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Balancing Chemical Equations

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The Balanced Equation l Atoms can’t be created or destroyed. l All the atoms we start with we must end up with. l A balanced equation has the same number of each element on both sides of the equation.

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l Balanced equations show how mass and atoms are conserved. The Balanced Equation

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C + O 2 CO l This equation is NOT balanced. l There is one carbon atom on the left and one on the right. l There are two oxygen atoms on the left and only one on the right. C + O C O O

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C + O 2 CO l We need one more oxygen atom in the products. l We can’t change the formula, because it describes what it is. C + O C O O

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l In order to have two oxygen atoms, another CO must be produced. l But where did the other carbon come from? C + O C O O O C

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l We must have started with two carbon atoms. C + O C O O O C C

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l The balanced chemical equation is 2 C + O 2 2 CO C + O C O O O C C

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Chemical Equations l Numbers and types of atoms must balance.

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Rules for Balancing 1. Write the correct formulas for all the reactants and products. 2. Count the number of atoms of each type appearing on both sides. 3. Balance the elements one at a time by adding coefficients (the numbers in front). 4. Check to make sure it is balanced.

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Never l Never change a subscript to balance an equation. l If you change the formula you are describing a different reaction.

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Never l Never put a coefficient in the middle of a formula. l 2 NaCl is okay; Na2Cl is not.

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Example H 2 +H2OH2O O2O2 Make a table to keep track of where you are in the balancing process.

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Example H 2 +H2OH2O O2O2 We need twice as much O in the product. ReactantsProducts H O 2 2 2 1

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Example H 2 +H2OH2OO2O2 We need 2 oxygen atoms on the product side, so a coefficient of 2 should be placed in front of water. 2

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Example H 2 +H2OH2OO2O2 We must recalculate the number of hydrogen and oxygen atoms on the right. Coefficients are used as multipliers. RP H O 2 2 2 1 2 4 2

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Example H 2 +H2OH2OO2O2 Now we need 4 hydrogen atoms on the reactant side. RP H O 2 2 4 2 2

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Example H 2 +H2OH2OO2O2 A coefficient of 2 should be placed in front of hydrogen gas. RP H O 2 2 4 2 22 4

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Example H 2 +H2OH2OO2O2 The equation is balanced; it has the same number of each kind of atom on both sides! RP H O 4 2 4 2 22

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Example H 2 +H2OH2OO2O2 This is the answer, RP H O 2 2 2 1 2 2 4 2 4 not this.

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Balancing Hints l Balance elements in the following order: metals nonmetals hydrogen oxygen

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Balancing Hints l If an atom appears more than once on a side, balance it last. l If you fix everything except one element, and it is even on one side and odd on the other, double the first number, then move on from there.

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Problem l Balance the following equation. CH 4 + O 2 CO 2 + H 2 O CH 4 + 2 O 2 CO 2 + 2 H 2 O

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Problem 2 AgNO 3 + Cu Cu(NO 3 ) 2 + 2 Ag l Balance the following equation. AgNO 3 + Cu Cu(NO 3 ) 2 + Ag

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Problem l Balance the following equation. Mg + N 2 Mg 3 N 2 3 Mg + N 2 Mg 3 N 2

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Problem l Balance the following equation. P + O 2 P 4 O 10 4 P + 5 O 2 P 4 O 10

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Problem l Balance the following equation. Na + H 2 O H 2 + NaOH 2 Na + 2 H 2 O H 2 + 2 NaOH

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Objectives l PSc.2.2.5 –Classify types of reactions such as synthesis, decomposition, single replacement, or double replacement.

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