Student Exploration: Chemical Equations

Vocabulary: Avogadro’s number, chemical equation, chemical formula, chemical reaction, coefficient, combination, combustion, conservation of matter, decomposition, double replacement, molar mass, mole, molecular mass, molecule, product, reactant, single replacement, subscript.

Prior Knowledge Questions (Do these BEFORE using the Gizmo.) 

1. A candle is placed on one pan of a balance, and an equal weight is placed on the other pan.

What would happen if you lit up the candle and waited for a while? ____________________

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2. Suppose the candle was placed in a large, sealed jar that allowed it to burn for several minutes before running out of oxygen. The candle and jar are balanced by an equal weight.

In this situation, what would happen if you lit up the candle and waited? ________________

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Gizmo Warm-up

Burning is an example of a chemical reaction. The law of conservation of matter states that no atoms are created or destroyed in a chemical reaction. Therefore, a balanced chemical equation will show the same number of each type of atom on each side of the equation.

To set up an equation in the Chemical Equations Gizmo, type the chemical formulas into the text boxes of the Gizmo. First, type in “H2+O2” in the Reactants box and “H2O” in the Products box. This represents the reaction of hydrogen and oxygen gas to form. water.

1. Check that the Visual display is chosen on each side of the Gizmo, and count the atoms.

A. How many hydrogen atoms are on the Reactants side? ____   Products side? ____

B. How many oxygen atoms are on the Reactants side? ____   Products side? ____

2. Based on what you see, is this equation currently balanced? _________________________

Introduction: To balance a chemical equation, you first need to be able to count how many atoms of each element are on each side of the equation. In this activity, you will practice counting the atoms that are represented in chemical formulas.

Question: How do we read chemical formulas?

1. Observe: Type “H2” into the Reactants box and hit Enter on your keyboard. Note that the formula is shown as H₂ below. The small “2” in H₂ is a subscript.

A. What does the “2” in H₂ represent? _______________________________________

B. In general, what do you think a subscript. in a chemical formula tells you? _________

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C. Try typing in other subscripts next to the H, such as 3, 4, and 5. Is your answer to question B still true? Explain. ____________________________________________

2. Count: Clear the Reactants box, and type in a more complex chemical formula: “Ca(OH)2.” Look at the number of atoms shown.

A. How many of each type of atom do you see?      Ca: _____      O: _____      H: _____

B. In general, what happens when a subscript. is found outside of parentheses?

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C. Try typing in other subscripts next to the (OH), such as 3, 4, and 5. Is your answer to question B still true? Explain. ____________________________________________

3. Practice: For each of the real chemical formulas below, calculate how many of each element there are. Check your answers for the first three formulas using the Gizmo.

AgCl₃Cu₂ Ag: _____ Cl: _____ Cu: _____

Ba(AsO₄)₂ Ba: _____ As: _____ O: _____

(NH₄)₃PO₄ N: _____ H: _____ P: _____ O: _____

MnPb₈(Si₂O₇)₃ Mn: _____ Pb: _____ Si: _____ O: _____

Introduction: In a chemical reaction, the reactants are the substances that enter into the reaction, and the products are the substances that are made in the reaction. A chemical reaction is balanced if the numbers of reactant atoms match the numbers of product atoms.

Goal: Learn to balance any chemical equation.

1. Observe: To model how hydrogen and oxygen react to make water, type “H2+O2” into the Reactants box and “H2O” into the Products box.

As the equation is written, which element is not in balance? ________________________

Explain: _________________________________________________________________

2. Balance: To balance a chemical equation, you are not allowed to change the chemical formulas of the substances involved in the reaction. You are allowed to change the number of molecules of each substance by adding coefficients in front of the formulas.

A. To balance the oxygen atoms, add a “2” in front of the “H2O” in the Products box.

How many oxygen atoms are found on each side of the equation now? _________

B. To balance the hydrogen atoms, add a “2” in front of the “H2” in the Reactants box.

How many hydrogen atoms are found on each side of the equation now? _________

C. Is this equation currently balanced? _________ Click Show if balanced to check.

3. Apply: Now enter a more complex chemical reaction: Ca(OH)₂ + HBr → CaBr₂ + H₂O. List the numbers of each element in the tables below:

A. Which elements are out of balance? ______________________________________ 

B. Add coefficients to balance first the bromine (Br) and then the hydrogen (H) atoms. When the equation is balanced, write the complete formula below:

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4. Practice: Chemical reactions are generally classified into five groups, defined below. Balance each equation, using the Gizmo for help.

Combination (or synthesis) – two or more elements combine to form. a compound.

· Na + O₂ → Na₂O _________________________________________

· La₂O₃ + H₂O → La(OH)₃ _________________________________________

· N₂O₅ + H₂O → HNO₃ _________________________________________

Decomposition – a compound breaks down into elements and/or simpler compounds.

· KNO₃ → KNO₂ + O₂ _________________________________________

· NaN₃ → Na + N₂ _________________________________________

· NH₄NO₃ → N₂O + H₂O _________________________________________

Combustion – a fuel reacts with oxygen to release carbon dioxide, water, and heat.

· CH₄ + O₂ → CO₂ + H₂O _________________________________________

· C₃H₈ + O₂ → CO₂ + H₂O _________________________________________

· C₆H₁₂O₆ + O₂ → CO₂ + H₂O _________________________________________

Single replacement – an element replaces another element in a compound.

· KCl + F₂ → KF + Cl₂ _________________________________________

· Mg + HCl → MgCl₂ + H₂ _________________________________________

· Cu + AgNO₃ → Cu(NO₃)₂ + Ag _________________________________________

Double replacement – two compounds switch parts with one another.

· AgNO₃ + K₂SO₄ → Ag₂SO₄ + KNO₃ ___________________________________

· Mg(OH)₂ + HCl → MgCl₂ + H₂O ___________________________________

· Al(OH)₃ + H₂SO₄ → Al₂(SO₄)₃ + H₂O ___________________________________

Introduction: Chemists are often interested in obtaining a certain mass of product from a chemical reaction without wasting any reactants. But how is this done? To calculate the masses of reactants needed for a desired mass of product, it is necessary to understand a unit of quantity called the mole.

Question: How do chemists know how much of each substance to mix?

1. Observe: The mass of a molecule of a substance is its molecular mass (M). Molecular mass is measured in universal mass units (u). One universal mass unit (1 u) is approximately the mass of a proton. Hydrogen gas has a molecular mass of 2.0158 u.

A. Type the formula “H2” into the Reactants box. What is the molar mass of hydrogen gas, H₂? ________________________

B. What is the relationship between the molecular mass and the molar mass of a substance? _________________________________________________________

A mole is defined as 6.0221415 × 10²³ molecules (or atoms) of a substance. This value, called Avogadro’s number, is special because a mole of a substance has a mass in grams that is equal to the molecular mass of the substance. Moles are handy because a mole of one substance contains the same number of particles as a mole of another substance.

2. Gather data: The balanced equation to synthesize water is: 2H₂ + O₂ à 2H₂O. Use the Gizmo to find the molar masses of each substance in this equation:

2H₂ __________ O₂ __________ 2H₂O __________

3. Analyze: Based on the molar masses, how can you tell that an equation is balanced?

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4. Apply: Suppose you had one mole of oxygen (O₂). How many moles of hydrogen (H₂) would react completely with the oxygen, and how many moles of H₂O would be produced?

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5. Calculate: Suppose you had 2.0158 grams of hydrogen (H₂).

A. How many moles of hydrogen do you have? ____________________ 

B. How many moles of oxygen would react with this much hydrogen? ______________

C. What mass of oxygen would you need for this reaction? _____________________

D. How many grams of water would you produce? _____________________

6. Challenge yourself: Suppose you wanted to make 100 grams of water. 

A. What is the molar mass of water (H₂O)? _____________________

B. How many moles of water are in 100 grams? _____________________

C. How many moles of hydrogen will you need? _____________________

D. How many moles of oxygen will you need? _____________________

E. How many grams of hydrogen and oxygen will you need?

Hydrogen: _____________________ Oxygen: _____________________

F. Is your answer reasonable? Why or why not? _______________________________ 

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7. Summarize: Why is it useful to use moles to measure chemical quantities? _____________

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