1. Describing Chemical Reactions
Prentice-Hall Chapter 11.1
Dr. Yager

2. Objectives Describe how to write a word equation
Describe how to write a skeleton equation
Describe the steps for writing a balanced chemical equation

3. Reactant + Reactant  Product + Product
Word Equations
To write a word equation, write the names of the reactants to the left of the arrow separated by plus signs. Write the names of the products to the right of the arrow, also separated by plus signs.
Reactant + Reactant  Product + Product

4. Methane + Oxygen  Carbon dioxide + Water

5. iron + oxygen  iron(III) oxide

6. The Word Equation (Not generally used)
Reactants → Products
Example:
Iron + Oxygen → Iron (III) oxide or
Iron + Oxygen → Iron (II) oxide
How do you tell the difference?

7. Hydrogen Peroxide  Water and Oxygen

8. Chemical Equations
A chemical equation is a representation of a chemical reaction where the formulas of the reactants (on the left) are connected by an arrow with the formulas of the products (on the right).

9. Here is the equation for rusting:
Skeletal Equations
A skeleton equation is a chemical equation that does not indicate the relative amounts of the reactants and products.
Here is the equation for rusting:
Fe + O2  Fe2O3

10. Skeleton Equation Reactants → Products Fe + O2 → Fe2O3
How do you get Fe2 from Fe and O3 from O2?
We do this by balancing equations.
4Fe + 3O2 → 2Fe2O3
You cannot make or destroy matter!
It also helps to know if it is solid, liquid or gas.
4Fe(s) + 3O2(g)→ 2Fe2O3(s)

11. Catalyst
A catalyst helps or speeds up a reaction, but is not used up in the reaction.
Example:
H2O2 → H2O + O2
This happens faster in the presence of MgO2:
MgO2

12. Without Catalyst With Catalyst

16. Balancing the Equation
To write a balanced chemical equation:
First, write the skeleton equation.
Then use coefficients to balance the equation so that it obeys the law of conservation of mass.

17. Balancing the Equation
4Fe + 3O2 → 2Fe2O3
The coefficients in yellow balance the equation.
Law of the Conservation of Matter: There are the same number and types of atoms on the left side as on the right side.
4Fe = 2Fe2
3O2 = 2O3

19. The Reaction to Make Water

20. Balanced chemical equation:
2 H2 (g) + O2 (g) → H2O (l)
2 x 2 “H” = 4 “H” x 2 “H” = 4 “H”
1 x 2 “O” = 2 “O” x 1 “O” = 2 “O”
Steps:
Write the unbalanced equation.
Inventory the atoms in reactants and products.
Balance the equation.
Strategies:
Balance elements left-to-right on the Periodic Table (metals first, then nonmetals)
Leave H and O until last.

21. __Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g)
1. Write the unbalanced equation.
__Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g)

22. __Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g)
2. Inventory reactants and products.
__Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g) Zn Cl H 1 Zn Cl H 1 2

23. __Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g)
3. Balance the equation by changing coefficients.
__Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g) 2 Zn Cl H 1 Zn Cl H 1 2
BALANCED ! 2
BALANCED !

24. __CH4 (g) + __O2 (g)  __CO2 (g) + __H2O (l)
Balance this reaction:
__CH4 (g) + __O2 (g)  __CO2 (g) + __H2O (l)

25. C H O 1 4 2 C H O 1 2 3 Balance this reaction:
__CH4 (g) + __O2 (g)  __CO2 (g) + __H2O (l) C H O 1 4 2 C H O 1 2 3

26. __CH4 (g) + __O2 (g)  __CO2 (g) + __H2O (l)1 4 2 C H O 1 2 3 4 4 4

27. Balance This Equation K Cl O 1 3 2 6 K Cl O 1 2 2 6
__KClO3 (s)  __KCl (s) + __O2 (g) 2 2 3 K Cl O 1 3 2 6 K Cl O 1 2 2 6

28. Al(OH)3(s) + H2SO4(aq)  Al2(SO4)3(aq) + H2O(l)6 Al S H O 1 5 7 2 2 Al S H O 2 3 13 2 1 3 3 8 12
6x2=12 10 18
12+6=18

29. Practice Problems _AgNO3 + _H2S → _Ag2S + _HNO3
_Zn(OH)2 + _H3PO4 → _Zn3(PO4)2 + _H2O
_FeCl3 + _Ca(OH)2 → _Fe(OH)3 + _CaCl2
_CuS + _O2 → _Cu + _SO2
_Fe2O3 + _H2 → _Fe + _H2O

30. Practice Problems 2AgNO3 + 1H2S → 1Ag2S + 2HNO3
_Zn(OH)2 + _H3PO4 → _Zn3(PO4)2 + _H2O
_FeCl3 + _Ca(OH)2 → _Fe(OH)3 + _CaCl2
_CuS + _O2 → _Cu + _SO2
_Fe2O3 + _H2 → _Fe + _H2O

31. Practice Problems 2AgNO3 + 1H2S → 1Ag2S + 2HNO3
3Zn(OH)2 + 2H3PO4 → 1Zn3(PO4)2 + 6H2O
_FeCl3 + _Ca(OH)2 → _Fe(OH)3 + _CaCl2
_CuS + _O2 → _Cu + _SO2
_Fe2O3 + _H2 → _Fe + _H2O

32. Practice Problems 2AgNO3 + 1H2S → 1Ag2S + 2HNO3
3Zn(OH)2 + 2H3PO4 → 1Zn3(PO4)2 + 6H2O
2FeCl3 + 3Ca(OH)2 → 2Fe(OH)3 + 3CaCl2
_CuS + _O2 → _Cu + _SO2
_Fe2O3 + _H2 → _Fe + _H2O

33. Practice Problems 2AgNO3 + 1H2S → 1Ag2S + 2HNO3
3Zn(OH)2 + 2H3PO4 → 1Zn3(PO4)2 + 6H2O
2FeCl3 + 3Ca(OH)2 → 2Fe(OH)3 + 3CaCl2
1CuS + 1O2 → 1Cu + 1SO2
_Fe2O3 + _H2 → _Fe + _H2O

34. Practice Problems 2AgNO3 + 1H2S → 1Ag2S + 2HNO3
3Zn(OH)2 + 2H3PO4 → 1Zn3(PO4)2 + 6H2O
2FeCl3 + 3Ca(OH)2 → 2Fe(OH)3 + 3CaCl2
1CuS + 1O2 → 1Cu + 1SO2
1Fe2O3 + 3H2 → 2Fe + 3H2O

36. 1. Propane gas reacts with oxygen to produce water vapor and carbon dioxide. Choose the correct word equation for this reaction.
propane + carbon dioxide  water + oxygen
propane + oxygen + water  carbon dioxide
propane + oxygen + water + carbon dioxide
propane + oxygen  water + carbon dioxide

37. 1. Propane gas reacts with oxygen to produce water vapor and carbon dioxide. Choose the correct word equation for this reaction.
propane + carbon dioxide  water + oxygen
propane + oxygen + water  carbon dioxide
propane + oxygen + water + carbon dioxide
propane + oxygen  water + carbon dioxide

38. 2. Which of the following is a skeleton equation?
H2 + CO  CH3OH
2H2 + CO  CH3OH
2H2 + CO2  CH3OH
hydrogen + carbon monoxide  methanol

39. 2. Which of the following is a skeleton equation?
H2 + CO  CH3OH
2H2 + CO  CH3OH
2H2 + CO2  CH3OH
hydrogen + carbon monoxide  methanol

40. 3. What coefficient for H2SO4 is required to balance the following equation?
Ca3(PO4)2 + ____ H2SO4  3CaSO4 + 2H3PO4 1 2 3 4

41. 3. What coefficient for H2SO4 is required to balance the following equation?
Ca3(PO4)2 + ____ H2SO4  3CaSO4 + 2H3PO4 1 2 3 4