1. Chemical Reactions

2. 7-1 Describing Reactions

3. Chemical Equations
To describe a chemical change, describe what is present before and after the change.
Reactants: substances that undergo a change
Products: new substances that form
Using Equations to Represent Reactions
Word equations:
Reactants → Products
Carbon + Oxygen → Carbon dioxide
Chemical formulas simplify word equations:
C + O2 → CO2
Read “The reaction of C and O yields carbon dioxide.
Conservation of Mass Reviewed
The mass of the reactants equals the mass of the products
Number of atoms in the reactants equals the number of atoms in the products
Mass is neither created nor destroyed

4. Balancing Equations
When hydrazine burns in the presence of oxygen, the reaction produces nitrogen, water vapor and heat.
This chemical equation is at illustrated in the table to the right in an unbalanced equation and in a balanced equation.
Study the table carefully.

5. Practice Balancing Chemical Equations

6. Another Example… Four steps to remember

7. Let’s try some! 3 3 ____ AlBr3 + ____ K  ____ KBr + ____ Al
Count the atoms:
1 Aluminum atom Potassium atom Potassium atom Aluminum atom
3 Bromine atoms Bromine atom
Is it balanced?
No!
Add coefficients to balance atoms:
1 Aluminum atom Potassium atoms 3 Potassium atoms Aluminum atom
3 Bromine atoms Bromine atoms
Is it balanced?
YES!

8. Let’s try another! ____ FeO + ____ PdF2  ____ FeF2 + ____ PdO
Count the atoms:
1 Iron Palladium 1 Iron 1 Palladium
1 Oxygen 2 Flourine 2 Flourine 1 Oxygen
Is it balanced?
YES!
Do we need to add coefficients?
NO!

9. Need another? Okay! 4 6 ____ P4 + ____ Br2  ____ PBr3
Count the atoms:
4 Phosphorus Bromines 1 Phosphorus
3 Bromines
Is it balanced?
No!
Add coefficients to balance atoms:
4 Phosphorus Bromines 4 Phosphorus
12 Bromines
Is it balanced?
YES!

10. Let’s try a harder one! 2 3 3
____ CoBr3 + ____ CaSO4  ____ CaBr2 + ____ Co2(SO4)3
Count the atoms:
1 Co Ca Ca Co
3 Br 1 S Br S
4 O O
Is it balanced?
No!
Add coefficients to balance atoms:
2 Co 3 Ca Ca 2 Co
6 Br 3 S Br 3 S O O
Is it balanced?
YES!

11. Now Try Balancing Chemical Equations on your own
More practice:
1. Na+H2O→NaOH+H2
Show your work.
2. HCl+CaCO3→CaCl2+CO2+H2O
Show your work
3. Al+Cl2→AlCl3
2Na+2H20→2NaOH+H2
2HCl+CaCO3→CaCl2+CO2+H2O
2Al+3Cl2→2AlCl3

12. Counting with Moles
Counting things in terms of atoms and molecules is not always practical for chemists because chemical reactions involve large number so these particles.
This is why chemists use a counting unit called a mole.
1 mole = 6.02X particles
Known as Avogadro’s number
Just as the mass of 12 eggs does not equal the mass of 12 oranges, a mole of carbon has a different mass than a mole of sulfur.

13. Moles Continued
Molar mass for an element is the same as its atomic mass expressed in grams.
Carbon’s atomic mass is 12.0 amu so its molar mass is 12.0 grams.
Molar mass for a compound can be calculated by adding up the atomic masses of its atoms and expressing the result in grams.

14. Molar Mass Conversions

15. Chemical Calculations
Chemical equations can be read as recipes for making new substances.
In chemical reactions, the mass of a reactant or product can be calculated by using a balanced chemical equations and molar masses of the reactants and products.
The chemical equation tells you how to relate amounts of reactants to amounts of products.

17. 7-2 Types of Reactions
Different reactants can produce the same product.
Consider that CO2 is produced in the following multiple ways:
dropping hydrochloric acid on limestone
heating limestone
gasoline

18. Classifying Reactions: 1. Synthesis & 2. Decomposition
Synthesis reaction: Two or more substances react to form a single substance A + B → AB
Sodium and Chlorine 2Na + Cl2 → 2NaCl
Hydrogen and Oxygen 2H2 + O2 → 2H2O
Decomposition reaction: The opposite of synthesis and one in which a compound breaks down (the reactant must be a compound but the products can be elements or compounds) AB → A + B
Water decomposes into hydrogen gas and oxygen gas 2H2O → 2H2 + O2
Calcium carbonate in limestone decomposes into lime, CaO, and carbon dioxide CaCO3 → CaO + CO2

19. Classifying Reactions Continued:
3. Single Replacement Double Replacement

20. Classifying Reactions Continued: 5. Combustion
A combustion reaction is one in which a substance reacts rapidly with oxygen often producing heat and light.
Example:
natural gas contains methane (CH4)
combined with an unlimited supply of oxygen
the products are carbon dioxide, water, heat and light

21. Reactions as Electron Transfers: oxidation and reduction
The discovery of subatomic particles enabled scientists to classify certain chemical reactions as transfers of electrons between atoms.
A reaction where electrons are transferred from one reactant to another is called an oxidation-reduction reaction, or redox reaction.

22. 7-3 Energy Changes in Reactions

23. Chemical Bonds and Energy
Chemical energy is the energy stored in the chemical bonds of a substance.
Chemical reactions involve there breaking of chemical bonds I the reactants ad the formation of chemical bonds in the products.
Propane grill = site of a chemical reaction
Reactants = propane and oxygen
Products = carbon dioxide and water
Since heat is produced (energy), heat can be included in the equation indicating heat is released in the reaction from the reactants
In the combustion of propane, the bonds in propane and oxygen molecules are broken while bonds in carbon dioxide and water molecules re formed.
Oxygen, carbon dioxide, and water molecules all have energy stored in their bonds too.
Breaking Bonds Forming Bonds

24. Reviewing EndoThermic Reactions = Energy Absorbed
Chemical reactions occur when bonds break and new bonds form. They usually absorb heat or liberate heat(release it).
Endothermic reactions absorb energy when the bonds break
Energy is absorbed from nearby matter
Often temperature changes
Baking soda combined with vinegar absorbs heat, making it feel colder
When heat is added constantly (egg frying)
Mercury(II) oxide is heated to a temperature of about 450ºC, it breaks down into mercury and oxygen (decomposition)

25. Reviewing Exothermic Reactions = Energy Released
The making or breaking of chemical bonds results in a net release of energy
Example: combustion
Energy typically releases as heat
Fuel and oxygen in the plane’s engine release heat causing the gases in the engine to expand. This moves the gases out of the plane which exerts a force that moves the plane forward.

26. Exothermic and Endothermic Reactions
In any reaction, the chemical energy reaches a peak before the reactants change into products. The peak represents the amount of energy require to break the chemical bonds of the reactants (activation energy).

27. Conservation of Energy
Exothermic reaction = chemical energy of the reactants is converted into heat + the chemical energy of the products
Endothermic reaction = heat + the chemical energy of the reactants is converted into chemical energy of the products
The total amount of energy is the same before and after both types of reactions…the law of conservation of energy

28. 7-4 Reaction Rates

29. Reactions over time
A reaction rate is the rate at which reactants change into products over time.
Reaction rates tell you how fast a reaction is going.
How fast the reactants are being consumed
How fast the products are being formed
How fast energy is being absorbed or released
Five Factors Affect Reaction Rates
Temperature, surface area, stirring, concentration, catalysts
(inhibitors)

30. Factors Affecting Reaction Rates
Temperature: High = faster
Surface Area: More = faster
Stirring: Speeds up, Washing clothes
Concentration: More = faster
Catalysts: Speeds up, Enzymes
Bonus: Inhibitors: Slows reaction, preservatives

31. 7-5 Equilibrium

32. Types of Equilibria
Equilibrium is a state in which the forward and reverse path of a change take place at the same rate.
Physical Equilibrium in physical changes:
When a physical change does not go to completion, a physical equilibrium is established between the forward and reverse changes.
Chemical Equilibrium in chemical changes:
In a reversible reaction, the conversion of reactants into products and the conversion of products into reactants can happen simultaneously.
When a chemical reaction does not go to completion, a chemical equilibrium is established between the forward and reverse reactions.
Synthesis of sulfur trioxide from sulfur dioxide and oxygen

33. Factors Affecting Chemical Equilibrium
When a change is introduced to a system in equilibrium, the equilibrium shifts in the direction that relieves the change.
Class Participation Opportunity: Research the life of Louis Le Chatelier ( )
Example: Synthesis of Ammonia (system containing nitrogen, hydrogen and ammonia in equilibrium)
Temperature:
Increase temp…system removes heat, decreasing the amount of ammonia
Pressure
Increase pressure…produces fewer gas molecules, more ammonia is produced
Concentration
Remove ammonia, system would favor the forward reaction, producing more ammonia