Friday, Aug. 30th: “A” DayTuesday, Sept. 3rd: “B” DayAgenda • Go over quiz/review worksheet • Section 8.2: “Balancing Chemical Equations” • Homework: • Sec. 8.2 review, pg. 274: #1-6 • Chemical Formulas/Equations Worksheet • Pg. 329: #15 – 24/ Pg. 330: # 1- 20 • “Writing Chemical Formulas” worksheet • Concept Review: “Balancing Chemical Equations”
Reactions Conserve Mass • A basic law of science is the conservation of mass. • Law of conservation of mass: in ordinary chemical or physical changes, mass is neither created nor destroyed. • If the products of a chemical reaction are all collected, their total mass will be the same as the total mass of the reactants.
Reactions Rearrange Atoms • The products and reactants of a chemical reaction are made up of the same number and kinds of atoms. • The atoms are just rearranged and connected differently. ? Na + ? H2O ? NaOH + ? H2 • To show that a reaction satisfies the law of conservation of mass, the equation must be BALANCED!
Balancing Equations • To balance an equation, the number of atoms of each element needs to be the same on each side of the arrow. • But, the formulas cannot be changed. • CO cannot be changed to CO2 to balance the oxygen atoms – these are two different molecules with different properties. • Place numbers, called coefficients, in FRONT of the formulas…
Coefficients • Coefficient: a small, whole number that appears as a factor in front of a formula in a chemical reaction. • A coefficient multiplies the number of atoms of each element in the formula. • For example: H2O: 1 water molecule contains 2 hydrogen atoms and 1 oxygen atom 2 H2O: 2 water molecules contain 4 hydrogen atoms and 2 oxygen atoms
Diatomic Elements • For this section, you will need to remember that the following elements are diatomic and exist as molecules: H2, O2, N2, F2, Cl2, Br2, I2 **Write these at the top of your notes**
How to Balance Chemical EquationsSkills Toolkit (pg. 268) • Identify reactants and products. • Write an unbalanced equation. • Count atoms. • Balance the atoms that appear in only 1 reactant and 1 product first. • Balance O2 and H2 last. • Treat polyatomic ions as a unit if they appear on both sides of arrow. • Insert coefficients to balance 1 element at a time. • Check to make sure you did it right!
Balancing an EquationSample Problem A, pg. 269 Balance the equation for the reaction of iron(III) oxide with hydrogen to form iron and water. 1. Identify reactants and products. Iron(III) oxide + hydrogen iron and water ? H2 Fe H2O • We need to figure out the formulas for our reactants and products.
Balancing an EquationSample Problem A, pg. 269 cont. • What is the formula for iron(III) oxide? • Iron(III) means that the charge on the iron cation is 3+ • Oxide is oxygen, whose ion charge is always 2- Fe3+ O2- Fe2O3 Fe2O3 + H2 Fe + H2O (unbalanced) • Write an unbalanced equation: Fe2O3 + H2 Fe + H2O
Sample Problem A, cont. Fe2O3 + H2 Fe + H2O 2. Count atoms. Balance Fe (Iron) 1st because it appears in only 1 reactant and 1 product. Fe2O3 + H22 Fe + H2O • Balance O2 (Oxygen) next: Fe2O3 + H2 2 Fe + 3 H2O • Lastly, balance H2 (Hydrogen): Fe2O3 + 3 H22 Fe + 3 H2O • Is everything balanced? YES
Additional Example Balance the following equation: C3H8 + O2 CO2 + H2O • Balance C (carbon) first: C3H8 + O23 CO2 + H2O • Next, balance H (hydrogen): C3H8 + O2 3 CO2 + 4 H2O • Lastly, balance O (oxygen): C3H8 + 5 O2 3 CO2 + 4 H2O 10 oxygen atoms = 6 + 4 oxygen atoms
Never Change Subscripts to Balance an Equation • Unbalanced Equation: H2 + O2 H2O Incorrect: H2 + O2 H2O2 • H2O H2O2 • H2O is water; H2O2 is hydrogen peroxide • They are NOT the same thing! • Instead, add coefficients to balance… Correct: 2 H2 + O2 2 H2O
Sample Problem BPg. 271 The reaction of ammonia with oxygen produces nitrogen monoxide and water vapor. Write a balanced equation for this reaction. • Identify reactants and products. • Reactants: NH3 and O2 • Products: NO and H2O • Write unbalanced equation: NH3 + O2 NO + H2O
Sample Problem B, Cont. NH3 + O2 NO + H2O • Count atoms. • Balance N (nitrogen) first: It’s already balanced – yeah! • Balance H (hydrogen) next: 2 NH3 + O2 NO + 3 H2O • Recheck the N, it is now unbalanced, fix it: 2 NH3 + O22 NO + 3 H2O • Lastly, try to balance the O (oxygen): 2 NH3 + O22 NO + 3 H2O 2 oxygen 5 oxygen atoms
Sample Problem B, Cont. 2 NH3 + 2.5 O22 NO + 3 H2O 5 oxygen 5 oxygen atoms • In order to get the oxygen atoms to be equal, on both sides, the O2 coefficient needs to be 2.5 • But, coefficients need to be small, whole numbers so this is incorrect. • To solve, simply double ALL of the coefficients: 4 NH3 + 5 O24 NO + 6 H2O • Is everything balanced? YES
Sample Problem C, Pg. 273 Aluminum reacts with arsenic acid, HAsO3, to form H2 and aluminum arsenate. Write a balanced equation for this reaction. • Identify reactants and products. • Reactants: Al and HAsO3 • Products: H2 and Al(AsO3)3 • Write unbalanced equation: Al + HAsO3 H2 + Al(AsO3)3
Sample Problem C, Cont. Al + HAsO3 H2 + Al(AsO3)3 • Count atoms. • First, balance the Al It’s already balanced – yeah! • Next, balance the arsenate polyatomic ion: Al + 3 HAsO3 H2 + Al(AsO3)3 • Next, balance the H (hydrogen): Al + 6 H AsO33 H2+ Al(AsO3)3 • This messes up the AsO3 ion, fix it: Al + 6 H AsO3 3 H2 + 2 Al(AsO3)3
Sample Problem C, Cont. Al + 6 H AsO3 3 H2 + 2 Al(AsO3)3 • Are we balanced yet? NO! • Now the Al (aluminum) is unbalanced, fix it: 2 Al + 6 H AsO3 3 H2 + 2 Al(AsO3)3 • How about now? Yes, we’ve finally balanced the equation!
Practice Makes Perfect • The best way to get better at balancing equations is to practice, a lot! • You don’t have to balance polyatomic ions as a unit if you don’t want to. You can balance the individual elements if you find that easier. • Sometimes, balancing equations is tricky and you need to use trial and error…
Homework • Sec. 8.2 review, pg. 274: #1-6 • Chemical Formulas/Equations WS • Pg. 329: #15 – 24 • Pg. 330: # 1- 20 • “Writing Chemical Formulas” worksheet • Concept Review: “Balancing Chemical Equations” Next time: Quiz over section 8.2: “Balancing Chemical Equations” High School Open House: Wednesday, Sept. 4th: 6:30 – 8:00 pm