1. Chapter 7
Chemical Reactions

2. Homework Assigned Problems (odd numbers only)
“Questions and Problems” to 7.31 (begins on page 200)
“Additional Questions and Problems” to 7.49 (page 221)
“Challenge Questions” (page 222)

3. Chemical Reactions Physical changes: Chemical changes:
Involves no changes in chemical identity of a substance
No changes in physical properties (color, physical state, freezing point, boiling point)
Chemical changes:
A chemical reaction in which one or more substances changes to a different substance
Properties that matter exhibits as it undergoes changes in chemical composition

4. Chemical Reactions Reactants  Products
Chemical properties determine whether or not a substance can be changed to another substance
Reactions involve chemical changes in matter resulting in new substances
Reactions involve rearrangement and exchange of atoms to produce new molecules
Elements are not changed during a reaction
Chemical properties determine whether or not a substance can be changed into another substance
Reactants  Products

5. Changes During Chemical Reactions
A chemical change occurs when new substances are made
Conversion of material(s) into one or more new substances
These substances will have different properties from the original material
New properties are visible (visual clues)
Color change, precipitate formation, gas bubbles, flames, heat release

6. Changes During Chemical Reactions
Fe Fe2O3
Li LiOH, H2
HCO CO2
Lithium reacts with water to generate lithium hydroxide and hydrogen gas
Bicarbonate ion reacting with water to generate carbon dioxide
Na NaOH, H2

7. Changes During Chemical Reactions
In a chemical reaction:
At least one new substance is produced
Atoms are never created or destroyed
Every atom present as a reactant has to be present as a product
The atoms in reactants rearrange to form new products
From the result of a chemical reaction (during the process)
bonds in the reactants are broken, atoms rearrange and reform into products

8. Chemical Equations
A chemical equation is a written statement that uses symbols and formulas (no words) to describe the changes during a chemical reaction
It shows substances at the beginning of a reaction (reactants)
It shows substances formed in the reaction (products)
It is an abbreviated way of representing a chemical reaction on paper

9. Writing a Chemical Equation
Chemical reactions can be written as:
Word equations
Formula equations
reactants
products
Word equations use the chemical names of the reactants and products
Formula equations use symbols and formulas of the substances involved

10. Balancing Chemical Equations
A balanced chemical reaction has the same number of atoms of each element on both sides of the arrow
Atoms are neither created nor destroyed
Every atom must be accounted for
Equations are balanced by placing a coefficient in front one or more of the substances in the equation
A chemical reaction obeys the Law of Conservation of Mass
If there are 10 carbon atoms in the reactants, there must be 10 carbon atoms in the reactants

11. Symbols Used in Equations
Symbols used after chemical formula to indicate physical state
(g) = gas
(l) = liquid
(s) = solid
(aq) = aqueous, dissolved in water

12. Writing Chemical Equations
When magnesium metal burns in air it produces a white, powdery compound magnesium oxide
Burning in air means reacting with O2
Write the word equation
The reactants are to the left of the arrow
The products are to the right of the arrow
Two or more reactants or products are separated by a plus sign
A word description
Reactants on the left and products on the right, separated by an arrow
magnesium + oxygen magnesium oxide

13. Writing Chemical Equations
Indicate the physical state of each substance
Use the correct chemical symbol to indicate liquids and solids
Metals are solids, except for Hg which is liquid
Use molecular form for gases (H2, O2, N2, all halogens)
Identify polyatomic ions
Elements in the solid and liquid states are represented in equations by the chemical symbols for the element
magnesium(s) + oxygen(g) magnesium oxide(s)

14. Writing Chemical Equations
Convert the word equation into a formula equation
Use the correct chemical symbol to indicate liquids and solids
There must be the same number of each kind of atom on the reactant and product side of the equation
Determine if the equation is balanced
If not equal, must BALANCE
Elements in the solid and liquid states are represented in equations by the chemical symbols for the element
The arrow means “to produce”
Always use molecular not empirical formulas
Use the correct chemical symbol for the elements
Elements that are gases at room temperature are identified by their correct molecular form
___Mg (s) +___O2 (g) ___MgO(s)

15. Balancing Chemical Equations
Balance equations by the use of a coefficient placed to the left of a substance
NEVER change the subscripts of a compound to balance an element
It changes the identity of the compound
Can change coefficients but never subscript numbers
Adding coefficients to the equation will adjust the number of reactant and or product molecules
A coefficient will denote the amount of a substance

16. Balancing a Chemical Equation Example 12 2
Coefficient 2
1 Mg 2
1 Mg
2 O 2
1 O
The coefficient affects both of the elements

17. Balancing a Chemical Equation: Example 2
When solid ammonium nitrite is heated it produces nitrogen gas and water vapor
Write the formula equation

18. Balancing a Chemical Equation Example 2
2 x N
2 x N
2 x O 2
1 x O
4 x H 4
2 x H

19. Balancing a Chemical Equation: Example 3
Nitrogen monoxide gas decomposes to produce dinitrogen monoxide gas and nitrogen dioxide gas
Write the formula equation

20. Balancing a Chemical Equation Example 3
1 x N
3 x N 3
1 x O
3 x O

21. Balancing a Chemical Equation Example 4
Liquid nitric acid decomposes to reddish-brown nitrogen dioxide gas, liquid water and oxygen gas.
Write the formula equation

22. Balancing a Chemical Equation Example 42 2 2 4 2
1 x N 4 2
1 x N 12 6
3 x O 12 7
5 x O 4 2
1 x H 4
2 x H

23. Types of Reactions
Reactions are separated into groups of similar reactions
Based on the form of the equation for the reaction
Synthesis (combination)
Decomposition
Single replacement
Double replacement
Combustion

24. Types of Reactions Synthesis Reactions General form of equation:
Reactions in which two or more substances combine to form a third substance
(one product forms)
General form of equation:
Can have
Elements combine
An element and a compound
Two compounds
A + B AB

25. Synthesis Reactions The combinations can include Examples Two elements
An element and a compound
Two compounds
Examples

26. Types of Reactions Decomposition Reactions General Form of Equation
Reactions in which one reactant breaks down into simpler (smaller) substances
Generally initiated by addition of energy (electric current or heating substances to high temperature)
Opposite of a Synthesis Reaction
General Form of Equation AB
A + B

27. Decomposition Reactions
Can be broken down to:
Smaller compounds
Elements
Both
Examples
Symbol for heat is the delta triangle
Light is symbolized by hV

28. Types of Reactions Single replacement reactions
One element replaces another element
Forms a new compound which frees the replaced element
Most reactions occur in an aqueous solution
General Form of Equation
A + BC
AC + B

29. Single Replacement Reactions
Three types
Metal replaces a metal
Metal replaces hydrogen
Nonmetal replaces nonmetal
Examples
metal
replaces
metal fi
metal
replaces
hydrogen
nonmetal
replaces
nonmetal

30. Type of Reactions
Two compounds exchange ions or atoms to form new compounds
Also called exchange reactions
Shows the exchange of “associates” when comparing the reactants and products
General Form of Equation
AB + CD
AD + BC

31. Double Replacement Reactions
Most of these reactions occur in aqueous solution
Most involve acids, bases, and ionic compounds
Products formed
Precipitate (a solid that is insoluble)
A gas
Water

32. Double Replacement Reactions
Examples
precipitate
gas
water

33. Summary of Reaction Types

34. Combustion Reactions
Occurs when a hydrocarbon combines with oxygen which produces carbon dioxide, water and heat (flame)
The reaction of oxygen with any substance
If a combustion reaction is possible then the substance will burn
Commonly known as burning, the reaction of oxygen with any substance

35. Combustion Reactions Examples The combustion of propane gas
Produces carbon dioxide and water
Produces heat (flame)
The combustion of sulfur
Also a combination reaction
Also produces heat (flame)
hydrocarbon
The combustion of sulfur also produces sulfur trioxide

36. Energy in Chemical Reactions
In a chemical reaction
A change in energy occurs as bonds are broken (reactants) and new ones form (products)
Nearly all chemical reactions absorb or produce heat
Measured by the heat of reaction or enthalpy
Enthalpy change is the amount of heat produced or consumed in a process (∆H )

37. Heat of Reaction Endothermic reactions absorb heat as they occur
If (∆H ) is positive, then heat is added to the reaction
Exothermic reactions produce heat as they occur
If (∆H ) is negative, then heat is evolved by the reaction
Discuss definitions concerning the amount of heat produced or consumed

38. Heat of Reaction Photosynthesis reaction Cell metabolism
Carbon dioxide reacts with water to produce glucose and oxygen
Cell metabolism
Glucose reacts with oxygen to produce carbon dioxide and water
∆H = kJ
The energy from the sun is absorbed to produce glucose from carbon dioxide and water
The reverse of the reaction changes it from one that absorbs energy to one that produced energy. The quantity of the change in energy does not change, only the sign itself changes
∆H = kJ

39. Calculation of Heat in Reactions
The combustion of sulfur dioxide
It reacts with oxygen to produce sulfur trioxide
Calculate the heat produced when 75.2 g of sulfur trioxide is produced
∆H = kJ
Reacts or combines
Given 75.2 g SO3
Heat in kJ produced
when SO3 is formed

40. Calculation of Heat in Reactions
Relation between g of SO3 and heat released
Grams
of SO3
Moles
of SO3
Heat
of rxn
Molar
mass kj
Write the necessary conversion factors
Set up the problem
46.5 kJ

41. End