1. Pearson Prentice Hall Physical Science: Concepts in Action
Chapter 7
Chemical Reactions

2. 7.1 Describing Reactions Objectives:
1. Interpret chemical equations in terms of reactants, products and conservation of mass
2. Balance chemical equations by manipulating coefficients
3. Explain what a mole is and calculate molar mass

3. Reactants & Products
During a chemical equation, reactants (the stuff you start with) are changed into products (the stuff you finish with)
The products actually are composed of the same stuff you started with, but the molecules have been rearranged by chemical bonding to produce completely new compounds with unique physical properties
A basic chemical equation looks like this:
REACTANTS PRODUCTS

4. A chemical reaction is when atoms interact with each other to form new substances
What you put in = reactant
What comes out = product
Mass of reactants = mass of products
MASS IS CONSERVED

5. Conservation of Mass
Definition: a chemical equation is a chemical reaction in which reactants and products (separated by an arrow) are expressed as formulas
The Law of Conservation of Mass states that mass is neither created nor destroyed during a chemical reaction

6. Balancing Equations/Manipulating Coefficients
In order to show that mass is conserved during a reaction, a chemical equation must be balanced
Definition: a coefficient is the number that appears BEFORE a formula
Recall that subscripts are used to balance a formula
Coefficients are used to balance the equation

7. Steps to balancing a chemical equation:
1. Write out the chemical equation
2. Balance the most unusual element or compound first, use subscripts (charges) as a guide
3. Continue with other elements, ending with the most common
Example: Balance the chemical equation
ZnCl HF → ZnF + HCl

8. Moles & Molar Mass
Definition: a mole is the counting unit that measures the amount of substance
A mole contains 6.02 x 1023 particles (molecules, atoms, ions) of substance
Definition: molar mass is the mass of one mole
Molar mass is calculated by adding up the atomic masses of each element (multiplied by the subscript)
Example: CO2 carbon=12g oxygen x 2 = 32g & = 44g
The molar mass of CO2 is 44 g

9. 7.2 Types of Reactions Objectives:
1. Classify chemical reactions as synthesis (also called combination), decomposition (decomp), single replacement (single displacement) double replacement (double displacement) or combustion reactions

10. Synthesis or Combination Reactions
There are millions of substances but only five general types of reactions
The first is a synthesis reaction where two substances combine to form a new more complex substance
general form A + B → AB
Example: 2H2 + O2 → 2H2O

11. Decomposition
The second reaction type is decomposition, where one large substance breaks down to two or more smaller substances
general form AB → A + B
Example: 2NaClO3 → 2NaCl + 3O2

12. Single Replacement (Displacement)
The next reaction type is a single replacement reaction where atoms of one element take the place of an element in a compound
general form XA + B → A + XB
example 3CuCl2 + 2Al → 2AlCl3 + 3Cu

13. Double Replacement (Displacement)
A reaction type with a similar name is the double displacement where two compounds exchange their elements
general form AX + BY → AY + BX
example : AgNO3 + HCl → AgCl + HNO3

14. Combustion
The final type of reaction is a combustion reaction where a compound (usually a hydrocarbon) and oxygen burn
With enough oxygen the products are carbon dioxide and water
without enough the products are carbon monoxide and water
general form A + O2 → H2O + CO2
example 2CH4 + 4O2 → 4H2O + 2CO2

15. 7.3 Energy Changes in Reactions
Objectives:
1. Describe the energy changes that take place during chemical reactions
2. Classify chemical reactions as exothermic or endothermic
3. Explain how energy is conserved during chemical reactions

16. Energy Changes & Chemical Reactions
Indications of a chemical reaction include color change, gas formation, and temperature change
Atoms are just rearranged as bonds are broken and new ones formed
Breaking bonds requires energy
Forming bonds releases energy

17. Definition: chemical energy is the energy stored in the chemical bonds of a substance
Chemical reactions involve the breaking of chemical bonds in the reactants & the formation of chemical bonds in the products
Q: Does breaking chemical bonds require energy or release energy?

18. Exothermic & Endothermic
Definition: Reactions that release energy are exothermic
Definition: Reactions that absorb energy are endothermic
ENERGY IS CONSERVED.
Example: for an exothermic reaction energy stored in bonds (chemical energy) = energy released during reaction

19. Conservation of Energy
Law of Conservation of Energy: during a chemical reaction, energy is neither created nor destroyed
In exothermic reactions, the chemical energy of the reactants is converted to heat plus the chemical energy of the products

20. 7.4 Reaction Rates Objectives: 1. Explain what a reaction rate is
2. Describe factors that affect chemical reaction rates

21. What a Reaction Rate is
Definition: the reaction rate is the rate at which reactants change into products over time
Reaction rates tell you how fast a reaction is going

22. Factors that Affect Reaction Rates
Reactions can happen at different speeds
Factors influencing reaction rates:
Temperature: rxn’s go faster at higher temp
Surface area: large surface area speeds up reaction
Large particles have less surface area than the same amount of small particles (blocks)

23. Stirring or agitation (shaking) increases exposure of reactants & increases reaction rate
Concentrated solutions react faster (better chance of atoms hitting each other)
Rxn’s go quicker at higher pressure
Catalysts speed up the reaction by lowering the energy needed (do not influence the products)
Enzymes are biological catalysts

24. 7.5 Equilibrium Objectives:
1. Identify and describe physical and chemical equilibria
2. Describe the factors affecting chemical equilibrium

25. Physical & Chemical Equilibrium
Definition: equilibrium is a state in which the forwards and reverse paths take place at the same rate
Some reactions are reversible, a double arrow indicates the reaction can go either way
Equilibrium is when the amount of reactants being turned into products is the same as the amount of products being turned into reactants
reactants ↔ products

26. When a physical change does not go to completion, a physical equilibrium is established between the forward and reverse changes
H2O(l) H2O(g)
Definition: a reversible reaction is a reaction in which the conversion of the reactants into products can happen simultaneously (at the same time)

27. When a chemical reaction does not go to completion, a chemical equilibrium is established between the forward and reverse reactions
2SO2(g) + O2(g) SO3(g)
Q: What happens during chemical equilibrium?

28. Factors Affecting Chemical Equilibrium
LeChatelier’s principle predicts what will happen to equilibrium as a change is introduced to the system
LeChatelier’s principle: if a change is made to a system at equilibrium the system will shift to oppose or relieve the change

29. Increasing temp favors the reaction that absorbs energy
Increasing pressure favors the reaction that produces less gas
Increasing concentration favors the reaction that produces less of that substance
CONDITION
Tempera-ture
Pressure
Concentra-tion

30. Example: N2 (g)+ H2 (g) ↔ NH3 (g) + heat
Which way will the equilibrium shift when the reaction is cooled? When the pressure is raised? When more hydrogen is added?