1. Chapter 7
Reactions

2. Chemical Changes Substances react and form a new substance.
Reactants go in.
Products come out.
Reactants Products
Represented by chemical equations

3. Chemical Equations For the process of burning: Word equation:
Carbon + Oxygen → Carbon dioxide
Chemical equation:
C + O2 → CO2

4. Law of Conservation of Mass
Mass is neither created nor destroyed in a chemical reaction.
It may change form (solid to liquid or gas).
If 1 atom of Carbon goes into a reaction, 1 atom of carbon must come out. It can’t be lost or multiplied.

5. Chemical Reactions C + O2 → CO2
→ atom of carbon goes in and 1 atom of carbon comes out
2 atoms of oxygen go in and 2 atoms of oxygen come out
Mass is neither created nor destroyed.

6. Chemical Reactions
N2H4 + O2 → N2 + H2O + → + Why is this wrong?

7. Balancing Equations N2H4 + O2 → N2 + H2O + → + + Why is this wrong?
2 atoms of nitrogen go in and 2 come out
4 atoms of hydrogen go in but only 2 come out
2 atoms of oxygen go in but only 1 comes out
Mass appears to be lost and that cannot happen!!
N2H4 + O2 → N2 + H2O

8. Balancing Equations
You can balance equations by changing the coefficients.
Coefficients are numbers that come in front of the formulas.
Never!!!! Change the subscripts in a formula!!
Changing subscripts makes it a different compound.

9. Balancing Equations N2H4 + O2 → N2 + H2O N2H4 + O2 → N2 + 2H2O
The 2 in front of water is a coefficient.
Now 2 atoms of nitrogen go in and 2 come out.
4 atoms of hydrogen go in and (2x2=4) 4 come out.
2 atoms of oxygen go in and (2x1+2) 2 come out.

10. Steps for Balancing Equations
Count the atoms of each element on each side of the equation.
Then change coefficients 1 at a time until the equation is balanced.
REMEMBER: You cannot lose or gain atoms!!

11. Practice Balance the following equations: Cu + O2 → CuO N2 + H2 → NH3
K + Br2 → KBr
Na + Cl → NaCl

12. Practice Answers Cu + O2 → CuO Cu + O2 → 2CuO 2Cu + O2 → 2CuO
I add a coefficient of 2 to get 2 oxygen atoms but that also makes me have 2 copper atoms as products. I then have to add a 2 in front of copper. Now 2 copper in and out and 2 oxygen in and out.

13. Practice Answers N2 + H2 → NH3 N2 + H2 → 2NH3
First I add a 2 to the product to end with 2 nitrogen atoms, but now I am ending with 6 hydrogen atoms.
N2 + 3H2 → 2NH3
Adding a 3 in front of hydrogen gives me 6 atoms in the beginning as well.

14. Practice Answers K + Br2 → KBr K + Br2 → 2KBr
First I add a 2 to the product to end with 2 bromine atoms, but now I am also ending with 2 potassium atoms.
2K + Br2 → 2KBr
Adding a 2 in front of potassium fixes this.

15. Practice Answers Na + Cl → NaCl
1 atom of each go in and 1 atom of each come out
The equation is already balanced.

16. Moles
Because chemical reactions often involve large numbers of small particles, chemists use a counting unit called the mole to measure amounts of a substance.

17. Moles 1 mole = 6.02 X 1023 particles
1 mole of flowers = 6.02 X 1023 flowers or 602,000,000,000,000,000,000,000 flowers
1 mole of apples = 6.02 X 1023 apples
1 mole of oxygen = 6.02 X 1023 atoms of oxygen

18. Molar Mass
For an element, the molar mass is the same as the atomic mass expressed in grams.
So for carbon, the atomic mass is 12.0 amu, and the molar mass is 12.0 grams.
Element atomic mass molar mass
Oxygen amu grams
Nitrogen amu grams

19. Mole-Mass Conversions
Once you know the molar mass of a substance, you can convert moles of that substance into mass (or mass into moles).
Carbon has a molar mass of 12.0 grams.
How many moles are in 22 grams of carbon?
22 g X 1 mol/12.0 g = 1.83 moles or 1.8 moles in significant digits
mol is the abbreviation for moles

20. Mole-Mass Conversions
How many moles are in 35 grams of oxygen (in significant digits)?

21. Mole-Mass Conversions
How many moles are in 35.0 grams of oxygen (in significant digits)?
35.0 g X 1 mol/ g = 2.19 mol O

22. Finding the mass of a compound
To find the mass of a compound:
CO2
Find the mass of each element in the compound.
C =
O =

23. Finding the mass of a compound
C = g
O = g
Then multiply the mass by the number of atoms of each element.
There is only 1 atom of carbon so x 1 =
There are 2 atoms of oxygen (O2) so x 2 =

24. Finding the mass of a compound
Then add the masses of each element together.
g O
g C
g CO2

25. Practice How many moles are in 55.0 grams of CO2?
55.0 g X 1 mol / g = 1.25 mol CO2
How many moles are in 25.0 grams of H2O?
How many grams are in 2.3 moles of CO2?

26. Practice How many moles are in 25.0 grams of H2O?
x = (mass of H2O)
25.0 g x 1 mol/ g = 1.39 mol
How many grams are in 2.3 moles of CO2?
x 2 = (mass of CO2)
2.3 mol x g/1 mol = g = 1.0 x 102

27. Calculations
In chemical reactions, the mass of a reactant or product can be calculated by using a balanced chemical equation and molar masses of the reactants and products.

28. Calculations How much oxygen is required to make 144 grams of water?
2H2 + O2 → 2H2O
First determine how many moles of water you are trying to make.
Molar mass of water = (2) = grams
144 g H2O x 1 mol H2O/ g H2O = 7.99 mol H2O

29. Calculations 144 g H2O = 7.99 mol H2O 2H2 + O2 → 2H2O
Now you can use the balanced equation to convert.
(7.99 mol H2O) x (1 mol O2 )/(2 mol H2O) = 4.00 mol O2
Finally convert moles back to grams.
(4.00 mol O2) x ( g O2)/(1 mol O2) =
63.9 g O2

30. Practice Calculations
How much magnesium do you need to make 34.5 grams of Magnesium Oxide?
2Mg + O2 → 2MgO
How much sodium chloride will 22.3 grams of sodium make?
2Na + Cl2 → 2NaCl

31. Practice Calculations
How much magnesium do you need to make 34.5 grams of Magnesium Oxide?
2Mg + O2 → 2MgO
Molar mass of MgO = = g/ 1 mol MgO
34.5 g MgO x 1 mol MgO/ g MgO x 2 mol Mg/2 mol MgO x g Mg/ 1 mol Mg= 20.8 g Mg

32. Practice Calculations
How much sodium chloride will 22.3 grams of sodium make?
2Na + Cl2 → 2NaCl
22.3 g Na x 1 mol Na/ g Na x 2 mol NaCl/ 2 mol Na x ( =58.443)g NaCl/ 1 mol NaCl = 56.7 g NaCl