Students who struggle to balance chemical equations usually make the same mistakes when learning this concept. Through action research, I have outlined four common difficulties faced by these students.
Go Straight to the mistakes:
An unstable foundation: The necessary pre-requisite knowledge
Balancing equations is a difficult topic to teach. Many students find this concept challenging but this is not a topic that can be glossed over. Just like chemical bonding, it is an integral topic for student success in chemistry as it sets the foundation for other content such as chemical reactions, the mole concept and organic chemistry.
In order for students to balance chemical equations well, they need to :
understand chemical bonding and structure
understand chemical formulae of compounds and
know how to write chemical formulae accurately.
For years, I tried many different strategies for balancing equations with my struggling students, from using physical modeling, to the algebraic method to the inspection method (also known as the ping pong method). However, without the necessary pre-requisite knowledge, their struggles persisted.
Finding the root of the problem
While conducting research on misconceptions in chemistry, I administered a series of worksheets to a group of high school chemistry students (ages 14-16) who had just learned to balance chemical equations, in an attempt to assess the nature of their struggles.
During the first phase, students were given word equations and asked to write the corresponding chemical equation, then for the second phase, they were asked to balance the equation they had written in phase 1 (no state symbols were required at this point).
For every incorrectly balanced equation, I assigned codes in order to categorize their errors.
Once I had these categorized, I conducted some more research, by interviewing teachers and consulting the literature. I was able to categorize these mistakes (or difficulties rather) into four groups.
Mistake#1: Students do not understand chemical formulae
The chemical formula of a chemical substance tells us three main things:
the different elements present in a compound,
the relative proportion of the atoms of each element and
the type of bonding present (ionic, covalent, metallic).
For example, the substance water is made up of many water molecules, each water molecule is made up of two atoms of hydrogen and one atom of oxygen covalently bonded together.
It is useful when teaching students chemical formulae that it is represented at the three level of representation in order for them to make the necessary conceptual connections Atkinson (2002) , which are best made visually.
Below is a diagram showing the explanation of the chemical formula of water and what it represents by linking the three levels of representation.
Balanced but not really...
Consider this example taken from one of my online sessions where a student was given a task to write and balance the equation for the reaction between zinc metal and hydrochloric acid (formula for hydrochloric acid was supplied) to produce zinc chloride and hydrogen gas.
Although this equation is technically balanced the student made some crucial errors, which makes it evident that:
1. the student does not have a good grasp of chemical bonding, as they would know that zinc metal is composed only of zinc atoms, and metals are not made up of molecules but a network of metal atoms closely packed within the metal lattice.
2. The student does not understand that hydrogen gas is a simple molecular substance (made up of homonuclear diatomic molecules) and molecules are composed of two or more atoms chemically bonded.
These are common errors seen time after time with students and demonstrates that:
students have difficulty differentiating subscripts and coefficients (more on that later) and,
2. students have difficulty with chemical language such as the meaning of terms such as atoms, elements, formula units, molecules and compounds.
3. there are misconceptions about chemical bonding and structure
4. all of the above.
The solution
1.Teach chemical bonding first!
I am a strong advocate for teaching chemical bonding and structure before students are taught how to write chemical formulae and balancing chemical equations, otherwise many of the issues students encounter will persist because they do not truly understand what they are doing or why they are doing it.
If you have taught bonding, maybe it's time you did a review to diagnose some student misconceptions and address those. A great way to start is by administering a two-tier diagnostic test.
2. Have students count atoms in chemical compounds.
It is as simple as giving your students the chemical formula of different substances ( covalent, ionic and metallic) and have them
determine the type of bonds present
determine the different elements present
count the number of atoms of each element present
This will assist them in "decoding" the information in the chemical formula which will be essential when they begin to balance equations.
Mistake #2: Writing the chemical formulae for ionic compounds
Students will obviously have difficulty balancing the equations if the chemical formulae are written incorrectly.
Students tend to have a harder time writing the chemical formula for ionic substances than they do for covalent substances. With covalent compounds, most of the time, the formula is in the name. It is easy for a student to understand that carbon-di-oxide means one carbon to two oxygen atoms.
When writing the chemical formula for ionic compounds, you may have noticed that some students tend to have a much harder time for two reasons:
1. they do not know the charges of the simple ions and therefore ignore them;
2. they do not know how to balance the charges of these ions.
Consider this example:
When teaching chemical bonding to a small group of students during an online session they were administered a short two-tier diagnostic test. One of the questions asked them to write the chemical formula for Aluminum Oxide.
One student who selected option A wrote:
The formula for aluminum is Al, and the formula for oxygen is O, therefore the formula for aluminum oxide is AlO.
The student reasoned that:
Aluminum has a 3+ charge and oxygen has a 2+ charge therefore we require three aluminum atoms and two oxygen atoms
You may have seen similar answers with your students. This issue becomes evident when students are given word equations and asked to write the corresponding chemical equation which is something they will often be required to do when moving on to topics such as reactions of acids and alkalis etc.
The Solution
In order to write chemical formulae, students need to cover a few basic requirements:
1. Students need to know the charges of the common ions.
Now, before you hand your students a list of common ions to memorize (and forget), they can be taught how to determine the charges of the ions for different elements based on their group number in the periodic table.
This is a much better than the rote learning approach teachers often go for, and this way, it is easy for students to quickly determine the charge of an ion that is unfamiliar to them if they know its position in the periodic table.
The group number determines the number of electrons that are lost (in the case of metals) or gained (in the case of non-metals), group 14 (IVA) and group 18 elements do not form ions.
The charges for transition metal ions are easy for students to figure out since the oxidation number (and the subsequent charge of the ion) can be deduced from the name of the metal. For example Iron(II) has a +2 oxidation number and thus a 2+ charge.
It is more effective and leads to more long-term retention and understanding when students know the why and how behind the charges of ions.
2. Students need to know how to balance charges to determine the chemical formula of a compound.
Chemical compounds are electrically neutral, i.e. the charges on the ions (which make up the ionic compound) need to cancel out each other (or balance out).
total positive charges = total negative charge.
Once students understand this concept they can be shown how to determine the chemical formula using two methods.
The balancing (Charges) method
The balancing method is exactly what it sounds like and is my method of choice when teaching students how to write chemical formulae.
Students need to determine the number of each ion in order to balance all the charges on the chemical compound therefore leading to an electrically neutral product.
The video below demonstrates the use of the balancing method to determine the chemical formula for Copper(II) chloride which I use for my one-to-one online tutoring.
If you would like to use this activity for your classes you can purchase it here.
With this method, students understand what they are doing and why they are doing it. They understand (and can visualize), that in order to obtain a neutral compound two negative charges (Cl-) are required to balance out the 2+ charge on copper(II).
The Crisscross Method
With this method students first write the magnitude of the charges on each ion, switch the numbers, then write these numbers as subscripts to reveal the formula of the ion.
I personally do not like this method because :
1. students are just switching numbers around and most times they do not know why they are doing so. They are following a formula (no pun intended) they do not understand.
More meaningful learning occurs when students know what they are doing and why they are doing it.
2. The crisscross method becomes trickier when dealing with non binary compounds (see section on Incorrect use of brackets when dealing with polyatomic ions).
3. Sometimes, students forget that the formula for ionic compounds needs to be expressed in its simplest ratio. For example:
This is not to say I do not teach the crisscross method at all. I usually teach both the crisscross method and the balancing (charge) method and allow students to choose the method they are most comfortable with.
But I have found, students when introduced to both prefer the balancing method as it is more easily rationalized than the crisscross method.
NB. It is important, that students are comfortable working out the formula for binary ionic compounds before moving onto formulae that involve complex ions.
Mistake#3: Incorrect use of brackets when dealing with Polyatomic ions
When writing the chemical formula for compounds involving polyatomic ions, some of the most common errors seen are exemplified below:
Students have difficulty knowing when to use brackets in their formula, and the reasons are:
1. they do not truly understand what is going on with polyatomic ions,
2.they do not understand how they're formed and
3.they do not know how to treat them when writing formulae.
The Solution
1. Explain polyatomic ions by comparing them to monatomic ions.
Monatomic ions such as Li+ or F- are formed when atoms are ionized by gaining or losing electrons.
Similarly, polyatomic ions are molecules that are ionized by either gaining or losing electrons.
Just like monatomic ions the number of electrons in the entire molecule is not equal to the number of protons.
The overall charge on a polyatomic ion is equal to the sum of the formal charges on each atom in the complex ion.
If you want to take this a step further, you can have students calculate the overall charge from the formal charges then draw Lewis structures for the polyatomic ions.
2. Explain when brackets are used by having students circle the formula of the polyatomic ion
Once students understand the bonding within the polyatomic ion, then you can stress that when writing the formula for compounds involving polyatomic ions, these should be treated as a single unit.
The best way I've found to do this is to have students write out the formula of the ion and circle it, leaving the charge on the outside. That way, they begin to see the polyatomic ion as a single unit.
Once that's done, have students balance the charges using the balancing method (or crisscross method) as previously discussed. You must stress that:
Brackets are only needed for polyatomic ions
If there is only one polyatomic ion no brackets are necessary
If there is more than one polyatomic ion in the compound then brackets are necessary.
Give students plenty of practice writing the chemical formula of compounds with polyatomic ions. Then you can mix up the examples with binary compounds.
Mistake #4: Confusing coefficients and subscripts
We have all faced this issue when first introducing this topic. When balancing equations, students attempt to change subscripts rather than adding coefficients. This is because, students do not realize that the coefficient and the subscript serve two distinct purposes.
However, in most cases, if the previous three mistakes are addressed, students tend to have less difficulty in this area.
It is best to represent the differences visually. I commonly use the example of water and hydrogen peroxide, as shown in the image below:
I tell my students that subscripts are part of the chemical formula of the compound and this does not change when balancing an equation. Changing the subscript changes the chemical identity of the substance.
The coefficient however indicates the number of moles of the substance present. You can only add coefficients to the front of a chemical compound in order to balance it.
If you have not covered moles with your students, you can tell them that the coefficients indicate the number of molecules of the compound needed to balance the equation (although technically there are no molecules of ionic compounds, this definition can be used as a placeholder until the mole concept is covered).
The Solution
The key to get students to understand the difference between the use of coefficients is to get them to practice by connecting the symbolic representation of molecules with the submicroscopic representations.
An example of this is shown in the video below:
Get this digital activity for your class here!
Here students get to visually understand the difference between the use of coefficients and subscripts in various examples.
Conclusion
Often times, we teach balancing equations in isolation, where students do not have a solid grasp of the pre-requisite knowledge. If we address the student difficulties highlighted in this post, students will be well on their way to mastering chemical equations.
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References:
Atkinson, R. K. (2002). Optimizing learning from examples using animated pedagogical agents. Journal of Educational Psychology, 94, 416–427.