Comprehensive Guide to Balancing Chemical Equations and the Law of Conservation of Mass

Learning Intentions and Success Criteria

• Learning Intention:

  • The primary objective is to learn how to balance chemical equations.

  • A secondary objective is to explain why chemical equations must adhere to the Law of Conservation of Mass.

• Success Criteria:

  • Ability to count atoms on both the reactant and product sides of a chemical equation.

  • Ability to balance chemical equations by adjusting coefficients.

  • Ability to explain how balanced equations provide evidence for the Law of Conservation of Mass.

The Law of Conservation of Mass

• Conceptual Foundation:

  • Chemical equations must be balanced because matter cannot disappear or appear from nowhere during a reaction.

  • The Law of Conservation of Mass Definition: Matter is neither created nor destroyed in a chemical reaction.

  • In a chemical process, atoms simply rearrange themselves into new formats or compounds.

  • Quantitative Requirement: The total number of atoms of each element before the reaction (reactants) must equal the total number of atoms of each element after the reaction (products).

Analysis of Unbalanced Equations

• Definition of an Unbalanced Equation: An equation where the number of atoms for at least one element differs between the left and right sides.

• Case Study: Formation of Water (Unbalanced):

  • Equation: $H_2 + O_2 \rightarrow H_2O$

  • Left Side (Reactants):

    • Hydrogen (H): $2$

    • Oxygen (O): $2$

  • Right Side (Products):

    • Hydrogen (H): $2$

    • Oxygen (O): $1$

  • Problem Identification: Because the oxygen atoms do not match ($2$ vs $1$), the equation is not balanced. This implies that one oxygen atom "disappeared," which breaks the Law of Conservation of Mass.

Mechanics of Balancing: Coefficients vs. Subscripts

• Coefficients:

  • Definition: Large numbers placed directly in front of chemical formulas.

  • Function: They indicate how many whole molecules or compounds are present in the reaction.

  • Purpose in Balancing: They change the total number of atoms without altering the chemical identity of the substance.

  • Comparison Example:

    • $H_2O$ represents one water molecule.

    • $2H_2O$ represents two water molecules. This increases the atom count while keeping the formula for water identical.

• Essential Rules for Balancing:

  • Rule 1: Change coefficients ONLY.

  • Rule 2: NEVER change subscripts.

  • Rationale: Changing a subscript changes the substance itself. For example, while $H_2O$ is water, changing the subscript to $H_2O_2$ creates hydrogen peroxide, an entirely different substance.

  • Incorrect Application: To balance oxygen in water, one might be tempted to write $H_4O_2$. This is incorrect; the correct application is $2H_2O$.

• Balanced Example: Formation of Water:

  • Equation: $2H_2 + O_2 \rightarrow 2H_2O$

  • Left Side (Reactants): H = $4$, O = $2$

  • Right Side (Products): H = $4$, O = $2$

Step-by-Step Balancing Method

• Example 1: Sodium and Chlorine ($Na + Cl_2 \rightarrow NaCl$)

  • Step 1: Count Atoms:

    • Left: Na = $1$, Cl = $2$

    • Right: Na = $1$, Cl = $1$ (Unbalanced)

  • Step 2: Balance Chlorine: Add a coefficient of $2$ before $NaCl$.

    • Equation: $Na + Cl_2 \rightarrow 2NaCl$

    • Left: Na = $1$, Cl = $2$

    • Right: Na = $2$, Cl = $2$ (Chlorine balanced, Sodium now unbalanced)

  • Step 3: Balance Sodium: Add a coefficient of $2$ before the reactant $Na$.

    • Equation: $2Na + Cl_2 \rightarrow 2NaCl$

    • Result: Both sides have Na = $2$ and Cl = $2$. The equation is balanced.

• Example 2: Magnesium and Oxygen ($Mg + O_2 \rightarrow MgO$)

  • Step 1: Count Atoms:

    • Left: Mg = $1$, O = $2$

    • Right: Mg = $1$, O = $1$ (Unbalanced)

  • Step 2: Balance Oxygen: Place a $2$ before $MgO$.

    • Equation: $Mg + O_2 \rightarrow 2MgO$

  • Step 3: Balance Magnesium: Place a $2$ before the reactant $Mg$.

    • Final Equation: $2Mg + O_2 \rightarrow 2MgO$

Introductory Practice Problems

• $\underline{\quad}H_2+\underline{\quad}Cl_2\rightarrow\underline{\quad2}HCl$

• $\underline{\quad} Fe + \underline{\quad} O_2 \rightarrow \underline{\quad} Fe_2O_3$

• $\underline{\quad} Al + \underline{\quad} O_2 \rightarrow \underline{\quad} Al_2O_3$

• $\underline{\quad} Na + \underline{\quad} O_2 \rightarrow \underline{\quad} Na_2O$

PhET Online Simulation Activity

• Purpose: To mathematically visualize the Law of Conservation of Mass in chemical formulas.

• Simulation URL: https://phet.colorado.edu/en/simulations/balancing-chemical-equations

• Part 1: Introduction Tasks:

  • Make Ammonia: $N_2 + 3H_2 \rightarrow 2NH_3$

  • Separate Water: $2H_2O \rightarrow 2H_2 + O_2$

  • Combust Methane: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$

• Part 2: Game Section:

  • Students must complete Level 1, Level 2, and Level 3, achieving a score out of 10 for each level.

Extensive Balancing Worksheet Problems

• Converting Word Equations to Symbols:

  • Carbon + Oxygen $\rightarrow$ Carbon Dioxide: $C + O_2 \rightarrow CO_2$

  • Magnesium + Chlorine $\rightarrow$ Magnesium Chloride: $Mg + Cl_2 \rightarrow MgCl_2$

  • Iron + Sulfur $\rightarrow$ Iron Sulfide: $Fe + S \rightarrow FeS$

• Symbol Equation Set (Total 37):

  1. $H_3PO_4 + KOH \rightarrow K_3PO_4 + H_2O$

  2. $K + B_2O_3 \rightarrow K_2O + B$

  3. $HCl + NaOH \rightarrow NaCl + H_2O$

  4. $Na + NaNO_3 \rightarrow Na_2O + N_2$

  5. $C + S \rightarrow CS_2$

  6. $Na + O_2 \rightarrow Na_2O$

  7. $N_2 + O_2 \rightarrow N_2O_5$

  8. $H_3PO_4 + Mg(OH)_2 \rightarrow Mg_3(PO_4)_2 + H_2O$

  9. $NaOH + H_2CO_3 \rightarrow Na_2CO_3 + H_2O$

  10. $KOH + HBr \rightarrow KBr + H_2O$

  11. $Na + O_2 \rightarrow Na_2O$

  12. $Al(OH)_3 + H_2CO_3 \rightarrow Al_2(CO_3)_3 + H_2O$

  13. $Al + S_8 \rightarrow Al_2S_3$

  14. $Cs + N_2 \rightarrow Cs_3N$

  15. $Mg + Cl_2 \rightarrow MgCl_2$

  16. $Rb + RbNO_3 \rightarrow Rb_2O + N_2$

  17. $C_6H_6 + O_2 \rightarrow CO_2 + H_2O$

  18. $N_2 + H_2 \rightarrow NH_3$

  19. $NaCl \rightarrow Na + Cl_2$

  20. $C_{10}H_{22} + O_2 \rightarrow CO_2 + H_2O$

  21. $Al(OH)_3 + HBr \rightarrow AlBr_3 + H_2O$

  22. $CH_3CH_2CH_2CH_3 + O_2 \rightarrow CO_2 + H_2O$

  23. $Li + AlCl_3 \rightarrow LiCl + Al$

  24. $C_2H_6 + O_2 \rightarrow CO_2 + H_2O$

  25. $NH_4OH + H_3PO_4 \rightarrow (NH_4)_3PO_4 + H_2O$

  26. $Rb + P \rightarrow Rb_3P$

  27. $CH_4 + O_2 \rightarrow CO_2 + H_2O$

  28. $Al(OH)_3 + H_2SO_4 \rightarrow Al_2(SO_4)_3 + H_2O$

  29. $Na + Cl_2 \rightarrow NaCl$

  30. $Rb + S_8 \rightarrow Rb_2S$

  31. $H_3PO_4 + Ca(OH)_2 \rightarrow Ca_3(PO_4)_2 + H_2O$

  32. $NH_3 + HCl \rightarrow NH_4Cl$

  33. $Li + H_2O \rightarrow LiOH + H_2$

  34. $Ca_3(PO_4)_2 + SiO_2 + C \rightarrow CaSiO_3 + CO + P_4$

  35. $NH_3 + O_2 \rightarrow N_2 + H_2O$

  36. $FeS_2 + O_2 \rightarrow Fe_2O_3 + SO_2$

  37. $C + SO_2 \rightarrow CS_2 + CO$

Bonus Revision Activity: Kahoot or Blooket

• Task: Partner work to create a revision game for the chemistry exam.

• Quiz Content Requirements:

  • Minimum of 8–10 questions.

  • Topic Coverage:

    1. Atomic Structure.

    2. Periodic Trends (specifically atomic radius and metallic character).

    3. Word equations and Symbol equations.

    4. Identification of Reactants vs. Products.

    5. Balancing equations techniques.

    6. The Law of Conservation of Mass.

Questions & Discussion

• Warm Up - Turn & Talk:

  • Question: Are the same number of atoms present on both sides of the initial reaction shown?

  • Question: Does it look like any atoms have disappeared or appeared?

  • Question: Does the reaction follow the Law of Conservation of Mass? Why or why not?

  • Question: Do you think the equation is balanced? Why or why not?

• Reflection:

  • The session concludes with a self-reflection phase on simulation results and game scores.