Homework

# Chemical Equilibrium

## Chapter 15: 1-2 Homework

Textbook assignment: Read Kotz and Triechel, Chemistry and Chemical Reactivity Chapter 15: Sections 1 and 2.

##### Study Notes
• 15.1 All chemical reactions are reversible given the right energy conditions, and even when a reaction is dominantly 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 forward and backward reaction become equal, individual molecules counting 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.

#### Key Formula

Rules for Reaction Rates
In a solution, the reaction quotient has the form

aA + aB → cC + dD

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 equilibrium 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.

#### Web Lecture

Read the following weblecture before chat: The Equilibrium Condition

#### 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

Use the Virtual Lab at the Chem Collective Cobalt Chloride and LeChatlier's Principle. If the lab does not load, try refreshing your browser window.

1. Click on the "Cobalt Lab" link in the upper right and follow the instructions in the assignment to work through the lab.
2. What happens as you change the temperature? How the the solution re-establish its equilibrium state?
3. Note that if you have Java installed, you can download the lab and run it locally.

#### Chat Preparation Activities

• Essay question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
• Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.

#### Chapter Quiz

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