2.2 Conservation of Mass in Chemical Equations

Chemical Reactions

Chemical reactions result in chemical changes.

• Chemical changes occur when new substances are created.
• The original substance(s), called reactants, change into new substance(s) called products.

Chemical change means new compounds are created.

• BUT no new matter is created or destroyed; atoms are just rearranged.
• All of the matter in the reactants = all of the matter in the products
• John Dalton, 200 years ago, realized that atoms simply rearrange themselves during chemical reactions.
• Number of each atom in reactants = number of each atom in products

Chemical reactions can be written in different ways.

• A word equation:
  • Nitrogen monoxide + oxygen --> nitrogen dioxide
• A symbolic equation:
  • 2NO(g) + O₂(g) --> 2NO₂(g)

Coefficients - Big numbers in front of each compound - indicate the ratio of compounds in the reaction - here, there is twice as much NO and NO₂ than there is O₂.

Conservation of Mass

The Law of Conservation of Mass

• In chemical reactions, atoms are neither created nor destroyed
• Developed by Antoine Lavoisier and his wife Maire-Anne in the 1700s
• Mass of reactants = Mass of products

Balancing chemical equations is a "trial and error" method; you systematically change the coefficients in front of the chemical formulas until you "balance" the number of each atom on each side of the equation.

Chemical equations do not come already balanced. This must be done before the equation can be used in a chemically meaningful way. All chemical calculations to come must be done with a balanced equation. A balanced equation has equal numbers of each type of atom on each side of the equation.

There is no apparent change in mass during a chemical reaction. This is what is expected from the balanced equation for a reaction. Atoms and mass are conserved.

2 Mg + O₂ → 2 MgO

There are 2 Mg atoms + 2 O atoms to start with and 2 Mg atoms + 2 O atoms in the end. There is no gain or loss of mass.

There are 2 molecules on the reactant side (2 MgO) and 3 particles on the product side (2 Mg atoms and 1 O₂ molecule) of the equation.

Conservation of Mass Video

Balancing Chemical Equations

Because of the Law of Conservation of Mass, we can count atoms and use math to balance the number of atoms in chemical equations.

• Word equation:Methane + oxygen--> water + carbon dioxide
  • Skeleton equation: CH₄(g) + O₂ (g)--> H₂O(l) + CO₂(g)
  • To balance the compounds, take note of how many atoms of each element occur on each side of the reaction arrow:

• To balance, attempt to find values that equate atoms on both sides

• Balanced equation: CH₄(g) + 2O₂ (g) --> 2H₂O(l) + CO₂(g)

Balance chemical equations by following these steps:

• Trial and error will work, but can be very inefficient
• Balance compounds first, elements last
• Balance one compound at a time
• Only add coefficients; NEVER change subscripts!
• If H and O appear in more than one place, attempt to balance them LAST
• Polyatomic ions (such as SO₄^2–) can often be balanced as a whole group
• Always double-check after you think you are finished!

Balance the following:

Answer

Explanation

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Practice

Give the simulation below a try by balancing the reactions in the introduction and then trying the game.

Writing Word Equations

Word equations require careful examination to be written correctly.

• The chemical symbol is used for most elements but not in a compound
  • Be careful of diatomic and polyatomic elements< such as O₂, P₄ and S₈
  • The "special seven" are all diatomic elements
    • H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂

• Several common covalent molecules containing hydrogen have common names that do not help in writing chemical formulas

• For example, methane = CH₄, glucose = C₆H₁₂O₆, Ethane = C₂H₆, Ammonia = NH₃

The pink shaded area in the picture below shows the diatomic atoms.

Writing Chemical Equations

The simplest form of chemical equation is a word equation

• Not much information other than the elements/compounds involved
• Potassium metal + oxygen gas --> potassium oxide
• reactants appear on the left of the arrow and products appear on the right

• Shows atoms, but not quantities of atoms
• K(s) + O₂ (g)--> K₂O(s)

• Balancing ensures that the number of each atom is the same on both sides of the reaction arrow
• Always use the smallest whole number ratio
• 4K(s) + O₂ (g)--> 2K₂O(s)

States of matter are indicated in chemical equations.

Letters indicate the state of each compound

(aq) = aqueous/dissolved in water

(s) = solid

(l) = liquid

(g)= gas

Practice Quiz

Complete this quiz to help you prepare for your assignments and the Unit 4 quiz.

Balancing Equations Review

Check your understanding of Balancing Equations here.