Chapter 7
Balancing Equations
What is the conservation of matter?
Matter is neither created or destroyed in a chemical reaction. The mass of any element at the beginning must equal the mass of that element at the end of the reaction.
Does the following equation obey the law of conservation of matter? Why or why not?
CuCl2 (aq) + Na2S (aq) Æ CuS (s) + NaCl (aq)
What does that Mean?
• In a chemical equation, the number of elements in the reactant must equal the number of elements in the product.
CuCl2 (aq) + Na2S (aq) Æ CuS (s) + NaCl (aq)
Objectives
• Identify what a subscript and coefficient is
• Determine what it means to “balance”
a Determine chemical equation
• Distinguish between a balanced and unbalanced chemical equation
Notes:
• Subscripts= In a chemical equation, these Subscripts
numbers indicate how many atoms are in the chemical formula for that substance.
Example: CO2, H2O2
• Coefficients= The large numbers in front of Coefficients
the chemical formula. They indicate how many parts of that substance are in a reaction. Example: 4CO2, 3H2O
For You To Do
• Consider the following chemical formula:
4Na2CO3
• Label the coefficients and subscripts
• Calculate the number of atoms:
# Na =
# C =
# O =
Activity
Balance the following chemical equations: 1. Zn (s) + HCl (aq) Æ ZnCl2 (aq) + H2 (g)
2. O2 (g) + H2 (g) Æ H2O (l)
3. O2 (g) + CH4 (g) Æ CO2 (g) +H2O (l)
4. NO2 (g) + H2O (l) Æ HNO3 (aq) + NO (g)
Check‐in
Looking back….
Does the following equation obey the law of conservation of matter? If not balance the equation:
CuCl2 (aq) + Na2S (aq) Æ CuS (s) + NaCl (aq)
•Identify the subscripts in the equation.
•Identify the coefficients in the balanced equation.