ChemThinks; Chemical Reactions

1. Starting materials in a chemical reaction are called reactants.

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

3. The arrow indicates a chemical reaction has taken place.

4. All reactions have one thing in common: there is a rearrangement of chemical bonds.

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

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

7. In every reaction there can never be any missing atoms or new atoms.

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

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 2 atoms of H and 2 atoms of O as reactants. This would make 1 molecule of H2O, but we’d have 1 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:
4 H2 + 2 O2 -> 2 H2O
Which is the same as:

11. This idea is called the Law of Conservation of Mass.

12. There must be the same mass and the same number of atoms before the reaction (in the reactants) and after the reaction (in the products).

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

14. In the unbalanced equation there are:

Reactants: 1 Cu atoms, 2 O atoms

Products: 1 Cu atoms, 1 O atoms

15. To balance this equation, we have to add 2 copper molecules to the products, because this reaction doesn’t make lone oxygen atoms.

16. When we added a molecule of CuO, now the number of oxygen atoms is balanced but the number of copper atoms don’t match. Now we have to add more copper atoms to the reactants.

17. The balanced equation for this reaction is

2 Cu + 2 O2 -> 2 CuO
This is the same thing as saying:
Reactants Products

# Cu atoms is 2 = #Cu atoms is 2

# O atoms is 2 = # O atoms is 2

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.) 1 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.) 2 KClO3  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.) 4 Al + 3 O2  2 Al2O3

SUMMARY


1) Chemical reactions always involve

2) The Law of Conservation of Mass says that the same atoms must be

3)To balance a chemical equation, you change the number in front of each substance until there are the same number of each type of atoms in both reactants and products.