High School Chemistry: Reaction Rates and Equilibrium - Chemical Equilibrium - Forward and Reverse Reactions Occur at Same Rate, Dynamic Balance

Chemical Equilibrium: The Balance of Reactions

1. What This Is (In Plain English)

Chemical equilibrium is when two or more chemical reactions happen at the same rate, like a seesaw balancing perfectly. This means that the amounts of reactants and products don't change over time.

Why does it matter in real life? Without chemical equilibrium, we wouldn't have things like:

• Air to breathe, because the oxygen and nitrogen molecules wouldn't balance out.
• Water to drink, because the hydrogen and oxygen atoms wouldn't mix in the right proportions.
• Even the taste of your favorite foods, because the chemical reactions that happen during cooking would be out of balance!

2. Key Ideas & Definitions

• Chemical Equilibrium: A state where the rates of forward and reverse reactions are equal, so the concentrations of reactants and products stay the same.
  • Definition: Imagine a seesaw balancing perfectly, with no net movement.
  • Example: A seesaw with two kids, one on each side, where the weight of each kid is equal.
• Forward Reaction: A chemical reaction that produces products from reactants.
  • Definition: Think of it like a factory producing goods from raw materials.
  • Example: A lemonade stand making lemonade from lemons, sugar, and water.
• Reverse Reaction: A chemical reaction that converts products back into reactants.
  • Definition: Imagine a recycling center turning old plastic bottles back into raw materials.
  • Example: A compost bin breaking down food scraps back into soil.
• Equilibrium Constant (K): A number that represents the ratio of product concentrations to reactant concentrations at equilibrium.
  • Definition: Think of it like a ratio of how many kids are on each side of the seesaw.
  • Example: If there are 5 kids on the left side and 3 kids on the right side, the equilibrium constant would be 5:3 or 1.67.
• Le Chatelier's Principle: When a system at equilibrium is disturbed, the equilibrium will shift to counteract the change.
  • Definition: Imagine a seesaw that gets pushed to one side, and then the other kids move to balance it out.
  • Example: If you add more reactants to a system at equilibrium, the equilibrium will shift to consume some of the added reactants.

3. How To Do It (Step-by-Step)

Step 1: Write the Equilibrium Expression

To write the equilibrium expression, you need to know the chemical equation and the equilibrium constant (K). The equilibrium expression is written as:

K = [products] / [reactants]

For example, let's say we have the chemical equation:

N2(g) + 3H2(g)-2NH3(g)

The equilibrium expression would be:

K = [NH3] / [N2] [H2]^3

Step 2: Calculate the Equilibrium Constant (K)

To calculate the equilibrium constant (K), you need to know the concentrations of the reactants and products at equilibrium. You can use the equilibrium expression to calculate K.

For example, let's say we have the following concentrations at equilibrium:

[NH3] = 0.5 M [N2] = 0.2 M [H2] = 0.1 M

We can plug these values into the equilibrium expression to calculate K:

K = [NH3] / [N2] [H2]^3 = 0.5 / (0.2 x 0.1^3) = 0.5 / (0.2 x 0.001) = 0.5 / 0.0002 = 2500

Step 3: Use Le Chatelier's Principle to Predict Shifts in Equilibrium

To use Le Chatelier's principle, you need to know the initial equilibrium and the change that is made to the system. You can then predict how the equilibrium will shift to counteract the change.

For example, let's say we have the following equilibrium:

N2(g) + 3H2(g)-2NH3(g)

The equilibrium constant (K) is 2500. If we add more N2 to the system, the equilibrium will shift to consume some of the added N2. We can use Le Chatelier's principle to predict the new equilibrium concentrations.

Step 4: Write the New Equilibrium Expression

To write the new equilibrium expression, we need to know the new concentrations of the reactants and products. We can use the equilibrium expression to write the new equilibrium expression.

For example, let's say we add 0.1 M N2 to the system. The new equilibrium expression would be:

K = [NH3] / [N2] [H2]^3 = [NH3] / (0.2 + 0.1) [H2]^3 = [NH3] / 0.3 [H2]^3

Step 5: Calculate the New Equilibrium Constant (K)

To calculate the new equilibrium constant (K), we need to know the new concentrations of the reactants and products. We can use the new equilibrium expression to calculate K.

For example, let's say we have the following concentrations at the new equilibrium:

[NH3] = 0.6 M [N2] = 0.3 M [H2] = 0.1 M

We can plug these values into the new equilibrium expression to calculate K:

K = [NH3] / [N2] [H2]^3 = 0.6 / (0.3 x 0.1^3) = 0.6 / (0.3 x 0.001) = 0.6 / 0.0003 = 2000

4. Watch Out! (Common Mistakes)

Mistake 1: Not Considering the Equilibrium Constant (K)

• Fix: Always consider the equilibrium constant (K) when writing the equilibrium expression and calculating the equilibrium constant.
• Analogy: Think of K as a seesaw that helps you balance the concentrations of reactants and products.

Mistake 2: Not Using Le Chatelier's Principle

• Fix: Always use Le Chatelier's principle to predict shifts in equilibrium when the system is disturbed.
• Analogy: Think of Le Chatelier's principle as a referee who helps you balance the seesaw.

Mistake 3: Not Writing the Correct Equilibrium Expression

• Fix: Always write the equilibrium expression with the correct concentrations of reactants and products.
• Analogy: Think of the equilibrium expression as a recipe that helps you mix the right ingredients.

5. Practice Problems

Problem 1

Write the equilibrium expression for the following chemical equation:

CaCO3(s)-CaO(s) + CO2(g)

Solution

K = [CO2] / [CaCO3]

Problem 2

Calculate the equilibrium constant (K) for the following chemical equation:

N2(g) + 3H2(g)-2NH3(g)

[NH3] = 0.5 M [N2] = 0.2 M [H2] = 0.1 M

Solution

K = [NH3] / [N2] [H2]^3 = 0.5 / (0.2 x 0.1^3) = 0.5 / (0.2 x 0.001) = 0.5 / 0.0002 = 2500

6. Cram Sheet

• Chemical equilibrium is when two or more chemical reactions happen at the same rate.
• The equilibrium constant (K) represents the ratio of product concentrations to reactant concentrations at equilibrium.
• Le Chatelier's principle states that when a system at equilibrium is disturbed, the equilibrium will shift to counteract the change.
• The equilibrium constant (K) is a ratio, not a concentration.
• The equilibrium expression is written with the correct concentrations of reactants and products.
• Le Chatelier's principle helps you predict shifts in equilibrium when the system is disturbed.

7. Where to Learn More

• Amoeba Sisters: A fun YouTube channel that explains chemistry concepts in a simple and engaging way.
• PhET simulations: Interactive simulations that help you explore chemistry concepts in a virtual lab.
• Chemistry LibreTexts: A free online textbook that provides detailed explanations and examples of chemistry concepts.