Chemical Equation Balancing Guide: Stoichiometry Tips

1. Chapter 3 Stoichiometry

2. Stoichiometry • The study of quantities of materials consumed and produced in chemical reactions.

3. 3.1 Chemical equations • The '+' is read as 'reacts with' and the arrow '' means 'produces'

4. Because atoms are neither created nor destroyed in a reaction, a chemical equation must have an equal number of atoms of each element on each side of the arrow (i.e. the equation is said to be 'balanced').

5. Balancing Equations • Write 'un-balanced' equation using formulas of reactants and products • Write 'balanced' equation by determining coefficients that provide equal numbers of each type of atom on each side of the equation (generally, whole number values) • Note! Subscripts should never be changed when trying to balance a chemical equation. Changing a subscript changes the actual identity of a product or reactant. Balancing a chemical equation only involves changing the relative amounts of each product or reactant.

6. Examples

8. We seem to be o.k. with our number of carbon atoms in both the reactants and products, but we have only half the hydrogen in our products as in our reactants. We can fix this by doubling the relative number of water molecules in the list of products:

9. Note that while this has balanced our carbon and hydrogen atoms, we now have 4 oxygen atoms in our products, and only have 2 in our reactants. We can balance our oxygen atoms by doubling the number of oxygen atoms in our reactants:

10. The physical state of each chemical can be indicated by using the symbols (g), (l), (s), and (aq) (for gas, liquid, solid, and aqueous respectively): Na (s) + H2O (l)  NaOH (aq) + H2(g)

11. Balance the following equations C2H5OH (aq) + O2 (g)  CO2 (g) + H2O (g) Fe (s) + O2 (g)  FeO2 (s) C2H4 (g)+ O2 (g)  CO2 (g) + H2O (l)

12. Answer 2Na (s) + 2H2O (l)  2NaOH (aq) + H2(g) Fe (s) + O2 (g)  FeO2 (s) C2H4 (g)+ 3O2 (g)  2CO2 (g) + 2H2O (l)

13. Which box represents the reaction between NO and O2 to produce NO2.

14. Homework Balancing / Writing Rxn wks

15. Classifying reactions movie

16. Chemical Reactivity Combination/Synthesis Reaction: 2 or more substances react to form one new product A + B  C + 

17. solid magnesium and oxygen gas react to produce solid magnesium oxide 2Mg (s) + O2(g)  2MgO (s) Metal nonmetal ionic compound Diatomic 2+ 2-

18. Decomposition Rxn • One substance undergoes a reaction to produce two or more substances. • Typically occurs when things are heated. AX  A + X  +

19. Solid calcium carbonate reacts to produce solid calcium oxide and carbon dioxide gas CaCO3 (s)  CaO (s) + CO2 (g) 2+ (2-) 2+ 2- 4+ 2(2-)

20. Single displacement • One element replaces a similar element in a compound A + BX  AX + B BX + Y  BY + X +  +

21. Solid copper is dissolved in aqueous silver nitrate to produce solid silver and aqueous copper II nitrate. Cu(s) + AgNO3 (aq) Ag(s) + Cu(NO3)2 (aq) Write the sentence for this reaction: Fe (s) + Cu(NO3)2 (aq)Fe(NO3)2 (aq)+ Cu (s)

22. Activity Series • We need to know what metals are most likely to oxidize others. • Example: We can’t store nickel nitrate in an iron container because the solution would eat through the container.

23. Activity Series • A list of metals arranged in order of decreasing ease of oxidation. • Page 139 table

24. Using activity series • Any metal on the list can be oxidized by the metal below it. • Give: FeCl2 + Mg • Find: will iron oxidize Magnesium metal? • I finger on Fe • 1 finger on Mg • Is the bound chemical below • Yes Fe is below Mg. • Then complete the reaction

25. Give: NaCl + Mg • Find: will sodium oxidize Magnesium metal? • I finger on Na • 1 finger on Mg • Is the bound chemical below • no • Then the reaction is not possible

26. What if you don’t have an Activity Series table? • Down Group 1 (I) the "Alkali Metals" the activity increases Cs > Rb > K > Na > Li • Down Group 2 (II) the activity increases e.g. Ca > Mg • On the same period, the Group 1 metal is more reactive than the group 2 metal • the group 2 metal is more reactive than the Group 3 metal, • All three are more reactive than the "Transition Metals". e.g. Na > Mg > Al (on Period 3) and K > Ca > Ga > Fe/Cu/Zn etc. (on Period 4)

27. Double Replacement Rxn/ Metathesis • The ions of two compounds exchange places in an aqueous solution to form two new compounds. AX + BY  AY + BX • One of the compounds formed is usually a precipitate, an insoluble gas that bubbles out of solution, or a molecular compound, usually water.

28. Double Replacement Rxn/ Metathesis AX + BY  AY + BX +  +

29. Write the sentence for these double replacement reactions KOH (aq) + H2SO4 (aq) K2SO4 (aq) + H2O (l) FeS (aq) + HCl (aq) FeCl2 (aq) + H2S (aq)

30. Combustion Reaction A substance combines with oxygen, releasing a large amount of energy in the form of light and heat. C3H8 (g)+5O2 (g)  3CO2 (g) + H2O (g) Usually CO2 (carbon dioxide) / CO (carbon monoxide) and water are produced.

31. Reactive elements combine with oxygen P4(s) + 5O2(g)  P4O10 (s) (This is also a synthesis reaction) • The burning of natural gas, wood, gasoline C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g)

32. Homework • Classifying types of Rxns worksheet

33. 3.3 Formula Weights • Although we can’t “count atoms” in a molecule directly, we can count them indirectly if we know their masses.

34. Formula Weights/ Molecular Weight • Sum of atomic masses of each atom in a molecule. F.W of H2SO4 = 2(H) + S + 4(O) 2(1) + 32 + 4(16) = 98amu 98 g/mol

35. We can describe composition in two ways1. number of atoms (amu) 2. % (by mass) of its elements. Percent Composition

36. Percent Composition We can find % mass of an atom in a compound from formula mass, by comparing each element present in 1 mole of compound to the total mass of 1 mole of compound

37. Percent Composition Equation % element = # of atoms element (atomic weight of the element ) * 100 Formula Weight

38. Example • Calculate the percentage of nitrogen in Ca(NO3)2

39. Think: % N = # N atoms (m.w N) X 100 m. w Ca(NO3)2

40. Answer % N = 2(14.02 N amu) X 100 164.12 Ca(NO3)2amu = 17%

41. Question • Calculate the percent composition of each element in C12H22O11

42. Homework Molar mass wks Percent composition wks

43. 3.4 The Mole!!!!! • The unit for dealing with, atoms, molecules, ions • Abbreviation = mol (oh the time you will save!)

44. History • Avogadro • Italian • 1776-1856

45. Molar Mass • A dozen eggs = 12 • A dozen elephants = 12 • But 12 eggs has a different weight than 12 elephants

46. Thus 1 mole of carbon is 6.02 x 1023 molecules but weighs 12 grams • 1 mole of sodium is 6.02 x 1023 molecules but weighs 23 g

47. If you had Avogadro's number of unpopped popcorn kernels, and spread them across the United States of America, the country would be covered in popcorn to a depth of over 9 miles.

48. If we were able to count atoms at the rate of 10 million per second, it would take about 2 billion years to count the atoms in one mole.

49. An Avogadro's number of standard soft drink cans would cover the surface of the earth to a depth of over 200 miles.

50. Molar mass of any substance is equal to its mass in atomic mass units (amu)