1. Chapter 4 Chemical Reactions

2. All chemical reactions
have two parts:
Reactants - the substances you start with
Products- the substances you end up with
The reactants turn into the products.
Reactants ® Products

3. In a chemical reaction The way atoms are joined is changed
Atoms aren’t created or destroyed.
Can be described several ways:
1. In a sentence
Copper reacts with chlorine to form copper (II) chloride.
2. In a word equation
Copper + chlorine ® copper (II) chloride

4. Symbols in equations
the arrow separates the reactants from the products
Read “reacts to form”
The plus sign means “and”
(s) after the formula = solid
(g) after the formula = gas
(l) after the formula = liquid

5. Symbols used in equations
(aq) after the formula - dissolved in water, an aqueous solution.

6. Symbols used in equations
indicates a reversible reaction (more later)
shows that heat is supplied to the reaction
is used to indicate a catalyst is supplied, in this case, platinum.

7. What is a catalyst?
A substance that speeds up a reaction, without being changed or used up by the reaction.

8. Skeleton Equation Uses formulas and symbols to describe a reaction
doesn’t indicate how many.
All chemical equations are sentences that describe reactions.

9. Convert these to equations
Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas.
Fe2S3 (s) HCl (g)  FeCl3 (s) H2S (g)

10. Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water.
HNO3 (aq) Na2CO3 (s)  H2O (l) + CO2 (g) + NaNO3 (aq)

11. Now, read these: Fe(s) + O2(g) ® Fe2O3(s)
Cu(s) + AgNO3(aq) ® Ag(s) + Cu(NO3)2(aq)
NO2 (g) N2(g) + O2(g)

12. HALOGEN HON:
When any halogen (Group 17), hydrogen, oxygen, or nitrogen are by themselves in an equation, they are shown as DIATOMIC ELEMENTS.
H2 O2 N2 F2
Cl2
Br2 I2

13. Balancing Chemical Equations

14. Balanced Equation Atoms can’t be created or destroyed
All the atoms we start with we must end up with
A balanced equation has the same number of each element on both sides of the equation.

15. ® C + O2 ® CO2 This equation is already balanced What if it isn’t? O +

16. ® O + C C O O C + O2 ® CO We need one more oxygen in the products.
Can’t change the formula, because it describes what it is (carbon monoxide in this example)

17. ® Must be used to make another CO But where did the other C come from?+ C O C O
Must be used to make another CO
But where did the other C come from?

18. C C O O + O C C O
Must have started with two C
2 C + O2 ® 2 CO

19. Where the numbers are mean different things…..
4Pb(NO3)2
3- only relates to the oxygen atom (called a “subscript”)
2-relates to everything inside the brackets(called a “subscript”)
4-relates to every atom in the compound (called the “coefficient”)

20. Rules for balancing:
Assemble, write the correct formulas for all the reactants and products
Count the number of atoms of each type appearing on both sides
Balance the elements one at a time by adding coefficients (the numbers in front) - save H and O until LAST!
Check to make sure it is balanced.

21. Never Never change a subscript to balance an equation.
If you change the formula you are describing a different reaction.
H2O is a different compound than H2O2
Never put a coefficient in the middle of a formula
2 NaCl is okay, Na2Cl is not.

22. Example
H2 + O2
H2O
Make a table to keep track of where you
are at

23. Example
H2 + O2
H2O R P 2 H 2 2 O 1
Need twice as much O in the product

24. Example
H2 + O2 2
H2O R P 2 H 2 2 O 1
Changes the O

25. Example
H2 + O2 2
H2O R P 2 H 2 2 O 1 2
Also changes the H

26. Example
H2 + O2 2
H2O R P 2 H 2 4 2 O 1 2
Need twice as much H in the reactant

27. Example 2
H2 + O2 2
H2O R P 2 H 2 4 2 O 1 2
Recount

28. Example 2
H2 + O2 2
H2O R P 4 2 H 2 4 2 O 1 2
The equation is balanced, has the same
number of each kind of atom on both sides

29. Example R P 4 2 H 2 4 2 O 1 2 2 H2 + O2 ® 2 H2O This is the answer
Not this

30. Balancing Examples _Mg + _N2 ® _Mg3N2 _P4 + _O2 ® _P4O10 2 2 3 5
_AgNO3 + _Cu ® _Cu(NO3)2 + _Ag
_Mg + _N2 ® _Mg3N2
_P4 + _O2 ® _P4O10 2 2 3 5

31. Odd Number Rule
If ,when balancing, there happens to be an odd number (other than 1) of an element on one side, and an even number on the other side, DOUBLE THE COEFFICIENT IN FRONT OF WHERE IT’S ODD, AND THEN START OVER (with that element)