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1. Balancing Chemical Reactions

In a chemical reaction, one or more reactants are transformed into products:

reactants → products

The purpose of a chemical equation is to express this relation in terms of the formulas of the actual reactants and products that define a particular chemical change. For example, the reaction of mercury with oxygen to produce mercuric oxide would be expressed by the equation

Hg + O2 → HgO

Sometimes, for convenience, it is desirable to indicate the physical state (gas, liquid or solid) of one or more of the species by appropriate abbreviations:

Hg(l) + O2(g) → HgO(s)

C(graphite) + O2(g) → CO2(g)

C(diamond) + O2(g) → CO2(g)

However, this is always optional.

Chemical equations must be balanced!

Basically, this means that chemical equations must be consistent with the law of conservation of mass:

Figure 4.10 Combustion Reaction

Ref: commons.wikimedia.org/

In the context of an ordinary chemical reaction, conservation of mass means that atoms are neither created nor destroyed. This requirement is easily met by making sure that there are equal numbers of all atoms on both sides of the equation.

When we balance an equation, we simply make it consistent with the observed fact that individual atoms are conserved in chemical changes.

A chemical Equation is made of reactants at the left side of the chemical equation. These reactants are compounds being reacting to produce new compounds called the products at right side of the chemical equation Both reactants and products are separated with an arrow.

If the reaction goes mostly to completion, then one arrow is used but if the reaction does not go to completion, then double arrows are used which means that both the reactants and the products are found in reaction in appreciative amount at the end of the reaction and the reaction does not give only products.

Figure 4.11 Balancing Chemical Requation

Ref: commons.wikimedia.org/

Reaction goes to completion: (Using One Arrow to Separate the Reactants from the Products)

Here are few more examples:

Figure 4.12 Balancing Chemical Equation

Ref: commons.wikimedia.org/

Some Reactions do not go to completion also known as chemical equilibrium: (Using Double Arrows to Separate the Reactants from the Products)

Figure 4.13 Chemical Equation

Ref: commons.wikimedia.org/

Furthermore, one to differentiate between numbers of moles (also called coefficients) appearing at the front of the products or the reactants which are changed till the chemical equations are balanced, and the numbers appearing within the molecular formula as subscripts which are fixed and should not be touched when balancing chemical equations.

Example:

Reference: https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Map%3A_Introductory_Chemistry_(Tro)/07%3A_Chemical_Reactions/7.03%3A_How_to_Write_Balanced_Chemical_Equations

General Procedure to balance chemical equation:

1. Balance the polyatomic anions on both sides of the chemical equation as one unit
2. Look at a standalone element or a molecule, balance them at the end
3. Balance the remaining atoms, and if necessary use fractional coefficients. If a fractional coefficients have been used, multiply both sides of the equation by the denominator to obtain whole numbers for the coefficients. Remember there are no fractional atoms
4. Count the numbers of atoms of identical type at both sides of the equation to make sure their amounts at both side are identical and hence the equation is balanced

Example:

C₄H₁₀    +    O₂       à     CO₂         +       H₂O        +      Heat

Let us make a table:

Now let us multiply the Carbon atom at the right side by 4 and Hydrogen atom by 5 to balance them at both sides.

C₄H₁₀    +    O₂       à    4 CO₂         +      5 H₂O        +      Heat

Now look at the chemical reaction. Everything is balanced except oxygen. General rule of thumb; if one atom at one side has an even number (i.e. like oxygen atom at the left side 2) and odd number at the other side (i.e. like oxygen atom at the right side 130, one has to go through fraction to balance the chemical equation:

C₄H₁₀    +   13/2  O₂       à    4 CO₂         +      5 H₂O        

Since nature does not support fractional atoms, both side of the chemical equations have to be multiplied with the denominator which is 2:

2 x [C₄H₁₀    +   13/2  O₂       à    4 CO₂         +      5 H₂O         

The final balance chemical equation is then:

 2 C₄H₁₀    +   13 O₂       à    8 CO₂         +      10 H₂O  

Practice: try this one!

Figure 4.14 Balancing Chemical Equation

Ref: commons.wikimedia.org/

What are the coefficients of the balanced equation of the above equation?

Try this Balancing app(free): “Reactions” with more than 400 equations for practicing balancing. Look for the app in app store on any mobile device.

Figure 4.15 Balancing App

Ref: commons.wikimedia.org/

In case of polyatomic ion containing equations, it is always recommended to consider them as unit rather than broken down to individual atoms if they appear on both sides of the equation.

LiOH  + H₃PO4   → Li₂HPO4 +H₂O (unbalanced)

To balance Li, we have to put coefficient 2 in front of LiOH. That will give total number H on the left hand side= 5 and O=2, keeping PO₄ unit separate  from other elements. We can put coefficient 2 in front of H₂O to form 4 H and 2 O, remaining one H is present in Li₂HPO4. PO₄ unit is same on left and right hand side. So the final number of atoms look like :

Left side: Li -2                      Right side: Li-2

O-2                                           O-2

H-5                                          H-5

PO4-1                                       PO4-1

Balanced equation is

2LiOH  + H₃PO4   → Li₂HPO4 +2H₂O (balanced)

Always lowest number of coefficients are used to balance a chemical equation. For example, 2H₂O → 2H₂ + O_2, this equation can also be balanced by 4H₂O → 4H₂ + O₂

But we always use lowest set of whole numbers to balance an equation.

The following video might help you to understand more this topic.

Now let us review the videos below:

Balancing Chemical Equations Practice Problems

Introduction to Balancing Chemical Equations

Balancing Chemical Equations Step by Step Practice Problems | How to Pass Chemistry

Chemical equations should exhibit the status of the chemical compounds involved in the chemical reactions such liquids (l), solids (s), gases (g) and aqueous (aq).

Furthermore, the status of the heat being absorbed (taken in) or given away:

2 C₄H₁₀(g)    +   13 O₂(g)       …..>    8 CO₂(g)         +      10 H₂O(g)   +  Heat  [Exothermic]

HgCO₃(s)           ∆            HgO(s)     +    CO₂(g) [Exothermic]

Heat  + NH₄Cl(g)    …..>  NH₃(g)     +  HCl(g)  [Endothermic]

The chemical equations of ionic compounds in aqueous solution (dissolved in water) represent very unique sets of balancing:

1. Complete molecular ionic chemical equation
2. Complete ionic chemical equation
3. Net ionic chemical equation

Complete molecular ionic chemical equation:

This is an ionic chemical equation that keeps the ionic compounds and they are not taken apart in ions.

Example:

NaI(aq)     +   Pb(NO₃)₂(aq)   …..>   PbI₂(s)     +    NaNO₃(aq)

Balancing this molecular ionic chemical equation:

2 NaI(aq)     +   Pb(NO₃)₂(aq)   ….>  PbI₂(s)     +   2 NaNO₃(aq)

Complete ionic chemical equation:

Each aqueous ionic compound in the ionic chemical equation will be taken apart to its original ions, keeping all solids, liquids and gaseous compounds untouched:

2 NaI(aq)     +   Pb(NO₃)₂(aq)   …..>   PbI₂(s)     +   2 NaNO₃(aq)

Figure 4.16 Precipitation Reaction

Reference: https://microbenotes.com/introduction-to-precipitation-reaction/

The videos below illustrate the net ionic chemical equations set up and balancing:

Net Ionic Equation

Molecular, Ionic, and Net Ionic Equations

How to Write Complete Ionic Equations and Net Ionic Equations

The simulation below helps understand the concept of balancing chemical equation:

Simulation Activity Instructions:

Learning Goals:  Students will be able to:

1. Describe what “reactants” and “products” in a chemical equation mean.
2. Explain the importance of knowing the difference between “coefficients” and “subscripts”.
3. Use pictures and calculations to show how the number of atoms for each product or reactant is found.
4. Identify the relationship between “reactants” and “products” atoms.
5. Balance a chemical equation using the relationships identified.
6. Given a chemical equation, draw molecular representations of the reaction and explain how the representations were derived.
7. Given a molecular drawing of a chemical reaction, write the equation and explain how the symbols were derived.

Develop your understanding: Open the Introduction screen, then explore to develop your own ideas about what it means to “balance a chemical equation”.

Explain your understanding:

1. How does the sim provide information to help you learn the goals?

2. What things did you have to research outside the sim (cite references)?

3. How can you use the simulation to check your learning?

4.  Use this balanced reaction to show that you can write the equation that makes chemical sense. Explain how your symbols were derived in paragraph form.

5. Use this reaction to show that you can draw molecular representations of a balanced reaction. Explain how your representations were derived in paragraph form.

Test your understanding: For each question, write your answer and give your reasoning with evidence from the simulation.

Question 1

Your answer and explanation:

Question 2

Your answer and explanation:

Question 3

Your answer and explanation:

Question 4

Your answer and explanation:

Question 5

Your answer and explanation:

Test your understanding and skill application:

Open the full simulation Balancing Chemical Equations, then open the Game screen. Use ideas you have from the learning goals. You may need to use the Intro and Balance Lab screens to learn.

1. Play Level 1of the Balancing Chemical Equations game
   1. Did you have to change your thinking or do you have other ideas to make you get a better score? Include screen captures from the simulation to help explain.

1. When you get a wrong answer, how did the “Show Why” help you?

1. When you complete the level 1 game, capture the screen with your score. Paste it below like this:

1. Play Level 2 of the game
   1. Explain what makes the level more difficult or different from previous levels. Do you have new strategy ideas or rules? Include screen captures from the simulation to help explain.

2. When you complete the level, capture the screen with your score and paste it below:

3. Play Level 3 of the game
   1. Explain what makes the level more difficult or different from previous levels. Do you have new strategy ideas or rules? Include screen captures from the simulation to help explain.

3. When you get a wrong answer, how did the “Show Why” help you?

3. When you complete the level, capture the screen with your score and paste it below:

Final Score:

After you play all levels of the game. Copy and paste your final results like this

Extra challenge: For an extra challenge, you can turn on the timer  and see if you can improve your skills.