CORE LAB #2 – PERTURBING EQUILIBRIUM
Course: Chemistry 3202
Unit: 1 – From Kinetics to Equilibrium
Activity: Investigation 13-B (p. 521), Perturbing Equilibrium, Parts 1 & 2
Purpose: To predict and observe the effects of changing factors in a system at equilibrium using Le
Châtelier’s Principle, while fostering appreciation for the nature of inquiry in science.
Outcomes:
114-2 Explain the roles of evidence, theories, and paradigms in the development of scientific
knowledge
213-5 Compile and organize data, using appropriate formats and data treatments to facilitate
interpretation of the data
212-5 Identify the theoretical basis of an investigation and develop a prediction and a hypothesis that
are consistent with the theoretical basis
213-8 Select and use apparatus and materials safely
323-5 Explain how different factors affect solubility, using the concept of equilibrium
Materials:
Goggles
Gloves
Test tubes
Disposable pipettes
Pipette bulbs
Part 1
0.1 mol/L K2CrO4(aq)
1 mol/L HCl(aq)
1 mol/L NaOH(aq)
1 mol/L FeCl3(aq)
1 mol/L BaCl2(aq)
Part 2
50 mL beaker
Scoopula
0.01 mol/L NH3(aq)
6.0 mol/L HCl(aq)
Phenolphthalein solution
NH4Cl(s)
Inquiry is such an important aspect of
science. Students need to get curious
about what they are learning and the
world around them. This lab has been
modified from the textbook version to
encourage students to make their
own discoveries.
Instructional Sequence:
Time
Pre-lab
5 min.
15 min.
10 min.
Teacher Task
Write equilibrium equations for parts 1 and 2
on blackboard
Review Le Châtelier’s Principle
Distribute handouts
Explain/identify materials
Circulate and observe
Direct students to alternate resources as
required
Summarize
Discuss discrepancies
Discuss other potential equilibrium stressors
Student Task
Review Le Châtelier’s Principle
Listen to instructions
Determine stressors of pertinent equilibria
Complete table by adding “additions” to
“starting solution”
Complete questions
Participate in class discussion
Assessment: Students will be observed throughout the lab period for group input and safety. Students
are also required to submit the tables, “process” and questions completed during the lab period.
Misconceptions: Students often incorrectly believe that:




All reactions are irreversible
Solutions which do not show any blatant outward signs of reaction are not reacting
Systems in equilibrium have equal amounts of reactants and products
Systems at equilibrium can only be affected by adding an existing reactant or product
Each of these common misconceptions is addressed through this activity.
Diverse Learners: This activity is particularly useful for students who are more comfortable with manual
manipulations or visual representations of concepts. Le Châtelier’s Principle is often viewed as an
abstract concept and the process of predicting and manipulating the shifts of an equilibrium aid in the
understanding of this concept. By completing the lab in groups, students who prefer a cooperative
learning environment or peer assistance also benefit.
Inquiry: In the prescribed course, this activity is a typical “cookbook” lab with extensive and specific
procedural instructions. To foster inquiry, the majority of the procedure has been removed to create a
more free-flowing style and students are encouraged to delegate tasks amongst the group. The end-oflab questions have been modified by asking students to construct their own table to organize their
results, also increasing the level of inquiry.
Extension: To extend on the outcomes of this activity, students could be asked to create or research a
“real-life” representation or model of equilibrium and factors affecting it. For example, predator-prey
relationships in an ecosystem or a physical modelling of equilibrium by shifting balls between persons,
etc.
INVESTIGATION 13-B – PERTURBING EQUILIBRIUM
Directions:
 Safety First: Always wear goggles and gloves when completing a chemistry experiment.
 Complete the provided table using the provided materials:
o Predict before you observe.
o Describe each reactant thoroughly (colour, clarity, state – space for “observations” has
been provided)
o Stock solutions are provided at the front of the classroom.
 Collect only what you need
 Be especially careful when transporting chemicals to your group table
o Each group member must complete their own table
 Be sensible in your additions! Don’t use litres of solution where drops would be sufficient!
 As you work, record the process you used to determine the required information on the blank
paper provided.
o Each group member must keep their own record
o Be sure to include who completed which tasks. Do not hesitate to delegate!
 Tables and process are due at the end of the period.
 Complete the following questions on the paper provided, to be submitted with tables and
process:
o Compare your predictions to your observations. Account for any differences
o Tabulate which tests/additions either added or removed a reactant or product and how
that affected the overall concentration of reactants and products.
I find handouts make collecting
lab work easier, but they also
help with safety concerns. A
clutter of backpacks and
textbooks can be a hazard in the
lab.
PART 1 The Chromate/Dichromate Equilibrium
H+(aq) + 2 CrO42-(aq) ↔ Cr2O72-(aq) + OH-(aq)
Starting solution: K2CrO4(aq)
Observations:
Addition/Observations
Expected
Shift
Expected
Colour
Observed
Colour
Observed
Shift
Observed
Colour
Observed
Shift
HCl(aq)
NaOH(aq)
FeCl3(aq)
*HCl(aq) + BaCl2(aq)
*Have your previous observations checked before proceeding to this step.
PART 2 Changes to a Base Equilibrium System
NH3(aq) + H2O(l) ↔ NH4+(aq) + OH-(aq)
Starting solution: NH3 + 2 drops phenolphthalein
N.B. – Phenolphthalein is pink in colour in basic solutions.
Observations:
Addition/Observations
NH4Cl(s)
HCl(aq)
Expected
Shift
Expected
Colour