1. Balancing equations: 1 st …review what we already know.

2. A formula is a representation of a molecule of an element or a molecule of a compound.

3. The subscript represents the number of atoms of a particular element in the molecule. 12 atoms of carbon 22 atoms of hydrogen 11 atoms of oxygen

4. If no subscript is shown. An understood one is indicated. 1 molecule of sulfuric acid 2 atoms of hydrogen 1 atom of sulfur 4 atoms of oxygen

5. A coefficient is a number in front of a formula and represents the number of molecules. If no coefficient is shown, one molecule is indicated. 2 molecules of sucrose 24 atoms of carbon 44 atoms of hydrogen 22 atoms of oxygen

6. An equation is a sentence showing what is happening in a chemical reaction.

7. The reactants are the substances (elements or compounds) that will react. They are on the left or starting side.

8. The products on the right side of the equation are what are formed or are produced during the reaction.

9. Chemical reactions do not create or destroy atoms, but only rearrange them Law of Conservation of Mass –During a chemical reaction matter is neither created nor destroyed –There must be as many atoms of each type on the left side of the equation as on the right side Proper coefficients are placed in front of formulas to balance atoms Subscripts cannot be changed –S 8 + 8O 2  8SO 2 http://www.nisd.net/secw w/science/science- taks/quiz23/mass%20qui z.htm 5 questions about conservation of mass. Allow pop-ups to view answers.

10. Balancing Chemical Reactions ReactantsProducts Element# Atoms N O N 2 + O 2 ---> N 2 O 2 1 1 1x2 1 1 1x1 2 1 These don’t match. So….

11. Balancing Chemical Reactions ReactantsProducts Element# Atoms N O N 2 + O 2 ---> N 2 O 1 1 1x2 1 1 1x1 2 2x1 2 1 2x2 4 2 4 2 Number of reactants equals number of products.

12. N 2 + O 2 ---> N 2 O 2, 1 ---> 2 N N O O O N N N N O N N 2 diatomic nitrogens and 1 diatomic oxygen become nitrous oxide or “laughing gas”.

13. Balancing Chemical Reactions ReactantsProducts Element# Atoms Al 1 1 H 1 2 Cl 1 3 6 2 1 111 To fix the Cl 3 To fix the H 2 To fix the Al 2 2 6 6

14. Which coefficients are needed to balance this equation? 2 To fix both the NO 3 and the K 1111111 111 To finish fixing the K and try out the CrO 4

15. ReactantsProducts Element# Atoms Pb1 Pb N2 N O10 O K2 K Cr1 Cr 1 1 1 2

16. Which coefficients are needed to balance this equation? 3 To fix our Cl 1 11 11 1 1 31 To fix our Ag 1

17. Which coefficients are needed to balance this equation? 1 111 3 To fix the Cl Now we have 3 H’s and 2 H’s

18. Which coefficients are needed to balance this equation? 1 111 6 To fix the Cl 2 To fix the H 3 To fix the Al 2

19. Which coefficients are needed to balance this equation? 3 To fix the SO 4 1111 11 111 3 To fix the Ca 2 To fix the Al and the OH 1 Finishes up the balanced equation. Check: Reactants 3Ca, 6O, 6H, 2Al, 3S and 12O Products: 3Ca, 3S, 12O, 2Al, 6O, 6H

20. Which one is already balanced correctly? The Al don’t match. The H don’t match. 1 1 1x3 for S and 4x3 for O

21. Which one is already balanced correctly? Al don’t match. Cl don’t match.

22. http://www.creative- chemistry.org.uk/gcse/revision/equations/0 2.htm This hyperlink has you balance equations. Instantly find out if you are correct or not.

23. As + NaOH → Na 3 AsO 3 + H 2 2 1 2 2 2 3 6 3 6 2 2

24. 241 2331 4 4 4 1 1 36 ?3 2 2 221

25. When the above equation is balanced, the coefficient for magnesium chloride is — A 0 B 1 C 2 D 4 2 ’04 Su #25

26. What are the coefficients that will balance this chemical equation? A 2, 1, 1 B 3, 4, 2 C 2, 2, 1 D 4, 3, 2 ’03 10 #45 1 lead 2 oxygens ≠ 3 oxygens Put a 2 here to make an even number of oxygens. 1 1 1 22 1 1 lead 2 lead 2 oxygens ≠ 4 oxygens 2 lead 4 oxygens = 4 oxygens

27. What is the coefficient for H 2 O when the equation is balanced? A 1 B 2 C 3 D 4 ’03 11 #19 1 1 1 1 1 To make the hydrogens ‘even’, We put the coefficient 2 here. And have a sum of 4 hydrogens, 2 potassiums and 2 oxygens 2 Then we’ll need to put coffecients on this side of the arrow to finish balancing it out. 22

28. To produce 4 molecules of sugar, a plant needs — F 6 molecules of hydrogen G 12 molecules of ATP H 18 molecules of water J 24 molecules of carbon dioxide ’03 10#10 Multiply the whole reaction by 4. 1 4( ) 4 x 6 carbon dioxide

29. Which chemical equation supports the law of conservation of mass? F 2H 2 O(l) H 2 (g) + O 2 (g) G Zn(s) + HCl(aq) ZnCl 2 (aq) + H 2 (g) H Al 4 C 3 (s) + H 2 O(l) CH 4 (g) + Al(OH) 3 (s) J CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) ’04 11 #32 4 hydrogens ≠ 2 hydrogens 2 oxygens 2 oxygens 1 zinc 1 zinc 1 hydrogens ≠ 2 hydrogens 1 chlorine 2 chlorines 4 aluminums 1 aluminum 3 carbons 1 carbon 2 hydrogens ≠ 3 hydrogens 1 oxygen 3 oxygens 1 carbons 1 carbon 4 hydrogens = 4 hydrogens 4 oxygens 4 oxygens

30. Why is the sum of the products’ energy in this reaction less than the sum of the reactants’ energy? A Energy is given off as heat. B The products absorb available energy. C Energy is trapped in the reactants. D The reactants’ energy is less than the melting point of glucose. ’04 11 #37 + = + ≠ Reactants = Products The energy not stored in the bonds of the carbon dioxide and water must have left in the form of heat.

31. According to the law of conservation of mass, how much zinc was present in the zinc carbonate? A 40 g B 88 g C 104 g D 256 g ’03 11 #39 64 + 192 = 152 + ? 64 + 192 – 152 = ?

32. If all the reactants in a chemical reaction are completely used, which of the following statements accurately describes the relationship between the reactants and the products? F The products must have a different physical state than the reactants. G The total mass of the reactants must equal the total mass of the products. H The reactants must contain more complex molecules than the products do. J The density of the reactants must equal the density of the products. Reactants become Products 2H 2 + O 2  2H 2 O 20 grams + 10grams = 30 grams ’04 Su #40

33. The chemical equation shows CaCO 3 being heated. Which of these statements best describes the mass of the products if 100 g of CaCO 3 is heated? A The difference in the products’ masses is equal to the mass of the CaCO 3. B The sum of the products’ masses is less than the mass of the CaCO 3. C The mass of each product is equal to the mass of the CaCO 3. D The sum of the products’ masses equals the mass of the CaCO 3. ’04 11 #29 Reactants → Products 100 g

34. When 127 g of copper reacts with 32 g of oxygen gas to form copper (II) oxide, no copper or oxygen is left over. How much copper (II) oxide is produced? F 32 g G 95 g H 127 g J 159 g ’04 11 #40 127 + 32 = ?

35. The illustrations show a conservation-of-mass experiment. The solution in the beaker lost mass because — F materials have less mass at high temperatures G the mass of the reactants and products was less than 100 g H sodium sulfate (Na 2 SO 4 ) is lighter than air J some of the water molecules turned into gas ’0410#30 Before Before(100 g) After After( less than 100 g) Some of the water evaporated because of the heat. REACTANTS →Products The sum of the reactants = the sum of the products.

36. According to this information, what is the chemical formula for aluminum sulfate? A AlSO 4 B Al 2 (SO 4 ) 3 C Al 3 (SO 4 ) 2 D Al 6 SO 4 ’04 11 #23

37. +1 +2 +3 +4 -3 -2 -1 0

38. Na P Li C

39. +1 +2 +3 +4 -3 -2 -1 0 Na +1 P -3 Li C

40. +1 +2 +3 +4 -3 -2 -1 0 Na +1 P -3 3 1 Li C

41. +1 +2 +3 +4 -3 -2 -1 0 Na +1 P -3 3 1 Li C

42. +1 +2 +3 +4 -3 -2 -1 0 Na +1 P -3 3 1 Li C

43. +1 +2 +3 +4 -3 -2 -1 0 Na 3 P Li C Balanced!

44. +1 +2 +3 +4 -3 -2 -1 0 Na 3 P Li +1 C ± 4

45. +1 +2 +3 +4 -3 -2 -1 0 Na 3 P Li +1 C ± 4 4 1

46. +1 +2 +3 +4 -3 -2 -1 0 Na 3 P Li +1 C ± 4 4 1

47. +1 +2 +3 +4 -3 -2 -1 0 Na 3 P Li 4 C Balanced!

48. According to this information, what is the chemical formula for aluminum sulfate? A AlSO 4 B Al 2 (SO 4 ) 3 C Al 3 (SO 4 ) 2 D Al 6 SO 4 ’04 11 #23

49. The chemical formula for calcium chloride is — F Ca 2 Cl G CaCl H CaCl 2 J Ca 2 Cl 3 ’04 11 #38