1) Chemical reactions always involve the breaking and forming of chemical bonds. 2) The Law of Conservation of Mass says that the same atoms must be present before and after the reaction. 3) 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.

1. name per date
BULGRIN Integrated Physical Science
Chemthink: CHEMICAL REACTIONS
a) Go to www.chemthink.com. Log in using your user name and password. (If you forgot, you need to WAIT
QUIETLY until your teacher is done introducing the lesson)
b) Scroll to CHEMICAL REACTIONS then the tutorial under the Chemical Reactions menu.
c) Complete the tutorial. As you complete the tutorial, write down the answers to the questions below.
d) Once you have successfully completed the tutorial and this worksheet, complete the Question Set.
CHEMICAL REACTIONS TUTORIAL QUESTIONS:
1. Starting materials in a chemical reaction are called
2. The ending materials in a chemical reaction are called
3. The arrow indicates a has taken place.
4. All reactions have one thing in common: there is a of chemical bonds.
5. Chemical reactions always involve old bonds, new bonds, or both.
6. In all reactions we still have all of the at the end that we had at the start.
7. In every reaction there can never be any atoms or
8. Chemical reactions only 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 atoms of H and atoms of O as reactants. This would
make molecule of H2O, but we’d have 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:
# of atoms in # of atoms in
Element
Reactants Products
H
O
11. This idea is called the
12. There must be the same and the same number of before the reaction (in the reactants)
and after the reaction (in the products).
13. What is the balanced equation for this reaction? ____ Cu + _____ O2  ____ CuO

2. 14. In the unbalanced equation there are: Reactants  Products
Cu atoms _____ Cu atoms ______
O atoms _____ O atoms ______
15. To balance this equation, we have to add ______ molecules to the products, because this reaction doesn’t make
lone _____ atoms.
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.
17. The balanced equation for this reaction is
____ Cu + _____ O2  ____ CuO
This is the same thing as saying: Reactants  Products
# Cu atoms _____ = # Cu atoms ______
# O atoms _____ = # O atoms ______
18. What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.)
_____ CH4 + _____ O2  _____ H2O + _____ CO2
# of atoms in # of atoms in
Element
Reactants Products
1 C 1
4 H 2
2 O 3
19. What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.)
_____ N2 + _____ H2  _____ NH3
# of atoms in # of atoms in
Element
Reactants Products
2 N 1
2 H 3

3. 20. What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.)
_____ KClO3  _____ KCl + _____ O2
# of atoms in # of atoms in
Element
Reactants Products
1 K 1
1 Cl 1
3 O 2
21. What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.)
_____ Al + _____ O2  _____ Al2O3
# of atoms in # of atoms in
Element
Reactants Products
1 Al 2
2 O 3
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 in front of each substance
until there are the same number of each type of in both reactants and products