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Friday, March 11, 2011

Chem Think; Chemical Reactions


1. Starting materials in a chemical reaction are called:
1A. Reactants

2. The ending materials in a chemical reaction are called
2A. products

3. The arrow indicates a __________ has taken place.
3A. A chemical change
4. All reactions have one thing in common: there is a ______ of chem. bonds
4A.rearrangement

5. Chemical reactions always involve ______ old bonds, ______ new bonds, or both.
5A. breaking, forming

6. In all reactions we still have all of the _______ at the end that we had at the start.
6A. atoms

7. In every reaction there can never be any __________atoms or ________
7A. missing, new

8. Chemical reactions only _______________________ in the atoms that are already there.
8A. rearrange the bonds

9. Let’s represent a reaction on paper. For example, hydrogen gas (H2) reacts with oxygen gas (O2) to form water (H2O):

H2 + O2 --->H2O

If we use only the atoms shown, we’d have two atoms of H and two atoms of O as reactants. This would make one molecule of H2O, but we’d have one atom of O leftover. However, this reaction only makes H2O.

Remember: reactions are not limited to 1 molecule each of reactants. We can use as many as we need to balance
the chemical equation.
A balanced chemical reaction shows:
a) What atoms are present before (in the reactants) and after (in the products)
b) How many of each reactant and product is present before and after.

10. So to make H2O from oxygen gas and hydrogen gas, the balanced equation would be:

____ H2 + _____O2 ---> _____ H2O

Which is the same as:



11. This idea is called the _____________
11A. Law of Conservation of Masses

12. There must be the same _____________ and the same number of _____________
before the reaction (in the reactants) and after the reaction (in the products).
12A. elements, atoms

13. What is the balanced equation for this reaction? 2Cu + 1O2 → 2CuO

14. In the unbalanced equation there are:
14A.
15. To balance this equation, we have to add ______ molecules to the products, because this reaction doesn’t make lone _____ atoms.
15A. CuO, oxygen

16. When we added a molecule of CuO, now the number of _____ atoms is balanced but the number of ____ atoms don’t match. Now we have to add more _____ atoms to the reactants.
16A. Oxygen, Cu, Cu

17. The balanced equation for this reaction is

2 Cu + 2 O2 ---> 2 CuO

This is the same thing as saying:


18. What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.)
1 CH4 + 2 O2 → 2 H2O + 1 CO2


19. What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.)
2 N2 + 3 H2 → 2 NH3

20. What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.)

2KClO3 → 2 KCl + 3 O2


21. What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.)

SU M M A R Y

1. Chemical reactions always involve:
1A. breaking and/or making bonds

2.The Law of Conservation of Mass says that the same atoms must be:
2A. present prior and after the chemical reaction

3. To balance a chemical equation, you change the _____ in front of each substance until there are the same number of each type of __________ in both reactants and products
3A. coefficients, atom

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