Lesson 28 Writing & Balancing Equations Objectives: - The student will write chemical equations representing reactions. - The student will balance chemical equations. PA Science and Technology Standards: A; A; B PA Science and Technology Standards: A; A; B PA Mathematics Standards: A, E, A

I.Atoms and Mass are conserved in chemical reactions reactions a. There are word equations and chemical equations for a reaction i. Hydrogen + Oxygen  Water i. Hydrogen + Oxygen  Water ii. H 2 + O 2  H 2 O ii. H 2 + O 2  H 2 O b. The only problem with the chemical equation we have written is that there is more oxygen on the left than on the right – this is not possible considering the law of conservation of mass. c. Also, when we measure the masses of the reactants and the products, there is no difference. d. We need to find some way to show that the reaction has occurred, but following this information

II. Coefficients indicate amounts of reactants and products a. We can place numbers in front of each of the reactants and products to indicate the amount of that reactant or product that is present in the reaction. b. We call these numbers coefficients c. We cannot change the subscripts associated with any of the elements involved in the reaction. d. Coefficients can only go in front of compounds or single elements present – not in the middle of a chemical formula.

e. Example i. Unbalanced --- H 2 + O 2  H 2 O i. Unbalanced --- H 2 + O 2  H 2 O ii. Balanced --- 2H 2 + O 2  2H 2 O ii. Balanced --- 2H 2 + O 2  2H 2 O f. The process of inserting coefficients to make sure that the law of conservation of mass is followed is called “balancing an equation”.

g. Steps for balancing an equation (start where you need to in order to solve the problem you are given – you don’t always need to start at the beginning) i. Read the problem 1. In a gas hot water heater, methane gas is combined with oxygen, and in the process, produces carbon dioxide and water vapor. ii. Write out the words in equation form. ii. Write out the words in equation form. 1. Methane + Oxygen  Carbon Dioxide + Water iii. Write the correct formulas for the different elements and compounds present. iii. Write the correct formulas for the different elements and compounds present. 1. CH 4 + O 2  CO 2 + H 2 O

iv. Count the number of elements on each side and make a chart. Make sure the elements are in the same order on both sides to keep from making the problem any more confusing. iv. Count the number of elements on each side and make a chart. Make sure the elements are in the same order on both sides to keep from making the problem any more confusing. 1. Left side:Right side: 1. Left side:Right side: C atoms – 1C atoms – 1 H atoms – 4H atoms – 2 O atoms – 2O atoms – 3 CH 4 + O 2  CO 2 + H 2 O

v. Insert coefficients for atoms of one element at a time so that the law of conservation of mass is satisfied 1. Tips for balancing equations a. First, balance the types of atoms that appear in only one reactant and one product b. Balance the remaining types of atoms one at a time c. Balance H atoms and O after most of the other elements have been balanced d. If the same polyatomic ions appear on both sides of the equation, treat them as if they were single units, like monatomic ions. e. If you get stuck – sometimes it is a good idea to start over from scratch.

vi. Repeat steps iii. and iv. until the law of conservation of mass holds for all of the elements in the equation. vii. Go back and check your math – simple mistakes can cause large problems. viii.Make sure that there are equal numbers of atoms of each element on both sides of the equation. viii.Make sure that there are equal numbers of atoms of each element on both sides of the equation. ix. Finally, check to see that all of the coefficients are in lowest terms. ix. Finally, check to see that all of the coefficients are in lowest terms.

i. Common Pitfalls Many students forget one or more of the final three steps. These are the places where mistakes made during the earlier stages of the problem are caught and corrected. Since it is relatively easy to make these mistakes, and just as easy to complete these three steps, always remember to apply them to every problem you complete! 1. Check your math! 2. Check for equal #’s of elements! 3. Check for lowest terms!

h. Second example I. Write the word equation, showing all of the reactants and products I. Write the word equation, showing all of the reactants and products Phosphorus + potassium chlorate  potassium chloride + phosphorus (V) oxide

i. More examples i. CaSi 2 + SbCl 3  Si + Sb + CaCl 2 i. CaSi 2 + SbCl 3  Si + Sb + CaCl 2

ii. C 2 H 2 + O 2  CO 2 + H 2 O

iii. Al + CH 3 OH  (CH 3 O) 3 Al + H 2 iii. Al + CH 3 OH  (CH 3 O) 3 Al + H 2

iv. Cellular respiration is the process that your body uses to get energy from the food you eat. In cellular respiration, sugars such as glucose, C 6 H 12 O 6, react with oxygen. The net result is an increase of energy and the production of carbon dioxide and water. Write the balanced equation for cellular respiration.

v. When calcium is added to water, calcium hydroxide and hydrogen gas are formed. When calcium hydroxide is heated, water and calcium oxide are the products. Write balanced chemical equations that describe this series of changes.

Writing and Balancing Equations PA Mathematics Standards: A, E, A Level 1 1.Balance each of the following equations. a.Zn + H 2 SO 4  ZnSO 4 + H 2  b.Na + Br 2  NaBr c.H 2 O + energy  H 2  + O 2  d.Cl 2 + KI  KCl + I 2 e.HNO 3 + LiOH  H 2 O + LiNO 3 f.N 2 + O 2  N 2 O g.NH 4 Cl + NaOH  NH 4 OH + NaCl h.HCl + K 2 CO 3  KCl + H 2 O + CO 2  2. Balance each of the following equations. Sodium + iodine  sodium iodide Zinc + hydrobromic acid  zinc bromide + hydrogen  Potassium hydroxide + heat  potassium oxide + water Magnesium + water  magnesium hydroxide + hydrogen 

Level 2 1.Balance each of the following equations. a.K 3 PO 4 + HCl  KCl + H 3 PO 4 b.Na + HNO 3  NaNO 3 + H 2  c.S + O 2  SO 3  d.Ca(ClO 3 ) 2 + heat  CaCl 2 + O 2  2.Balance each of the following equations. a.Potassium iodide + lead (II) nitrate  potassium nitrate + lead (II) iodide b.Iron (III) oxide + carbon  carbon monoxide  + iron c.Mercury (II) oxide + heat  mercury + oxygen  d.Calcium + aluminum chloride  calcium chloride + aluminum e.Mercury (I) nitrate + sodium carbonate  sodium nitrate + mercury (I) carbonate. f.Potassium bromide + aluminum nitrate  potassium nitrate + aluminum bromide g.Calcium phosphate + aluminum sulfate  calcium sulfate + aluminum phosphate h.Rubidium + acetic acid  rubidium acetate + hydrogen 

Level 3 Write and balance chemical equations for each of the following reactions. 1.Nitrogen + hydrogen  ammonia  2.butane (C 4 H 10 ) + oxygen + heat  carbon dioxide  + water  3.aluminum oxide  aluminum + oxygen  4.ethyl alcohol (C 2 H 5 OH) + oxygen + heat  carbon monoxide  + water  5.nitrogen + oxygen  dinitrogen pentoxide  6.octane (C 8 H 18 ) + oxygen + heat  carbon dioxide  + water  7.aluminum sulfate + phosphoric acid  aluminum phosphate + sulfuric acid 8.diphosphorus pentoxide + water  phosphoric acid 9.ammonia + nitric oxide  nitrogen  + water 10.iron (III) oxide + carbon monoxide  iron + carbon dioxide  11.copper + nitric acid  copper (II) nitrate + nitric oxide  + water 12.iron (II) sulfide + oxygen  iron (III) oxide + sulfur dioxide 