We're going to be honest: we do Physics. That's why this is called The Physics Classroom website. And when we do the Teacher's Notes section for our Concept Builders, we typically have a lot to say ... and a lot of resources to point you to. We're not claiming to be ignorant of chemistry; we just don't have a lot of chemistry resources here at The Physics Classroom to point you to. And so this page is going to be a lot shorter than our usual page that accompanies our Physics Concept Builders. That's our honest confession.

Equilibrium constant expressions relate the equilibrium concentrations of reactants and products to a temperature-dependent, reaction-dependent constant. The value of the constant is independent of the amount of reactants and products in the initial mix. The reversible system will always achieve an equilibrium position that is consistent with the equilibrium constant expression. The task of determining the reactant and product concentrations at equilibrium from the initial concentrations is typically facilitated by the use of an ICE Table. This Concept Builder provides practice and some guidance in the task of using an ICE Table to determine equilibrium concentrations from initial concentrations. Once determined, the equilibrium constant value is calculated.

The Concept Builder includes a total of 48 different questions. These questions are organized into 12 Question Groups and spread across three difficulty levels. The difficulty levels are differentiated as follows:

• Apprentice Difficulty Level: Question Groups 1-4 ... Initial concentrations for all reactants and products are given. The equilibrium concentration is given the reactant. The reaction involves coefficients of 1 such that the equilibrium expression does not have any powers other than 1.
• Master Difficulty Level: Question Groups 5-8 ... Initial concentrations for all reactants and products are given. The equilibrium concentration is given for one of the products (the one that has a coefficient of 1). The reaction involves coefficients other than 1 such that the equilibrium expression has powers other than 1.
• Wizard Difficulty Level: Question Groups 9-12 ... The container volume and the initial number of moles for all reactants and producs are given. The # of moles is given for one of the products (for which the coefficient is not 1). The reaction involves coefficients other than 1 such that the equilibrium expression has powers other than 1.

Each Question Group in this Concept Builder contains four nearly identical questions. The specific question a student receives is selected at random. The ordering of questions is also randomized and the order in which questions are delivered to students is also randomized. The result is every students' experience is different enough that two side-by-side students will have to do more than copy answers to be successful. 

We recommend that students use at least three significant digits in their answers for K. The answer checking algorithm makes sure that the student-inputted answer is within 1% of the keyed answer (known to several digits). Using three significant digits on answers may be a deparature from the rules of significant digits but it avoids errors owed to rounding. 

Teachers using the Concept Builder with their classes should preview the activity (or view the Questions on a separate page) in order to judge which difficulty levels would be most appropriate for their students.

 

The most valuable (and most overlooked) aspect of this Concept Builder is the Help Me! feature. Each question group is accompanied by a Help page that discusses the specifics of the question. This Help feature transforms the activity from a question-answering activity into a concept-building activity. The student who takes the time to use the Help pages can be transformed from a guesser to a learner and from an unsure student to a confident student. The "meat and potatoes" of the Help pages are in the sections titled "How to Think About This Situation:" Students need to be encouraged by teachers to use the Help Me! button and to read this section of the page. A student that takes time to reflect upon how they are answering the question and how an expert would think about the situation can transform their naivete into expertise.