2.5: Equilibrium and Le Chatelier's Principle (Experiment)-Home Version

Objectives

• To observe the effect of an applied stress on chemical systems at equilibrium.

A reversible reaction is a reaction in which both the conversion of reactants to products (forward reaction) and the re-conversion of products to reactants (backward reaction) occur simultaneously:

Forward reaction:

\[\ce{A + B-> C + D}\]

\[\text{Reactants} \ce{->} \text{Products}\]

Backward reaction:

\[\ce{C + D -> A + B}\]

\[\text{Products} \ce{->} \text{Reactants}\]

Reversible reaction:

\[\ce{A + B <=> C + D}\]

Consider the case of a reversible reaction in which a concentrated mixture of only \(A\) and \(B\) is supplied. Initially the forward reaction rate (\(\ce{A + B -> C + D}\)) is fast since the reactant concentration is high. However as the reaction proceeds, the concentrations of \(A\) and \(B\) will decrease. Thus over time the forward reaction slows down. On the other hand, as the reaction proceeds, the concentrations of \(C\) and \(D\) are increasing. Thus although initially slow, the backward reaction rate (\(\ce{C + D -> A + B}\)) will speed up over time. Eventually a point will be reached where the rate of the forward reaction will be equal to the rate of the backward reaction. When this occurs, a state of chemical equilibrium is said to exist. Chemical equilibrium is a dynamic state. At equilibrium both the forward and backward reactions are still occurring, but the concentrations of \(A\), \(B\), \(C\), and \(D\) remain constant.

A reversible reaction at equilibrium can be disturbed if a stress is applied to it. Examples of stresses include increasing or decreasing chemical concentrations, or temperature changes. If such a stress is applied, the reversible reaction will undergo a shift in order to re-establish its equilibrium. This is known as Le Chatelier’s Principle.

Consider a hypothetical reversible reaction already at equilibrium: \(\ce{A + B <=> C + D}\). If, for example, the concentration of \(A\) is increased, the system would no longer be at equilibrium. The rate of the forward reaction (\(\ce{A + B -> C + D}\)) would briefly increase in order to reduce the amount of \(A\) present and would cause the system to undergo a net shift to the right. Eventually the forward reaction would slow down and the forward and backward reaction rates become equal again as the system returns to a state of equilibrium. Using similar logic, the following changes in concentration are expected to cause the following shifts:

• Increasing the concentration of \(A\) or \(B\) causes a shift to the right.
• Increasing the concentration of \(C\) or \(D\) causes a shift to the left.
• Decreasing the concentration of \(A\) or \(B\) causes a shift to the left.
• Decreasing the concentration of \(C\) or \(D\) causes a shift to the right.

In other words, if a chemical is added to a reversible reaction at equilibrium, a shift away from the added chemical occurs. When a chemical is removed from a reversible reaction at equilibrium, a shift towards the removed chemical occurs.

A change in temperature will also cause a reversible reaction at equilibrium to undergo a shift. The direction of the shift largely depends on whether the reaction is exothermic or endothermic. In exothermic reactions, heat energy is released and can thus be considered a product. In endothermic reactions, heat energy is absorbed and thus can be considered a reactant.

Exothermic:

\[\ce{A + B <=> C + D +} \text{ heat}\]

Endothermic:

\[\ce{A + B +} \text{ heat} \ce{<=> C + D}\]

As a general rule, if the temperature is increased, a shift away from the side of the equation with “heat” occurs. If the temperature is decreased, a shift towards the side of the equation with “heat” occurs.

In this lab, the effect of applying stresses to a variety of chemical systems at equilibrium will be explored. The equilibrium systems to be studied are given below:

1. Adding a common ion-Common ion effect-Dissolution equilibrium
   Adding saturated sodium bi-carbonate solution to saturated sodium chloride solution
2. Red cabbage indicator under H+ concentration change (acidity change) -Acid-Base equilibrium
3. Thermochromic substances under heat change (like in mood rings or color changing spoons)
4. Phase equilibria-Heat change

By observing the changes that occur (color changes, precipitate formation, etc.) the direction of a particular shift may be determined. Such shifts may then be explained by carefully examining the effect of the applied stress as dictated by Le Chatelier’s Principle.

Procedure

Materials and Equipment.
Sodium chloride (table salt) 5 grams dissolved in 10 ml, Sodium bicarbonate (6-7 grams dissolved in 10 ml). Red cabbage solution prepared in the PH lab, Vinegar or lime juice, water, optional-mood ring or color changing spoons or any thermo chromic substance that changes color in cold water to hot water (watch the video given below)

Safety

No safety precautions are needed except heat safety when preparing the cabbage solution by boiling cabbage pieces in water.

Experimental Procedure

Record all observations on your report form. These should include, but not be limited to, color changes and precipitates. Note that solution volumes are approximate for all reactions below.

Part 1: Saturated Sodium Chloride Solution

1. Place 3-mL of saturated \(\ce{NaCl}\) (aq) into a small test tube.
2. Carefully add concentrated clear saturated sodium bicarbonate solution \(\ce{NaHCO3}\) (aq) drop-wise to the solution in the test tube until a distinct change occurs. Record your observations.

Part 2: Acidified Cabbage pH Indicator

1. Place 3-mL of the red cabbage solution into a test tube.
2. Add an equal amount of 3 ml vinegar. Observe the color change
3. Now add drop-by-drop of sodium bicarbonate until it goes back to the original color.
4. Play around by repeating additions 2 and 3 to see the color changes. 

Part 3: Thermochromic Materials

• Watch the following video on thermochromic materials and do a little bit search on the type and nature of such reactions. Find a chemical equation as an example with corresponding color changes.

   

Part 4: Phase Changes (What?)

1. Place an ice-cube in a container on the table.
2. Once it melts, place the container back in the freezer. Take it out and observe after 4 hours
3. Identify the stress that shifts the phase equilibria.

 

Pre-laboratory Assignment: Chemical Equilibrium and Le Chatelier’s Principle

1. Consider the reversible reaction:

\[\ce{A + B <=> C + D}\]

• What happens to the forward and reverse reaction rates when equilibrium is achieved?

• What happens to the reactant (\(A\) and \(B\)) and product (\(C\) and \(D\)) concentrations when equilibrium is achieved?

2. Le Chatelier's Principle states that if a stress is applied to a reversible reaction at equilibrium, the reaction will undergo a shift in order to re-establish its equilibrium. Consider the following exothermic reversible reaction at equilibrium:

\[\ce{2A <=> B + C}\]

In which direction (left or right) would the following stresses cause the system to shift?

• decrease the concentration of A
• increase the concentration of B
• lower the temperature

3. In this lab you will explore the effect of Le Chatelier's Principle on several chemical systems at equilibrium. These are supplied in the Theory Section. Consider the third system you will study: the Aqueous Ammonia Solution.

• Write the balanced equation for this reversible reaction.
• Suppose you added some excess ammonium ions to this system at equilibrium.
  • In which direction would a shift occur?
  • What color change might you expect to observe?

4. List all the equipment you will use in this lab.

Lab Report: Chemical Equilibrium and Le Chatelier’s Principle

Part 1 - Saturated Sodium Chloride Solution

Equilibrium System:

Observations upon addition of \(\ce{NaHCO3}\):

In which direction did this stress cause the equilibrium system to shift? Left or Right

Which ion caused the shift? Explain.

Part 2 - Acidified Cabbage Indicator

Equilibrium System:

Observations upon addition of \(\ce{H+}\):

In which direction did this stress cause the equilibrium system to shift? Left or Right

Which ion caused the shift? Explain.

Observations upon addition of \(\ce{NaHCO3}\):

In which direction did this stress cause the equilibrium system to shift? Left or Right

Which ion caused the shift? Explain.

Part 3 - Thermochromic Materials

Search and find one thermo chromic reaction from literature and write it down as an equilibrium equation with the corresponding colors on each side.

What stress causes the color change?

In which direction did this stress cause the equilibrium system to shift if it is heated? Left or Right with the specific color change

Part 4 - Phase Change

Equilibrium System:

Write an equation to show the equilibrium between the ice and water equilibrium

Draw a phase diagram of water showing the fusion (melting) curve highlighted.

What stress causes the equilibrium shift.

 

Contributors and Attributions

The experiment is from Chem 10 Experiments shared under a CC BY-NC license and was authored, remixed, and/or curated by Santa Monica College. Manjusha Saraswathiamma, Minnesota State Community and Technical College, Moorhead, has modified this to fit into a homeschool environment with less hazardous and cost-effective lab supplies. Manjusha would like to acknowledge the creators of the YouTube video embedded on this page.