Mass and Volume Relationships Balancing Chemical Equations Overview
Mass and Volume Relationships Balancing Chemical Equations
Overview: 1. Description of the Chemical Equation 2. Coefficients vs. Subscripts 3. The four basic steps to balancing a chemical equation 4. Law of Combined Volumes
Chemical Sentences: Equations Chemical equations represent the sentences in the language of chemistry. P 4(s) + 6 Br 2(l) 4 PBr 3(g) They are the means of communicating a chemical change using the symbols and formulas to represent the elements and compounds involved in a chemical reaction.
Chemical Sentences: Equations Reactants are the species present before the reaction. Products are the species present after the reaction: Reactants → Products The arrow (→) means “yield(s)” or “react(s) to produce”.
Chemical Sentences: Equations The following are used to denote the states of matter of a species in an equation: (s) = solid (l) = liquid (g) = gas (aq) = aqueous solution
Chemical Sentences: Equations Coefficients are numbers used to balance a chemical equation. Never change the subscripts!!!
Chemical Equations are Balanced In a balanced chemical reaction • no atoms are lost or gained. • the number of reacting atoms is equal to the number of product atoms.
A Balanced Chemical Equation In a balanced chemical equation, • the number of each type of atom on the reactant side is equal to the number of each type of atom on the product side. • numbers called coefficients are used in front of one or more formulas to balance the number of atoms. Al + S 1 Al 1 S 2 Al + 3 S 2 Al 3 S Al 2 S 3 Not Balanced 2 Al Unequal number of Al atoms 3 S Al 2 S 3 Unequal number of S atoms Balanced using coefficients = 2 Al Equal number of Al atoms = Equal number of S atoms 3 S
Steps to Balancing Equations There are four basic steps to balancing a chemical equation. 1 a. Write the correct formula for the reactants and the products. (Write a skeleton equation) b. Indicate the state of the matter 2. Find the number of atoms for each element on the left side. Compare those against the number of the atoms of the same element on the right side. 3. Determine where to place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element 4. Check your answer to see if: The numbers of atoms on both sides of the equation are now balanced. The coefficients are in the lowest possible whole number ratios. (reduced)
Steps to Balancing Equations Example: N 2 H 4 + N 2 O 4 N 2 + H 2 O Step 1 a&b : Write a skeleton equation; Indicate the state of the matter: N 2 H 4(l)+ N 2 O 4 (l) N 2 (g) + H 2 O (l) Step 2: Find the number of atoms on the left side and compare to right 4 N, 4 H, 4 O N 2 H 4(l)+ N 2 O 4 (l) 2 N, 2 H, 1 O N 2 (g) + H 2 O (l) Step 3: Determine where to place coefficients Note that the product side needs 4 O: Multiply the H 2 O by 4
Writing a Balanced Equation Steps to Balancing Equations 4 N, 4 H, 4 O N 2 H 4(l)+ N 2 O 4 (l) 2 N, 8 H, 4 O N 2 (g) + 4 H 2 O (l) The product side now has 8 H compared to the 4 H on the reactant side. What reactant can we multiply to get 8 reactant H? Try multiplying N 2 H 4 by 2 6 N, 8 H, 4 O 2 N 2 H 4(l)+ N 2 O 4 (l) N 2 (g) + 4 H 2 O (l) recount *Now use the N 2 to balance the N: multiple N 2 by 3 2 N 2 H 4(l)+ N 2 O 4 (l) 3 N 2 (g) + 4 H 2 O (l) Step 4: Check your answer: Both sides: 6 N, 8 H, 4 O
A Balanced Chemical Equation State the number of atoms of each element on the reactant side and the product side for each of the following balanced equation. P 4(s) + Br 2(l) PBr 3(g) P 4(s) + 6 Br 2(l) 4 PBr 3(g) 4 P atoms 12 Br atoms = = 4 P atoms 12 Br atoms
A Balanced Chemical Equation State the number of atoms of each element on the reactant side and the product side for each of the following balanced equation. Al(s) + Fe 2 O 3(s) Fe(s) + Al 2 O 3(s) 2 Al(s) + Fe 2 O 3(s) 2 Fe(s) + Al 2 O 3(s) 2 Al atoms 2 Fe atoms 3 O atoms = = = 2 Al atoms 2 Fe atoms 3 O atoms
Volume Relationships in Chemical Equations Law of Combined Volumes: When all measurements are made at the same temperature and pressure, the volumes of gaseous reactants and products are in a small whole-number ratio. 3 H 2 + N 2 2 NH 3
Summary 1. Chemical equations represent the sentences in the language of chemistry 2. Reactants are the species present before the reaction. 3. Products are the species present after the reaction: 4. Coefficients are numbers used to balance a chemical equation. Never change the subscripts!!! 5. Four basic steps to balancing a chemical equation. 1. 2. Write a skeleton equation Find the number of atoms for each element on the left side. Compare them to those on the right side. 3. Determine where to place coefficients in front of formulas so that each side has the same number of atoms 4. Check your answer 6. Law of Combined Volumes: When all measurements are made at the same temperature and pressure, the volumes of gaseous reactants and products are in a small wholenumber ratio.
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