ChemThink; 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 CHANGE 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 1 atoms of H and 2 atoms of O as reactants. This would
make WATER 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:
2 H2 + 4 O2 = 2 H2O Which is the same as:
11. This idea is called the LAW OF THE CONSERVATION OF MASS
12. There must be the same ATOMS and the same number of MASS before the reaction (in the reactants) and after the reaction (in the products).
13. What is the balanced equation for this reaction? TWO Cu + FOUR O2 = 2 CuO
14. In the unbalanced equation there are:
-->Reactants Cu atoms ONE O atoms TWO
-->Products Cu atoms TWO O atoms TWO
15. To balance this equation, we have to add Cu molecules to the products, because this reaction doesn’t make lone Cu atoms.
16. When we added a molecule of CuO, now the number of OXYGEN atoms is balanced but the number of Cu atoms don’t match. Now we have to add more Cu atoms to the reactants.
17. The balanced equation for this reaction is:
TWO Cu + TWO O2  TWO CuO
-->Reactants: Cu atoms TWO O atoms TWO
-->Product:s Cu atoms TWO O atoms TWO
18. What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.)
ONE CH4 + TWO O2 = TWO H2O + ONE CO2
19. What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.)
ONE N2 + THREE H2 = TWO NH3
20. What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.)
TWO KClO3 = TWO KCl + THREE O2
21. What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.)
FOUR Al + THREE O2 = TWO Al2O3

SUMMARY
Chemical reactions always involve: Breaking bonds, creating bonds, or both.
The Law of Conservation of Mass says that the same atoms must be That chemical reactions rearrange bounds but the same atoms must be there before and after the chemical reaction.
To balance a chemical equation, you change the Coefficients in front of each substance until there are the same number of each type of atom in both reactants and products