2. Writing Chemical Formulas Balancing Chemical Equations

3. Question: Why do we never change the subscript?

4. Chemical substances are represented by chemical formulas.

5. Chemical formulas contain the element symbol for each atom present…

6. …and subscripts which tell how many of each atom is in the molecule.

7. If only one of an atom is present, there is no subscript.

8. Water: H 2 O 2 hydrogen atoms 1 oxygen atom O H H

9. Carbon dioxide: CO 2 1 carbon atom 2 oxygen atoms

10. Carbon dioxide is a safe gas to breathe. Carbon monoxide, CO, is not.

11. What is the difference? CO 2 CO

12. If you change the number of atoms in a chemical formula, you change the molecule into something else.

13. NEVER CHANGE THE SUBSCRIPT!

16. If you write a formula incorrectly, you can set off a reaction that gives off unexpected, even toxic products.

17. So, NEVER CHANGE THE SUBSCRIPT!

18. Balancing Equations:

19. Question: Why do we need to write chemical equations to show the Law of the Conservation of Mass?

20. Chemists use chemical formulas in equations to show how atoms are recombined in a chemical reaction.

21. Carbon and oxygen form carbon dioxide:

22. C + O 2 CO 2

23. Two hydrogen gas molecules and one oxygen molecule combine to form water:

24. 2 H 2 + O 2 2H 2 O

25. Examine the reaction that forms water, H 2 O.

26. Pure oxygen and hydrogen do not occur in nature as single atoms, but as double molecules: O 2 and H 2

27. Why?

28. Since they must be combined as double molecules, they must be written as H 2 and O 2 in any equation.

29. Of course, we would NEVER…

30. Look at the equation again: 2 H 2 + O 2 2H 2 O

31. Consider the as an =

32. There are two oxygen atoms on the left side which is the reactant side.

33. There must be two oxygen atoms on the right side, which is the product side.

34. Those two oxygen atoms will form two water molecules. 2 H 2 O

35. There must also be an equal number of hydrogen atoms on the left and right sides.

36. In order for the to mean = I must put a 2 in front of the H 2

37. I now have 2 oxygen atoms and 4 hydrogen atoms on each side of the equation.

38. Is the equation balanced?

39. 2 H 2 + O 2 2H 2 O 4 H 4 H 2 O 2 O

41. For any chemical equation: Reactants on left Products on right

42. The basic form: reactants products

43. Rules for balancing equations: The amount of matter in a chemical reaction does not change.

44. Rules for balancing equations: The mass of the reactants will equal the mass of the products.

45. Rules for balancing equations: The number of atoms on the left must equal the number of atoms on the right.

46. Rules for balancing equations: To balance the equation, change the number in front of a molecule, NEVER the subscript.

47. Question: How does a balanced equation prove the Law of the Conservation of Mass?