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Balancing Chemical Reactions
Chemical reactions are like recipes in that the quantity and types of ingredients, or reactants , can be related to the quantity and type of cooked food, or product(s) formed. Balancing chemical reactions then allows one to determine stoichiometry calculations by understanding the ratio between reactants and/or products. This worksheet includes some rules and guidelines to help you balance chemical equations.
Rules
1.) The formulas of the reactants and products cannot be changed, do not alter subscripts or charges. 2.) The only numbers that can be changed are the numbers indicating how many molecules or atoms, which are called coefficients. 3.) A coefficient is assumed to be one if there is not a number in front of the molecule or atom. 4.) In order to be balanced, there must be an equal number of each type of atom on both the reactant and product side of the reaction. 5.) It is generally required that the coefficients are whole numbers.
Guidelines
1.) In reactions dealing solely with ions, one can leave the polyatomic ions as groups for ease of balancing. 2.) In reactions dealing with only ions and water, water can be considered as a combination of a hydrogen ion and hydroxide ion. 3.) If given a reaction with polyatomic ions that are broken down, one cannot leave the polyatomic ions as groups. 4.) “Atom accounting” makes this easier by using a table, and is detailed in the following guideline points and examples. 5.) Start with all coefficients of one and total the number of each type of atom or species. 6.) The more atoms in a given molecule, the larger the effect it has on balancing, so begin with these. 7.) End with molecules or atoms that consist of only one type, since the number can be changed independently of the other atom types. 8.) If a coefficient comes out to a fraction, multiply all coefficients by the fraction denominator to result in all whole number coefficients.
Atom Accounting
How many nitrogen atoms are in 1 N 2 O 5 molecule?
In one molecule, there are two nitrogen atoms, as notated by the subscript 2.
How many nitrogen atoms are in 2 N2O5 molecules?
2 molecules ∙ 2 nitrogen = 4 nitrogen atoms
How many phosphate groups and oxygen atoms are in 1 formula unit of Cu 3 (PO 4 ) 2?
1 formula unit ∙ 2 phosphate groups ∙ 4 oxygen atoms = 8 oxygen atoms
How many oxygen atoms are in 3 formula units of Cu 3 (PO 4 ) 2?
3 formula units ∙ 2 phosphate groups ∙ 4 oxygen atoms = 24 oxygen atoms
How many oxygen atoms are in 1 molecule of CO 2 and 1 molecule of H2O?
(1 molecule CO 2 ∙ 2 oxygen atoms) + (1 molecule H 2 O ∙ 1 oxygen atom) = 3 oxygen atoms
How many oxygen atoms are in 4 molecules of CO 2 and 7 molecules of H 2 O?
(4 molecules CO 2 ∙ 2 oxygen atoms) + (7 molecule H 2 O ∙ 1 oxygen atom) = 15 oxygen atoms
How many oxygen atoms are in 7 formula units of Cu 3 (PO 4 ) 2 and 4 formula units of Na 2 SO 4?
(7 formula units ∙ 2 phosphate groups ∙ 4 oxygen atoms) + (4 formula units ∙ 4 oxygen atoms) = 72 oxygen atoms
Practice Problems
Balance the following chemical equations:
1.) ___N 2 O 5 → ___N 2 + ___O 2
2.) ___C 5 H 12 + ___O 2 → ___CO 2 + ___H 2 O
3.) ___C 8 H 18 + ___O 2 → ___CO 2 + ___H 2 O
4.) ___HC 2 H 3 O 2 + ___KNO 2 → ___KC 2 H 3 O 2 + ___HNO 2
5.) ___Fe(NO 3 ) 3 + ___MgO → ___Fe 2 O 3 + ___Mg(NO 3 ) 2
6.) ___(NH 4 ) 3 N + ___Cr(CrO 4 ) 2 → ___(NH 4 ) 2 CrO 4 + ___Cr 3 N 4
7.) ___Cu 3 (PO 4 ) 2 + ___Na 2 SO 4 → ___Na 3 PO 4 + ___CuSO 4
8.) ___AlCl 3 + ___H 2 O → ___Al(OH) 3 + ___HCl
9.) ___Al(HCO 3 ) 3 + ___AlPO 4 → ___Al 2 (CO 3 ) 3 + ___H 3 PO 4
10.) ___HOCl + ___C → ___H 2 O + ___CO 2 + ___Cl 2
Balancing Chemical Equations Answers
1.) 2, 2, 5 2.) 1, 8, 5, 6 3.) 2, 25, 16, 18 4.) 1, 1, 1, 1 5.) 2, 3, 1, 3 6.) 4, 3, 6, 1 7.) 1, 3, 2, 3 8.) 1, 3, 1, 3 9.) 1, 1, 1, 1
- 4,1,2,1,
