Chemical Reactions CHAPTER 10 10.2 Methods for Balancing Chemical Equations

Cellular respiration What is the number of C, H and O atoms on each side of the reaction arrow?

Cellular respiration Reactants Products Carbon (C) Oxygen (O) Hydrogen (H)

6 8 12 Cellular respiration Reactants Products Carbon (C) Oxygen (O) Hydrogen (H) 12

6 1 8 3 12 2 Cellular respiration Reactants Products Carbon (C) Oxygen (O) 8 3 Hydrogen (H) 12 2

6 1 8 3 12 2 Not balanced! Cellular respiration Reactants Products Carbon (C) 6 1 Oxygen (O) 8 3 Hydrogen (H) 12 2 Not balanced!

Cellular respiration (unbalanced equation) Let’s find a method to balance a chemical equation

Write down the unbalanced chemical equation. C6H12O6 + 6O2 6CO2 + 6H2O

C6H12O6 + 6O2 6CO2 + 6H2O C appears in 1 compound Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. C6H12O6 + 6O2 6CO2 + 6H2O C appears in 1 compound H appears in 1 compound O appears in 2 compounds C appears in 1 compound H appears in 1 compound O appears in 2 compounds

C6H12O6 + 6O2 6CO2 + 6H2O C appears in 1 compound Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. C6H12O6 + 6O2 6CO2 + 6H2O Balance C appears in 1 compound H appears in 1 compound O appears in 2 compounds C appears in 1 compound H appears in 1 compound O appears in 2 compounds Note: We could also balance H first.

C6H12O6 + 6O2 6CO2 + 6H2O ? ? C appears in 1 compound 6 C Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. ? C6H12O6 + 6O2 6CO2 + 6H2O ? Balance C appears in 1 compound 6 C C appears in 1 compound 1 C

C6H12O6 + 6O2 6CO2 + 6H2O 1 6 C appears in 1 compound 6 C Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. 1 C6H12O6 + 6O2 6CO2 + 6H2O 6 The number of C atoms is now balanced Balance C appears in 1 compound 6 C C appears in 1 compound 6 C

C6H12O6 + 6O2 6CO2 + 6H2O 1 6 ? C appears in 1 compound Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. Continue with the rest of the elements. If a free element is present, it is balanced last. 1 C6H12O6 + 6O2 6CO2 + 6H2O 6 ? H does not appear in CO2, so the coefficient for CO2 is unaffected. C appears in 1 compound H appears in 1 compound O appears in 2 compounds C appears in 1 compound H appears in 1 compound O appears in 2 compounds Balance Note: Balance O last; oxygen is a free element in O2.

C6H12O6 + 6O2 6CO2 + 6H2O 1 6 ? H appears in 1 compound 12 H Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. Continue with the rest of the elements. If a free element is present, it is balanced last. 1 C6H12O6 + 6O2 6CO2 + 6H2O 6 ? H appears in 1 compound 12 H H appears in 1 compound 2 H Balance

C6H12O6 + 6O2 6CO2 + 6H2O 1 6 6 H appears in 1 compound 12 H Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. Continue with the rest of the elements. If a free element is present, it is balanced last. 1 C6H12O6 + 6O2 6CO2 + 6H2O 6 6 H appears in 1 compound 12 H H appears in 1 compound 12 H Balance

C6H12O6 + 6O2 6CO2 + 6H2O 1 ? 6 6 C appears in 1 compound Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. Continue with the rest of the elements. If a free element is present, it is balanced last. 1 C6H12O6 + 6O2 6CO2 + 6H2O ? 6 6 C appears in 1 compound H appears in 1 compound O appears in 2 compounds C appears in 1 compound H appears in 1 compound O appears in 2 compounds Balance

C6H12O6 + 6O2 6CO2 + 6H2O 1 ? 6 6 O appears in 2 compounds Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. Continue with the rest of the elements. If a free element is present, it is balanced last. 1 C6H12O6 + 6O2 6CO2 + 6H2O ? 6 6 O appears in 2 compounds (1 x 6) + (? x 2) O appears in 2 compounds (6 x 2) + (6 x 1) Balance

C6H12O6 + 6O2 6CO2 + 6H2O 1 ? 6 6 O appears in 2 compounds Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. Continue with the rest of the elements. If a free element is present, it is balanced last. 1 C6H12O6 + 6O2 6CO2 + 6H2O ? 6 6 O appears in 2 compounds (1 x 6) + (? x 2) = 6 + (? x 2) = 18 O appears in 2 compounds (6 x 2) + (6 x 1) = 12 + 6 = 18 We know that the number of O atoms must be the same on both sides of the reaction arrow. Balance

C6H12O6 + 6O2 6CO2 + 6H2O 1 6 6 6 O appears in 2 compounds Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. Continue with the rest of the elements. If a free element is present, it is balanced last. 1 C6H12O6 + 6O2 6CO2 + 6H2O 6 6 6 O appears in 2 compounds (1 x 6) + (? x 2) = 6 + (6 x 2) = 18 O appears in 2 compounds (6 x 2) + (6 x 1) = 12 + 6 = 18 Balance

Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. Continue with the rest of the elements. If a free element is present, it is balanced last. Check each element to make sure that the equation is balanced. 1 C6H12O6 + 6O2 6CO2 + 6H2O 6 6 6 6 C 12 H 18 O 6 C 12 H 18 O

C6H12O6 + 6O2 6CO2 + 6H2O C6H12O6 + 6O2 6CO2 + 6H2O 1 6 6 6 Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. Continue with the rest of the elements. If a free element is present, it is balanced last. Check each element to make sure that the equation is balanced. Make sure the coefficients are the smallest possible whole numbers. 1 C6H12O6 + 6O2 6CO2 + 6H2O 6 6 6 C6H12O6 + 6O2 6CO2 + 6H2O

Another example CH4 + 2O2 CO2 + 2H2O Write down the unbalanced chemical equation. CH4 + 2O2 CO2 + 2H2O

CH4 + 2O2 CO2 + 2H2O C appears in 1 compound H appears in 1 compound Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. CH4 + 2O2 CO2 + 2H2O C appears in 1 compound H appears in 1 compound O appears in 1 compound C appears in 1 compound H appears in 1 compound O appears in 2 compounds

CH4 + 2O2 CO2 + 2H2O C appears in 1 compound H appears in 1 compound Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. CH4 + 2O2 CO2 + 2H2O Balance C appears in 1 compound H appears in 1 compound O appears in 1 compound C appears in 1 compound H appears in 1 compound O appears in 2 compounds Note: We could also balance H first.

CH4 + 2O2 CO2 + 2H2O ? ? C appears in 1 compound 1 C Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. ? CH4 + 2O2 CO2 + 2H2O ? Balance C appears in 1 compound 1 C C appears in 1 compound 1 C

CH4 + 2O2 CO2 + 2H2O 1 1 C appears in 1 compound 1 C Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. 1 CH4 + 2O2 CO2 + 2H2O 1 Balance C appears in 1 compound 1 C C appears in 1 compound 1 C Already balanced

CH4 + 2O2 CO2 + 2H2O 1 1 ? C appears in 1 compound Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. Continue with the rest of the elements. If a free element is present, it is balanced last. 1 CH4 + 2O2 CO2 + 2H2O 1 ? C appears in 1 compound H appears in 1 compound O appears in 1 compound C appears in 1 compound H appears in 1 compound O appears in 2 compounds Balance Note: Balance O last; oxygen is a free element in O2.

CH4 + 2O2 CO2 + 2H2O 1 1 ? H appears in 1 compound 4 H Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. Continue with the rest of the elements. If a free element is present, it is balanced last. 1 CH4 + 2O2 CO2 + 2H2O 1 ? H appears in 1 compound 4 H H appears in 1 compound 2 H Balance

CH4 + 2O2 CO2 + 2H2O 1 1 2 H appears in 1 compound 4 H Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. Continue with the rest of the elements. If a free element is present, it is balanced last. 1 CH4 + 2O2 CO2 + 2H2O 1 2 H appears in 1 compound 4 H H appears in 1 compound 4 H Balance

CH4 + 2O2 CO2 + 2H2O 1 ? 1 2 C appears in 1 compound Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. Continue with the rest of the elements. If a free element is present, it is balanced last. 1 CH4 + 2O2 CO2 + 2H2O ? 1 2 C appears in 1 compound H appears in 1 compound O appears in 1 compound C appears in 1 compound H appears in 1 compound O appears in 2 compounds Balance

CH4 + 2O2 CO2 + 2H2O 1 ? 1 2 O appears in 1 compound 2 Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. Continue with the rest of the elements. If a free element is present, it is balanced last. 1 CH4 + 2O2 CO2 + 2H2O ? 1 2 O appears in 1 compound 2 O appears in 2 compounds (1 x 2) + (2 x 1) Balance

CH4 + 2O2 CO2 + 2H2O 1 ? 1 2 O appears in 1 compound 2 Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. Continue with the rest of the elements. If a free element is present, it is balanced last. 1 CH4 + 2O2 CO2 + 2H2O ? 1 2 O appears in 1 compound 2 O appears in 2 compounds (1 x 2) + (2 x 1) = 2 + 2 = 4 Balance

CH4 + 2O2 CO2 + 2H2O 1 2 1 2 O appears in 1 compound 4 Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. Continue with the rest of the elements. If a free element is present, it is balanced last. 1 CH4 + 2O2 CO2 + 2H2O 2 1 2 O appears in 1 compound 4 O appears in 2 compounds (1 x 2) + (2 x 1) = 2 + 2 = 4 Balance

Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. Continue with the rest of the elements. If a free element is present, it is balanced last. Check each element to make sure that the equation is balanced. 1 CH4 + 2O2 CO2 + 2H2O 2 1 2 1 C 4 H 4 O 1 C 4 H 4 O

Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. Continue with the rest of the elements. If a free element is present, it is balanced last. Check each element to make sure that the equation is balanced. Make sure the coefficients are the smallest possible whole numbers. 1 CH4 + 2O2 CO2 + 2H2O 2 1 2 CH4 + 2O2 CO2 + 2H2O

Strategy to balance a chemical equation: Write down the unbalanced chemical equation. Identify the element that occurs in only one compound on both sides, and balance it first. Continue with the rest of the elements. If a free element is present, it is balanced last. Check each element to make sure that the equation is balanced. Make sure the coefficients are the smallest possible whole numbers.