1. Chapter 7 “Chemical Reactions”

2. Describe how to write a word equation.
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. All chemical reactions…
have two parts:
Reactants = the substances you start with
Products = the substances you end up with
The reactants will turn into the products.
Reactants ® Products

4. Products
Reactants

5. In a chemical reaction
Atoms aren’t created or destroyed (the Law of Conservation of Mass)
A reaction can be described several ways:
In a sentence every item is a word
Copper reacts with chlorine to form copper (II) chloride.
In a word equation some symbols used
Copper + chlorine ® copper (II) chloride

6. In a skeleton equation – symbols and formulas are used, but the equation might not be balanced
N2O3(g) + H2O(l)  HNO2(aq)
In a balanced equation – symbols and formulas are used and there are the same number of atoms of each element on both sides of the equation

7. Symbols in equations? The plus sign means “and”
the arrow (→) separates the reactants from the products (arrow points to products)
Read as: “reacts to form” or yields
The plus sign means “and”
(s) after the formula means solid: Fe(s)
(g) after the formula means gas: CO2 (g)
(l) after the formula means liquid: H2O(l)

8. ­ ↑ used after a product indicates a gas has been produced: H2↑
(aq) after the formula means dissolved in water, an aqueous solution: NaCl (aq) is a salt water solution
­ ↑ used after a product indicates a gas has been produced: H2↑
¯ used after a product indicates a solid has been produced: PbI2↓

9. Symbols used in equations
(double arrow) indicates a reversible reaction
shows that heat is supplied to the reaction
means a catalyst is supplied (in this case, platinum is the catalyst)

10. Enzymes are biological or protein catalysts in your body.
What is a catalyst?
A substance that speeds up a reaction, or lowers the temperature when it can happen, but, is not changed or used up by the reaction.
Enzymes are biological or protein catalysts in your body.

11. The Skeleton Equation Uses formulas and symbols to describe a reaction
but doesn’t indicate how many; this means they are NOT balanced
All chemical equations are a description of the reaction.

12. Write a skeleton equation for:
Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas.

13. Write a skeleton equation for:
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)

14. Write a skeleton equation for:
Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water.

15. Write a skeleton equation for:
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)

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

17. Balanced Chemical Equations
Atoms can’t be created or destroyed in an ordinary reaction:
All the atoms we start with we must end up with (meaning: balanced!)
A balanced equation has the same number of each element on both sides of the equation.

18. Rules for balancing:
Write the correct formulas for all the reactants and products, using “+” and “→”
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) where you need more –
save balancing the H and O until LAST!
(hint: save O until the very last)
Double-Check to make sure it is balanced.

19. Never
Never change a subscript to balance an equation (You can only change coefficients)
If you change the subscript (formula) you are describing a different chemical.
H2O is a different compound than H2O2
Never put a coefficient in the middle of a formula; they must go only in the front
2NaCl is okay, but Na2Cl and Na2Cl are not.

20. Practice Balancing Examples
_AgNO3 + _Cu ® _Cu(NO3)2 + _Ag
_Mg + _N2 ® _Mg3N2
_P + _O2 ® _P4O10
_Na + _H2O ® _H2 + _NaOH
_CH4 + _O2 ® _CO2 + _H2O

21. Practice Balancing Examples
2 AgNO3 + 1 Cu ® 1Cu(NO3)2 + 2 Ag
3 Mg + 1N2 ® 1Mg3N2
4 P + 5 O2 ® 1P4O10
2 Na + 2 H2O ® 1 H2 + 2 NaOH
1 CH4 + 2 O2 ® 1 CO2 + 2 H2O

22. Types of Reactions There are probably millions of reactions.
they will fall into several categories.
We will learn: the 5 major types.

23. 5 Types of Equations Combination reactions R + S  RS
Decomposition reactions
RS  R + S
Single replacement
T + RS  TS + R
Double Replacement reactions
RS + TU  RU + TS
Combustion reactions (add oxygen)
Complete  CO2 + H2O
Incomplete  CO + H2O

24. 1. Combination Reactions
Combine = put together
2 substances combine to make one compound (also called “Synthesis”)
Ca + O2 ® CaO
SO3 + H2O ® H2SO4
often the reactants are two elements.
Mg + N2 ® Mg3N2

25. Formulas and balancing:
First write the correct formulas –subscripts can be written at this point, but not later while balancing!
Next balance by changing only the coefficients

26. 2. Decomposition Reactions
decompose = fall apart
one reactant breaks apart into two or more elements or compounds.
NaCl Na + Cl2
CaCO CaO + CO2
Energy (heat, sunlight, electricity, etc.) is usually required

27. 2. Decomposition Reactions
binary compounds break apart into the elements:
H2O H2 + O2
HgO Hg + O2

28. 3. Single Replacement Reactions
One element replaces another
Reactants must be an element and a compound.
Products will be a different element and a different compound.
Na + KCl ® K + NaCl
F2 + LiCl ® LiF + Cl2
(Cations switched)
(Anions switched)

29. 3. Single Replacement Reactions
Metals will replace other metals (and they can also replace hydrogen)
K + AlN ® K3N + Al
Zn + HCl ® ZnCl2 + H2
Think of water as: HOH
Metals replace the first H, and then combines with the hydroxide (OH).
Na + HOH ® NaOH + H2O

30. 3. Single Replacement Reactions
To predict whether or not a single replacement reaction will happen:
Some chemicals are more “active” than others
More active replaces less active
Table 7.2 on p. 191 –shows the Activity Series of Metals
Higher on the list replaces those lower.

31. The “Activity Series” of Metals
Lithium
Potassium
Calcium
Sodium
Magnesium
Aluminum
Zinc
Chromium
Iron
Nickel
Lead
Hydrogen
Bismuth
Copper
Mercury
Silver
Platinum
Gold
Higher activity
Metals can replace other metals, provided they are above the metal they are trying to replace
(for example, zinc will replace lead)
Metals above hydrogen can replace hydrogen in acids.
Metals from sodium upward can replace hydrogen in water.
Lower activity

32. The “Activity Series” of Halogens
Higher Activity
Halogens can replace other
halogens in compounds, provided they are above the halogen they are trying to replace.
Fluorine
Chlorine
Bromine
Iodine
Lower Activity
2NaCl(s) + F2(g) 
2NaF(s) + Cl2(g)
???
MgCl2(s) + Br2(g) 
No Reaction!
???

33. 4. Double Replacement Reactions
Two things replace each other.
Reactants must be two ionic compounds, in aqueous solution
NaOH + FeCl3 ®
The positive ions change place.
NaOH + FeCl3 ® Fe+3 OH- + Na+1 Cl-1
= NaOH + FeCl3 ® Fe(OH)3 + NaCl

34. 4. Double Replacement Reactions
Have certain “driving forces”, or reasons
Will only happen if one of the products:
a) doesn’t dissolve in water and forms a solid (a “precipitate”), or
b) is a gas that bubbles out, or
c) is a molecular compound (which will usually be water).

35. How do you know which type?
Look at the reactants: (E=element, C=compound)
E + E = Combination
C = Decomposition
E + C = Single replacement
C + C = Double replacement

36. 5. Combustion Reactions Combustion means “add oxygen”
When a compound made of only C and H (and maybe O) reacts with oxygen – commonly called “burning”
Complete combustion forms products of CO2 and H2O.
Incomplete combustion forms products of CO (or possibly just C) and H2O.

37. Combustion Reaction Examples:
C4H10 + O2 ® CO2 + H2O(complete)
C4H10 + O2 ® CO + H2O(incomplete)
C6H12O6 + O2 ® CO2 + H2O(complete)
C8H8 + O2 ® CO + H2O(incomplete)

38. SUMMARY: An equation... Describes a reaction
Must be balanced in order to follow the Law of Conservation of Mass
Can only be balanced by changing the coefficients.
Has special symbols to indicate the physical state, if a catalyst or energy is required, etc.

39. Reactions Come in 5 major types.
Identify the type by looking at the reactants.
Single Replacement happens based on the Activity Series
Double Replacement happens if one product is: 1) a precipitate (an insoluble solid), 2) water (a molecular compound), or 3) a gas.

40. 5 Types of Equations Combination reactions R + S  RS
Decomposition reactions
RS  R + S
Single replacement
T + RS  TS + R
Double Replacement reactions
RS + TU  RU + TS
Combustion reactions (add oxygen)
Complete  CO2 + H2O
Incomplete  CO + H2O