Slow: Oxidation: Nails rusting Paper turning yellow

  5. Fast: Slow: Slower:

  6. Reaction rates • Change in concentration of a reactant or product per unit of time (mol/L·s) • Brackets around formulas denote molar concentrations---[NO2] is read “concentration of NO2 in mol/L • Average rate = Δconcentration Δt

  7. Calculation of rate Experimental data for H2 + Cl2→ 2HCl Time(s) [H2] [Cl2] [HCl] 0.00 0.030 0.050 0.000 4.00 0.020 0.040 0.020 Calculate the reaction rate for H2 Rate = Δconcentration Δ t

  8. Potential Energy diagrams A temporary state where bonds are reforming. Show the DE during a reaction. Activated Complex Activation energyEact Energy -H

  9. POTENTIAL ENERGY DIAGRAMSshow relationship between potential energy and reaction path

  10. Activation energy---minimum energy needed to reach activated complex • Lower the energy the faster the reaction

  11. Collision theory • States that atoms, molecules, ions must collide in order to react • Must collide with: a. correct orientation b. enough energy to form activated complex or transition state

  12. A. Correct orientation

  13. B. Enough energy to break bonds and form activated complex activated complex

  14. A reaction won’t happen if: Insufficient energy to break bonds. N2 O2 N2 O2 Molecules are not alignedcorrectly.

  15. Activation Energy • The activation energyEact • Is the minimum energy needed for a reaction to take place upon proper collision of reactants. activated complex

  16. Factors that affect reaction rates • Nature of the reactants • Concentration • Surface area • Temperature • Catalyst

  17. 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 bases • dissolving substances makes reactions go faster

  18. http://www.seilnacht.com/film/english.html • http://www.open2.net/sciencetechnologynature/worldaroundus/akalimetals.html

  19. Concentration reactant concentration increases then # of collisions increases so reaction rate increases More Reactants:

  20. 8 blocks: 34 surfaces 8 blocks: 24 surfaces Surface area more surface area → more collisions →faster reaction Powdered sugar reacts faster than cube sugar

  21. Grain Elevator Accidents http://www.angelo.edu/faculty/kboudrea/demos/lycopodium/lycopodium.htm http://www.osha.gov/as/opa/foia/hot_6.html

  22. Temperature Higher Temperature: Faster molecules  More collisions More Energy  More collisions Reacting molecules move faster, providing colliding molecules with greater activation energy

  23. Uncatalysed reaction Catalyst Adding a Catalyst Lower Eact faster reaction

  24. Uncatalysed reaction Catalysed reaction Lower activation energy Lower Eact faster reaction

  25. Catalysts do not change the product Catalysts are not used up Enzymes are biological catalysts

  26. Catalytic Converter

  27. Catalytic Converter

  28. Gases • Why 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?

  29. Reaction mechanisms • A complex reaction consists of 2 or more elementary steps (Hess’s Law) • Complete sequence is referred to as a reaction mechanism • Intermediates are products in one step that are consumed in another • The slowest step in the reaction mechanism determines the rate • A catalyst goes through the reaction unchanged

  30. Rate law • equation that expresses the mathematical relationship between the rate of reaction and the concentration of the reactants • Expressed as moles of product made per unit of time • Rate = 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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