Introduction to Equilibria

Equilibrium Constant

Key Words:

dynamic equilibrium

equilibrium

equilibrium constant

Keq

Le Châtelier's principle

strong bases

triple point

Multiple Choice

Is this equation balanced?

• Yes

• No

Multiple Choice

Is this equation balanced?

• Yes

• No

Multiple Choice

In the equation shown, what are the product(s)?

• N₂ + H₂

• NH₃

Multiple Choice

In the equation shown, what are the reactant(s)?

• N₂ + H₂

• NH₃

Multiple Choice

How many Hydrogen (H) atoms are in 4NH₃?

• 4

• 1

• 3

• 12

Multiple Choice

What does the number 4 represent in 4NH₃?

• Subscript

• Superscript

• Coefficient

Multiple Choice

How many Nitrogen (N) atoms are in 4NH₃?

• 4

• 1

• 3

• 12

Multiple Choice

What does the number ₃ represent in 4NH₃?

• Subscript

• Superscript

• Coefficient

Multiple Choice

What does the number ₃ represent in 4NH₃?

• Subscript

• Superscript

• Coefficient

Multiple Choice

Where do you find the reactants in a chemical equation?

• Left hand side

• Right hand side

Open Ended

Magnesium + Oxygen = Magnesium Oxide

Name a reactant in the reaction above.

Open Ended

Magnesium + Oxygen = Magnesium Oxide

Name a product in the reaction above.

Lesson 1 Part 1: equilibrium

I can write equilibrium equations

Reversible reactions go back and forth between reactants (left hand side) to products (right hand side).

Equilibrium

A system is in equilibrium when the amount of each part of the system isn't changing.

Dynamic Equilibrium

When a system is in dynamic equilibrium, changes still happen but the system remains at equilibrium.

Open Ended

Where on a phase diagram do you think equilibrium exists? Explain your thinking.

Phases and Equilibrium

The boiling and melting point lines on a phase diagram indicate conditions of a dynamic equilibrium.

Multiple Choice

Can all three phases of water be in dynamic equilibrium together?

• Yes

• No

The Triple Point

Solid, liquid, and gas are all in equilibrium at the triple point.

It's often hard to get those conditions, however — for example, carbon's triple point is around 10 atm and 4600 K! That's almost as hot as the surface of the sun.

Dynamic Equilibrium versus Completion

Reactions either reach dynamic equilibrium or go to completion.

one of the chemical reactions reaches dynamic equilibrium. In the other reaction, all reactants are turned into products; in other words, the reaction goes to completion.

Reversible and Irreversible Reactions

The arrow or arrows in a chemical reaction indicate whether or not a reaction is reversible.

Multiple Choice

Molarity is measured in _____.

• moles per g.

• mols per L.

• moles per mm.

• moles per mL.

The Equilibrium Constant

The equilibrium constant is the ratio of products to reactants.

The brackets ([ ]) mean concentration in units of molarity. So the equilibrium constant is the concentrations of the products divided by the concentrations of the reactants.

Practice writing Keq's

https://www.khanacademy.org/science/chemistry/chemical-equilibrium/equilibrium-constant/e/writing-equilibrium-constant-expressions-exercise

Value of Keq

The value of Keq tells you whether products or reactants are favored at equilibrium.

The value of K_eq tells you whether products or reactants are favored when the reaction is at equilibrium.

See if you can figure out which match together.

Graphing Equilibrium Reactions

Graphing Equilibrium Reactions

In an equilibrium reaction, both reactants and products are present when the reaction is complete.

Multiple Choice

What best describes what happens at the beginning of the reaction?

• The concentration of products decreases.

• The concentration of reactants decreases.

Multiple Choice

What best describes what happens about halfway through the time shown on this graph?

• The level of products continually increases.

• The levels of products and reactants become constant.

Analyzing Reaction Graphs

Equilibrium is reached when the graph for concentration versus time is flat.

Upsetting Equilibrium in a Reaction

Adding more reactants or products to a reaction at equilibrium will upset the equilibrium.

If a chemical reaction is at equilibrium and you add more reactants or products, the system will no longer be at equilibrium. 

Le Chatelier's Principle

Le Châtelier's principle states that when a system in equilibrium is upset, it will adjust itself back to an equilibrium state.

You do something to a reaction, it will try to do the opposite to fight the change!

Le Châtelier's Principle

• In your experiment with nitrogen (N₂), hydrogen (H₂), and ammonia (NH₃), the system began in equilibrium. But when you increased the amount of nitrogen (N₂) or hydrogen (H₂), the system's equilibrium was disrupted. Soon, some of these extra reactants formed more ammonia (NH₃), which moved the system back into equilibrium.

  
  

• If a reaction at equilibrium is upset, the reaction will respond in a way that reverses the change so that equilibrium is restored.

Applying Le Châtelier's Principle

If a reaction at dynamic equilibrium is disturbed by the addition of more products or more reactants, the reaction will adjust so that the previous ratio of products to reactants remains constant.

Open Ended

What other things can you do to upset the dynamic equilibrium? Consider what we learned about with the ideal gas law from last unit.

Le Chatelier's Principle

If dynamic equilibrium is disturbed by changing the conditions, then the position of the equilbrium moves to counteract the change.

Conditions that can be changed: TEMPERATURE, PRESSURE

 AND CONCENTRATION.

Temperature

• If a reaction is exothermic (releases heat), an increase in the temperature will force the equilibrium to the left, causing the system to absorb heat and thus partially of setting the rise in temperature.

• The opposite effect occurs for endothermic reactions, which are shifted to the right by rising temperature

Analyzing Data

Imagine that you're working for a company that's trying to make as much ammonia (NH₃) as possible, using this reaction:

N₂(g) + 3H₂(g)  2NH₃(g)

You experiment to determine Keq at different temperatures in order to figure out what temperature will enable the company to produce the most ammonia. Below are your results. Which temperature results in the largest value of K_eq? 

Pressure

When pressure is increased on a reaction that involves gases and is at equilibrium, the reaction will shift in the direction that contains fewer moles of gas.

As pressure is increased, the reaction "shifts to the right" because there are fewer moles of gas on the right side of the equation. If pressure on the system is decreased, the equilibrium will "shift to the left," making more reactants.

Complete Practice over todays lesson

turn into the bin

Lesson 1 Part 2: Acid Base Equilibrium

What do we already know about Acids and Bases

Acids

The chemical formulas for many acids have something in common.

Look closely at the formulas for these acids. What do they all have in common?

Acids increase the H+ concentration in a solution.

What makes acids sour? It's actually the presence of H⁺ ions.

The simplest definition of an acid is as a substance that increases the number of H⁺ ions in an aqueous solution when it dissolves in the solution.

Bases

The chemical formulas for many bases have something in common.

Acids are one kind of solution. Another important kind of solution is a base. Bases include bleach and the liquids used to clean drains. Bases are usually slippery and can be dangerous.

You have learned that acids contain hydrogen atoms. Bases also have something in common.

Bases increase the OH– concentration of a solution.

All of the bases on the previous page have OH^– in them. These ions are called hydroxide ions.

When a base dissolves, it increases the number of OH^– ions in the solution; however, not all bases actually contain OH^– ions.

Try this on your whiteboard

Acid Dissociation

Dissociation is what occurs when a compound splits into ions.

When an acid dissolves in water, it dissociates into ions.

Remember that an acid is defined as a substance that increases the amount of H⁺ ions in solution.

Base Dissociation

Bases containing OH– also dissociate into ions in solution.

Remember that bases increase the amount of hydroxide, OH^–, ions in a solution.

Bases Without OH– Ions

You may be wondering how a base that does not contain an OH^– ion can increase the OH^– ion concentration in a solution.

To see how this is possible, consider this example of the overall reaction of ammonia, NH₃, with water, H₂O:

NH₃ + H₂O <--> NH₄⁺ + OH^–

Electricity

Think back to our previous unit and our discussion about water conducting electricity. Was all water able to do this?

Electrical Conductivity is based around Ions!

Acids and Bases are able to conduct electricity because they dissociate into Ions.

The more they are able to dissociate (break apart into ions) the stronger they are considered.

Strong Acids

Strong acids dissociate completely.

Hydrochloric acid (HCl) conducts much more electricity than does acetic acid, citric acid, or water. This means hydrochloric acid dissociates into more ions than do these other acids.

The strength of an acid is based on how much it dissociates. The more completely it dissociates, the stronger it is.

Weak Acids

Weak acids only partially dissociate.

Besides hydrochloric acid, each of the solutions in your experiment was a weak acid.

Weak acids are acids that do not dissociate completely. Acetic acid, citric acid, and pure water are all weak acids. Other weak acids include hydrofluoric acid (HF) and hydrocyanic acid (HCN).

Strong Base

A strong base dissociates completely into ions.

Examples of strong bases are potassium hydroxide (KOH) and sodium hydroxide (NaOH).

KOH <--> K⁺ + OH^–

Bases with OH^– ions in them are usually strong bases.

Weak Base

A weak base does not dissociate completely into ions.

Examples of weak bases are ammonia (NH₃) and calcium carbonate (CaCO₃). Weak bases react with water to release OH^– ions in solution.

NH₃ + H₂O  <--> NH₄⁺ + OH^–

Concentration and Danger

The strength of an acid or base does not tell you whether it is dangerous.

Just because an acid is strong does not necessarily mean it is dangerous. Nor should a weak acid be considered safe. The level of danger of an acid or base depends on many things. One factor is how concentrated the solution is.

Water

Water is both a weak acid and a weak base.

It may have surprised you that water conducts electricity, but water does so because it dissociates into ions:

H₂O <--> H⁺ + OH^–

Because water releases both H⁺ and OH^– ions, it can function as both a weak acid or a weak base.

Acid Dissociation and Equilibrium

Dissociation of a weak acid is an equilibrium reaction.

For an acid, the equilibrium constant is written as Ka instead of Keq. K_a is the acid dissociation constant. 

Acid Strength and Ka

Calculating Ka tells you the strength of an acid.

The value of Ka can tell you whether an acid is strong or weak.

Try this Ka Equation on your whiteboard

HCN _(aq) <--> H⁺_(aq) + CN^-_(aq)

Base Dissociation Constant

Kb is the base dissociation constant. For strong bases, K_b is very large. For weak bases, K_b is very small.

Remember that weak bases react with water in order to increase the concentration of OH^–. But because all acids and bases are in aqueous solution (in water, H₂O), water is not written in the equilibrium equation expression

Mini-Lab: pH testing Lab