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Lecture 12

Reaction Reates and Chemical Equilibrium

Shaun Williams, PhD

Reaction Rates

More Conditions that Effect Reaction Rates

Example of Reaction Rates

Photographs showing two reactions in sealed containers. After 10 seconds, one of the containers has generated more gas therefore the reaction in that flask, which has a higher concentration, has progressed faster.

Collision Theory

  • States that in order for a reaction to occur, reactant molecules must collide in the proper orientation and with sufficient energy
  • Which of the pictures on the right has the molecules in the proper orientation to collide?
When more reactants are present, there are a larger number of collision possible which have the correct orientation and energy.

Energy Diagrams

Plots of energy versus time. For endothermic reactions, the energy begins low and ends high while for an exothermic reaction, the energy begins high and ends low.

Activation Energy

Activated Complex

Energy Diagram for a Reaction

Plot of energy versus reaction progress showing the energy rising to a maximum value at the activated complex. The energy then drops to the product.

Conditions that Affect Reaction Rates

Concentration

Concentration and Reaction Rates

When more reactants are present, there are a larger number of collision possible which have the correct orientation and energy.

Temperature

Temperature and Collision Theory

A plot of fractions of collisions versus collision energy for gases at two temperatures. The the higher temperature, the curve is broader and has shifted to a higher energy.

Catalysis

  • Lower the activation energy for the reaction by forming new activated complexes with lower activation energies
  • Remain unchanged after the reaction
Adding a catalyst to a reaction lowers the energy of the activated complex so much that it forms a little valley. So the energy rises to a max, falls to the new intermediate complex, rises to a max, and then falls to the product's energy.

Enzymes

A graphic showing a substrate attaching to an enzyme, the enzyme-substrate complex, and then the products being ejected from the enzyme.

Enzyme Catalysis

A graphic showing a sucrose molecule fitting into the active site on the sucrase enzyme. The sucrose is then broken into a glucose molecule and a fructose molecule.

Reaction Intermediates

Chemical Equilibrium

Observing a Chemical Equilibrium

As the dinitrogen tetroxide equilibrium mentioned on the previous slide progresses, the gas goes from being a pale yellow (dinitrogen tetroxide) to being a brown color (nitrogen dioxide). After a certain amount of time, the container stops changing color as the reaction has reached equilibrium.

Plots showing the concentration of NO2 rising to a plateau and N2O4 falling to a constant value. Meanwhile the rate of the forward reaction and backward reaction start off very different but eventually merge at some time.

Position of Equilibrium

The Equilibrium Constant

Equilibrium Constant

Bar graphs showing the amount of reactants dropping as the reaction progresses to equilibrium while the amount of product increase.

The Meaning of the Value of \(K_{eq}\)

Value of \(K_{eq}\) Position of Equilibrium
\(K_{eq} \gg 1\) Lies to right. Reaction is product favored.
\(K_{eq} \ll 1\) Lies to left. Reaction is reactant favored.
\(K_{eq} = 1\) Lies in middle. Similar amounts of reactants and products.

Predicting the Direction of a Reaction

Predicting the Direction of a Reaction (cont.)

Heterogeneous Equilibrium

Heterogeneous Equilibrium (cont.)

Pictures of two sealed vials containing different volumes of liquid bromine. In both containers, the color and therefore amount of bromine vapor is the same.

Le Châtelier's Principle

Changes in Concentration

  • For a system at equilibrium, when the concentration of a reactant or product is increased, the equilibrium will shift to consume the added substance.
  • When the concentration of a reactant or product is reduced, the equilibrium will shift to produce more of the removed substance.
A picture showing that when more iron(III) nitrate is added to potassium thiocyanate the equilibrium shifts making a darker equilibrium color.

Effects of Changes in Concentration

General reaction: \[ \chem{A(g)+B(g) \rightleftharpoons C(g)+D(g)} \]

Add reactant Add product Remove reactant Remove product
shift right shift left shift left shift right

Changes in Volume

A image showing that when the volume of a system is reduced, the equilibrium shifts to the side of the reaction with the least number of moles of gas.

Example of Changes in Volume

Photographs showing the equilibrium between dinitrogen tetroxide (yellow gas) and nitrogen dioxide (red gas). When the gas is compressed to samller volume, it darkens and the molecules get more dense. Then the color lightens as it reaches a new equilibrium.

Equilibrium Shifts Due to Changes in Volume

Relative Number of Gaseous Molecules Increase Volume Decrease Volume
Reactants > Products shift right shift left
Reactants < Products shift left shift right
Reactants = Products no shift no shift

Changes in Temperature

Example of Changes in Temperature

\[ \chem{heat + N_2O_4(g) \rightleftharpoons 2NO_2(g)} \]

For this reaction, in boiling water the equilibrium mixture is very reddish-brown. In ice water, the equilibrium mixture is very pale yellow.

Equilibrium Shifts Due to Temperature Changes

Type of Reaction Equation Increase temperature Decrease temperature
endothermic \( \chem{heat + A + B \rightleftharpoons C+D} \) shift right, \(K_{eq}\) increases shift left, \(K_{eq}\) decreases
exothermic \( \chem{A + B \rightleftharpoons C+D+heat} \) shift left, \(K_{eq}\) decreases shift right, \(K_{eq}\) increases

Catalysts and Increasing Product Yield

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