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Reaction rate

• Change in concentration of a reactant or product over time

• Amount of reactant used up or product form / time

Measuring rate of reaction

• Measuring a decrease in mass

  • For reactions that produce gases

  • As gas releases the mass decreases

• Measuring volume of gas released

  • Gas syringe

• Timing how long a precipitate takes to form

  • When the product of a reaction is a precipitate

  • Measure how long it takes for a mark to become obscured by the precipitate

Calculating rates from graphs

• Time vs amount of reactant used means gradient represents reaction rate

Collision theory

• States that two conditions must be met for reaction

  • Orientation

    • Particles must collide in the correct orientation

  • Energy

    • Colliding particles need at least the minimum amount of KE (E_a)

  • Means that a lot of collisions do not cause reaction

5 Factors that affect rate of reaction

• Surface area of solids

  • More particles on the surface are available to collide and reaction

  • Higher frequency of effective collisions

• Concentration of solutions

  • Particles will on average be closer

  • Particles will collide more frequently so more effective collisions

• Pressure of gases

  • Particles will pushed closer together

  • Increases the number of effective collisions

• Temperature

  • Increases the KE and speed of particles

  • Move faster and collide more frequently

  • More particles will have the necessary energy to overcome E_a

• Catalysts

  • Provides alternate pathway

  • Lower activation energy means particles require less KE to react

  • Greater proportion of particles will have E_a

  • More effective collisions

Maxwell-Boltzmann distribution curve

• Some molecules move slowly because they have low KE

• Others move very fast due to high KE

• Majority have moderate KE and speeds

• Only molecules with KE greater than or equal to E_a have enough energy to react when they collide

Features of a Maxwell-Boltzmann distribution

• Key features

  • Curve originates at the origin - no molecules have zero KE

  • Rises sharply to a peak that represents the most common KE

  • Total area under the curve is equal to the total number of molecules

  • Peak of the curve shows the most probable KE

  • On average, KE of all molecules is a bit higher than peak energy

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Increasing temperature

• As the temperature rises, a greater number of molecules attain the KE needed to surpass the Ea

• Curve shifts to the right

  • More molecules have sufficient KE

• Despite the number of molecules being constant

  • Area under the curves is unchanged

    

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Catalysts

• Substance that accelerates rate of chemical reaction without being permanently altered

• Achieve this by providing an alternate pathway

• Key features

  • Highly efficient

  • Usually very specific

  • Participate in the reaction

Heterogeneous catalysts

• Is in a different phase from the reactants

• Often involves a solid catalyst with gaseous or liquid reaction

• Occurs on the catalysts surface

• e.g. Haber process

Homogeneous catalysts

• In the same phase as the reactants

• Involves an aqueous catalyst in a solution of aqueous reactants

• Reaction occurs throughout the mixture

• e.g. sulfuric acid in H₂O₂ and K

Heterogeneous catalysis

• Heterogeneous catalysis

  • Absorption

    • Reactant molecules attach to the catalyst surface

  • Activation

    • Bonds in the reactants weaken and break forming reactive species

  • Reaction

    • New bonds form between the reactive species

  • Desorption

    • Product molecules detach from the catalyst surface

  • Mechanism

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Homogeneous catalysis

• Reactants combine with the catalysts to form an intermediate species

• Intermediate reacting to form products and regenerate the catalyst

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Catalysts and M-B distributions

• Effects of catalysts can also be understood using Maxwell-Boltzmann distribution curves

• By lowering the Ea catalysts increase the proportion of molecules with sufficient Ea

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Economic benefits of catalysts

• Lower production costs

  • Allow reactions to occur at lower temperatures

  • Lower energy consumption

• Increased efficiency

  • Faster reaction rates mean more product can be produced in a shorter amount of time

• Improved product quality

  • Can influence the properties of final product

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