WebLecture: The Equilibrium Condition
Kotz and Triechel, Chemistry and Chemical Reactivity Chapter 15: Sections 1-2.
- 15.1 All chemical reactions are reversible given the right energy conditions, and even when a reaction is dominately running "forward", there is always some activity in each direction: reactants convert to products, but some products convert to reactants. When the rate of the forard and backward reaction become equal, individual molecules conting to react, but the overall concentration stops changing. The reaction has reached an equilibrium state.
- 15.2 We can determine the reaction quotient at any moment for a reaction as a ratio of some function of concentration of products to some function of the concentration of reactants when the reaction is at equilibrium. The equilibrium constant is the value of the reaction quotient when the reaction reaches equilibrium.
- In a solution, the reaction quotient has the form
aA + aB → cC + dD
Q = [C]c[D]d / [A]a[B]b
Note that the stoichiometric coefficient becomes the exponent on the concentration of the associated molecule, and that the concentration of water, when it is a reactant or product, is usually ignored.
- If a solid is involved, the concentration of the solid is [S] = 1. It can effectively be ignored: the amount of a solid does not affect the reaction quotient or equilibrium constant.
- If the reaction is between gases, then the concentrations of the gas can also be presented by the partial pressures of the gas:
A (g) + B(g) ↔ 2 C(g)
Kp = PC2 / (PA*PB)
The value of the equilibirum constant can tell us whether the reaction is product-favored or reactant-favored at equilibrium.
- If K > 1, the reaction is product-favored: the product concentrations will be greater than the reactant concentrations at equilibrium.
- If K < 1, the reaction is reactant-favored: the reactant concentrations will be greater than the product concentrations at equilibrium.
The value of the reaction quotient at any time, compared to the equilibrium constant (which is just the reaction quotient at equilibrium) can tell us which way the reaction is running.
- If Q < K: the reaction must run forward to decrease the amount of reactants and increase the amount of products so that Q → K.
- If Q > K: the reaction must run backwards to decrease the amount of products and increase the amount of reactants so that Q → K.
- If Q = K: the reaction is in equilibrium.
Videos for Chapter 15: Principles of Chemical Reactivity: Equilibria
Review the Videos at Thinkwell Video Lessons.
- Under "Chemical Equilibrium"
- Principles of Chemical Equilibrium
- The Concept of Equilibrium
- The Law of Mass Action and Types of Equilibrium
- Converting Between Kc and Kp
- Using Equilibrium Constants
- Approaching Chemical Equilibrium
- Predicting the Direction of a Reaction
Homework problems: See your Moodle assignment!
AP #11 GUIDED INQUIRY — Titration Methods — Phase III
Repeat your experiment if necessary to confirm your results, then submit your formal report.
- AP2009 11 Determination of appropriate indicators for various acid-base titrations pH determination
- APGIE Investigation 8 Redox Titrations: How Can We Determine the Actual Percentage of H2O2 in a Drugstore Bottle of Hydrogen Peroxide?
- HSCKM V-1 Determine the Effect of Concentration on pH and the pH Range of Indicators
- HCSKM V-2 Determine the Molarity of Vinegar by Titration
- IGHCE Lab 11.1 Concentration effects on pH
- IGHCE Lab 11.2 pH in salt solutions
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