2. Unit 6 Chemical Reactions Chapters 11 & 12

3. Diatomic Elements Elements that come in pairs if they are listed by themselves 7 diatomic elements H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2

4. Word Equations Describe chemical reaction in writing Reactants are written to the left of the arrow Products written to the right separate more than one reactant or product by a plus sign Iron + Oxygen  Iron (III) Oxide (The arrow means yields or produces.)

5. Chemical Equations The use of chemical formulas to describe chemical reactions. Skeleton equation: chemical equation that does not indicate the relative amounts of the reactants and products. (An Unbalanced EQN) Fe (s) + O 2 (g) Fe 2 O 3 (s) s = solid; g = gas; l = liquid; aq = aqueous

6. Balancing Chemical Equations 1.Write the skeleton equation 2.Coefficients 2.Coefficients are placed in front of the formulas to balance the equation. Never change subscripts!!

7. Rules for Writing and Balancing Equations: 1.Determine correct formulas for all reactants and products. 2.Write the skeleton equation by placing formulas for the reactants on the left and formulas for the products on the right with a yield sign (  ) in between. (If 2 or more reactants or products are involved, separate their formulas with plus (+) signs.

8. 3.Determine number of atoms of each element in the reactants and products. Count a polyatomic ion as a single unit if it appears unchanged on both sides of the equation. 4.Balance the elements one at a time by using coefficients. When no coefficient is written, it is assumed to be 1.

9. 5.Check each atom or polyatomic ion to be sure they are equal on both sides of the equation. 6.Make sure all the coefficients are in the lowest possible ratio

10. Hydrogen and oxygen react to form water. Write a balanced equation for this reaction. Step 1: H 2 + O 2 H 2 O Step 2: H 2 + O 2 2H 2 O Step 3: 2H 2 + O 2 2H 2 O

11. Review Problems: 1.Write a sentence that describes this chemical reaction: –Na + H 2 O NaOH + H 2 2.Sulfur burns in Oxygen to form sulfur trioxide. –Write the skeleton equation for this chemical reaction –Balance the skeleton equation

12. More Review Problems Balance the Following: Al 2 (SO 4 ) 3 + Ca(OH) 2  Al(OH) 3 + CaSO 4 H 3 PO 4 + NaOH  Na 3 PO 4 + HOH

13. Chemical Reactions The five general types of reactions are: 1.Combination (Synthesis) Reactions 2.Decomposition Reactions 3.Single-Replacement Reactions 4.Double-Replacement Reactions 5.Combustion

14. Combination Reactions Also called synthesis reactions Two or more reactants combine to form a single product Fe + S FeS Do you think that you can reverse this process??

15. Decomposition Reaction A single compound is broken down into 2 or more simpler products CaCO 3 CaO + CO 2

16. Single-Replacement RXN When atoms of one element replace the atoms of a second element in a compound. Mg + Zn(NO 3 ) 2 Mg(NO 3 ) 2 + Zn

17. Double-Replacement RXN Involve an exchange of positive ions between two reacting compounds. FeS + 2HCl H 2 S + FeCl 2

18. Combustion Reaction When an element or compound reacts with oxygen, often producing heat or light. (When it involves a hydrocarbon, the products are always carbon dioxide and water.) C 6 H 6 + O 2 CO 2 + H 2 O

19. A + B = AB(Marriage) AB = A+ B (Divorce) A + BC = AC + B(Affair) AB + CD = AD + CB(Switch Partners) C x H y + O 2 CO 2 + H 2 O

20. A Welder’s problem: A welder will use oxygen and acetylene (C 2 H 2 ) in a torch. The following reaction occurs: C 2 H 2 + O 2  CO 2 + H 2 O Balance the reaction so the welder can determine what the ratio of acetylene to oxygen that should be in his torch. What is that ratio? What type of reaction is this?

22. Using Everyday Equations A balanced chemical equation provides the same kind of quantitative information that a recipe does. Chemists use balanced chemical equations as a basis to calculate how much reactant is needed or product is formed in a reaction

23. STOICHIOMETRY calculation of quantities in chemical reactions.

24. Interpreting Chemical EQNs N 2 + 3H 2 2NH 3 What kinds of information can be derived from this equation? Think in terms of 1) atoms, 2) molecules, 3) moles, 4) mass, and 5) volume.

25. 1.Number of Atoms: –Two atoms of nitrogen react with 6 atoms of hydrogen to produce two atoms of nitrogen and six atoms of hydrogen. (Multiply the coefficients by the subscripts.) 2.Number of Molecules: –One molecule of nitrogen reacts with three molecules of hydrogen to produce two molecules of ammonia. (Coefficients)

26. 3.Moles: –One mole of nitrogen reacts with three moles of hydrogen to produce two moles of ammonia. (Coefficients) 4.Mass: –Although the number of moles of the reactants is not equal to the number of moles of product, the total number of grams is equal. Mass is conserved. (Law of Conservation of Mass)

27. 5.Volume: –If you assume standard temperature and pressure, each mole of gas represents 22.4 L. –Therefore, 22.4L of N 2 reacts with 67.2L H 2 to form 44.8L of NH 3 Only mass and atoms are conserved (equal) in every chemical reaction.

28. Chemical Calculations Mole to Mole Mole to Gram Gram to Gram g g mm mm g mm

29. gg mm Stoichiometry Road Map

30. Mole-Mole Calculations How many moles of ammonia (NH 3 ) are produced when 0.60 moles of nitrogen reacts with hydrogen? Step 1: Write Chemical EQN N 2 + H 2  NH 3 Step 2: Balance EQN N 2 + 3H 2  2NH 3

31. Step 3: Find known & unknown, map, then calculate Mole Ratio: 1 mol N 2 2 mol NH 3 Unknown: ? mol NH 3 Known: 0.60 mol N 2 0.60 mol N 2 x 2 mol NH 3 = 1.2 mol NH 3 1 mol N 2

32. Gram-Gram Calculations Calculate the number of grams of NH 3 produced by the reaction of 5.40 g of hydrogen with an excess of nitrogen. N 2 + 3H 2 2NH 3 5.40 g? grams g g mm gg m m 1 2 3

33. 5.40 g H 2 x 1 mol H 2 = 2.70 mol H 2 2.0 g H 2 Step 2 : Get a mole ratio from the equation. 3 mol H 2 2 mol NH 3 then 2.7 mol H 2 ? mol NH 3 Step 3 : Solve for the unknown number of moles. (2.7 x 2) / 3 = 1.8 moles NH 3 Step 1 : Change grams to moles.

34. Step 4 : Change moles of the unknown to grams. 1.8 mole NH 3 x 17.0 g NH 3 = 30.6 g NH 3 1 mol NH 3

35. Examples K 2 PtCl 4 (aq) + NH 3 (aq)  Pt(NH 3 ) 2 Cl 2 (s)+ KCl(aq) what mass of Pt(NH 3 ) 2 Cl 2 can be produced from 65 g of K 2 PtCl 4 ? How much KCl will be produced? How much from 65 grams of NH 3 ?

36. Making Chocolate Chip Cookies Ingredients in Kitchen (I have a BIG kitchen): 40 lbs of butter 2 lbs of salt 1 gallon of vanilla extract 80 lbs of chocolate chips 200 lbs of flour 150 lbs of sugar 10 lbs baking soda 2 eggs What’s going to determine how many cookies I can make?

37. Limiting Reactants limits or determines the amount of product that can be formed in a reaction. The reactant that isn’t used up is called the excess reagent

38. To determine the limiting reagent requires that you do two stoichiometry problems. Figure out how much product each reactant makes. The one that makes the least is the limiting reagent The other is the excess reagent

39. Other problem types include mass-volumevolume-volume mass-volume, volume-volume, particle-mass and particle-mass calculations. In any of these problems, the given quantity must be converted to moles before proceeding.

40. Example What is the maximum number of grams of Cu 2 S that can be formed when 80.0g of Cu reacts with 25.0g S? What is the limiting reagent? What is the excess reagent?