Acids and Bases and Equilibrium Oh my!

H+ OH- <> H+ OH-

The Acids, Bases, and Equilibrium, Oh, my! formative assessment (FA) asks students to model the effect of a strong acid on the dissociation equilibrium of water. This FA targets Chemical Mechanism by examining how students interpret LeChatelier’s principle as it relates to pH. The central idea of this formative assessment is that the dissociation of water is based on equilibrium and looks at how adding more H+ ions or more OH- ions would affect that equilibrium. The questions in the assessment are predominantly eliciting in that they ask students to narrow in on one aspect of acids and bases reacting with water. There is plenty of opportunity during the formative assessment for advancing questions to get students to explain the correct thinking.

In this FA task, students use cards to represent, H+, OH-, and undissociated water (H+ and OH- stapled together) to guide their activity. Each group of students receives the PDF icon Acids, Bases and Equilibrium, Oh My! worksheet and a plastic bag with 10 H+ cards, 10 OH- cards, and 10 H2O cards each consisting of an H+ and an OH- PDF icon Cards stapled together using the card template. Their task is to imagine that the cards represent a beaker of water to which they are going to add more acid (H+ ions). The goal is for students to answer the question: How would the system respond to try to lower the concentration of H+?


Students lay out the cards on a desktop as depicted above and imagine that they have a beaker of pure water represented as the HOH cards, a flask of acid, H+ cards, and a flask of base, OH- cards. The teacher will have prepared for this activity by printing and cutting out the cards using two pages of cards per group, and stapling or taping together ten H2O molecules. Ten H+ cards, ten OH- cards, and ten H2O molecules make up a set per group of students. It helps to keep each set in a plastic bag or envelope. The exercise took about one 47 minute period.

This FA task was tested with 11th grade Chemistry students at one of Boston’s exam schools. The curriculum is college preparatory. The school enrolls about 1800 students in grades 7 to 8 from diverse backgrounds and socio-economic levels. In the class in which it was tested, the students had recently started a chapter on acids and bases, beginning with the properties of acids and bases and the Arrhenius definitions. Students were then presented with the equation for the dissociation of water, trying to convey the minute amount of water that dissociates at any one time. Students were given the equation:

H2O ⇆ [H+] + [OH-]

and the equilibrium equation:

Kw = 1 x 10-14 = [H+][OH]

Students were then shown how to calculate pH. The attached formative assessment was implemented at this point in order to help students grasp what was actually going on in this equilibrium.


Teaching reflections

There is a range of responses from the student work. I think having a way to physically manipulate the equilibrium expression worked well when the students followed the directions. Even if they did not follow completely, most of them got the idea that there was a reaction involved that was more than just the addition of an acid or base.

What didn’t work as well as I hoped was the connection to the concept of equilibrium. I was hoping that students would see that there was a reaction going on all the time between the water molecules, hydrogen ions, and hydroxide ions and the added ions, and that wasn’t the case for all students. Some students just saw that adding hydrogen ions increased the hydrogen ion concentration.

Some students got hung up on laying the cards out on the table. What I recommend and will do in the future is give explicit instructions on laying the cards out and project the picture of laid out cards as a template (see above).  I don’t know how to link it correctly myself. I think that would move the lesson along so the students can spend more time thinking about the reaction.

Prior to this lesson, I emphasized the very low concentrations of hydrogen and hydroxide ions in pure water; how small a concentration of 1 x 10-7 M actually is. That is a difficult concept for some students, especially if they have trouble with scientific notation. Another thing to try is using more water molecules and less H+ or OH- cards per group to further emphasize this point.

Overall, I feel that this lesson helped most of my students understand that an equilibrium reaction is the basis of the acid and base chemistry they learn in the high school curriculum. I think having a way to physically manipulate the equilibrium expression worked well when the students followed the directions. Even if they did not follow completely, they got the idea that there was a reaction involved that was more than just the addition of an acid or base.


Examples of student work 

Student 1

Student 11

Student 12

This student and the group he was in did not set up the cards correctly and were confused as to what they were supposed to do. They were just adding hydrogen ions to the solution and observing that that meant the solution was getting more acidic. They did not understand the involvement of the dissociation of water in the process. I had to give them more explicit directions about setting up the cards. I then walked them through the first step of the process. Student 1 could then see that a reaction was going on because he stated he had “leftover hydrogen that won’t bond to OH-“.


Student 2

Student 21

Student 22

This student had the cards set up and knew that the reaction formed water, but did not understand that there was a balance of hydrogen and hydroxide ions. She answers question 3 stating that the amount of hydrogen ions remains the same and the amount of hydroxide ions increases as you add acid to water, so it appears that she did not make a connection to the activity and her own previous answers about the number of hydrogen ions increasing compared to hydroxide ions. Student 2 started to write the correct answer, but crossed it out. I think that she may have switched the answers to questions 3 (about adding acid) and 4 (about adding base), but I can’t be sure.


Student 3

Student 31

Student 32

This student didn’t answer all the questions on the number of cards left after each round. However, the student did conclude that adding acid increased [H+] higher than the [OH-] concentration. The minimal answers provided makes me think that this student did not understand that adding acid or base disrupted the equilibrium of the solution, but only understood that acid added hydrogen ions and base added hydroxide ions.