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Balancing Equations Warm-Up 9/22 1. List 4 signs of chemical change you observed in the reaction you carried out last class period. 2. State the law of conservation of mass. 3. Chemical equations must always be written as “balanced”. What does balanced mean in these context? Power Standard 1.2 Students understand chemical and physical changes of matter. Today’s Learning Targets • I can list the main types of chemical reactions. • I can identify reactants and products in a chemical equation. • I can balance chemical equations when all reactants and products are given. • I can classify reactions by major type and can predict products of simple reactions. • I can apply the state symbols (s), (l), (g) and (aq) in equations. D. Describing Chemical Change 1. Word Equations—Show the names of reactants and products. Example: Sodium + Chlorine Sodium chloride 2. Chemical equations—Show the formulas of reactants and products. Example: Na + Cl2 NaCl (Not Balanced Yet!) 3. Skeleton equations—Equations that are not yet balanced to represent what actually occurs in the real world (see example above) 4. State symbols— • (s)--solid, (g)--gas, (l)--liquid, (aq)—in water solution • are listed after each formula to give the physical states of the substances. • Example: Na(s) + Cl2(g) NaCl (s) 5. Balancing chemical equations A. Everyday example • 2 slices of bread + 1 slice of cheese + 1 slice of turkey 1 sandwich (multiple parts combine to 1 new whole “product”) B. What does it mean for an equation to be “balanced”? • Equal numbers of atoms of each type on both sides of the arrow. c. Why must equations be balanced? • Equations always occur in a balanced way since atoms are not created or destroyed in chemical reactions d. How are chemical equations balanced? Steps for writing balanced equations. 1. Write a skeleton equation including correct formulas for all reactants and products. At this point in our course you will be given the skeleton equation. Example: Mg + O2 MgO 2. Count the number of atoms of each element on each side of the arrow. Left: 1 Mg, 2 O Right: 1 Mg, 1 O 3. Place coefficients in front of chemical formulas to make the number of atoms of each element equal on both sides of the arrow. 2Mg + O2 2MgO (This means there are now 2 units of MgO) Left: 2 Mg, 2 O Right: 2 Mg, 2 O 4. Make sure coefficients are in the lowest whole number ratio. They are! 5. Add the appropriate state symbols. Do this if the state symbols are known. 2Mg (s) + O2 (g) 2MgO (s) Examples 1. C + O2 CO 2C + O2 2 CO 2. Pb(NO3)2 + KI KNO3 + PbI2 • Pb(NO3)2 + 2 KI 2 KNO3 + PbI2 II. Types of Reactions A. Classifying reactions • Reactions are classified into several general types B. Combination/Synthesis Reactions • Key Characteristic: 2 or more reactants 1 product • General Form: A + B AB • Example: 2Na + S Na2S C. Decomposition Reactions • Key Characteristic: 1 reactant 2 or more products • General Form: AB A + B • Example: 2NaHCO3(s) Na2CO3(s) + H2O(g) + CO2(g) 4. Combustion Reactions • Key Characteristic: 1 reactant is oxygen gas (O2) • General Form: CH compound + O2 CO2 + H2O • Example: CH4 +2O2 CO2 + 2H2O Warm-Up 9/26 Balance the following equations. Identify the reaction type. 1. 2. 3. 4. NaOH Na2O + H2O KClO3 KCl + O2 K + Cl2 KCl C3H8 + O2 CO2 + H2O Warm-Up Solution Balance the following equations. Identify the type of reaction. 1. 2K + Cl2 2KCl Combination 2. 2KClO3 2KCl + 3O2 Decomposition