1. Chemical Reactions

2. Objectives:
The student will recognize the four signs that a chemical reaction is taking place.
The student will be able to identify the basic components of a chemical equation that describes a chemical reaction.
The student will verify the law of conservation of mass by balancing chemical equations.
Focus:
A solid produced by a chemical reaction in solution that separates from the solution is called
a. a precipitate. c. a molecule.
b. a reactant. d. the mass of the
product.
Assignment: None
A sign that a chemical reaction is taking place is release of energy in the form of ________or ________.
a. light, heat c. heat, chemical
b. light, d. sound,
chemical chemical

3. All chemical reactions have two parts
Reactants - A substance that participates in a chemical reaction. (the substances you start with)
Products - A substance that forms in a chemical reaction (the substances you end up with)
The reactants turn into the products.
Chemical equations describe chemical reactions in the following format.
Reactants → Products *

4. Chemical reactions occur when substances undergo chemical changes to form new substances
During a chemical reaction, a chemical change takes place.
Atoms are rearranged.
Knowledge about what products are produced in a chemical reaction is obtained by laboratory analysis.
A chemical reaction has NOT occurred if the products have the same chemical properties as the reactants.
The signs of a chemical change help us to recognize when chemical change occurs.

5. Signs of a chemical change
Emission of light or heat
Formation of a gas
Formation of a precipitate
Color change

6. Describing chemical reactions
The law of conservation of mass applies in chemical reactions.
Atoms aren’t created or destroyed.
The way atoms are joined is changed but the atoms also maintain their individual identity
This is communicated by using chemical formulas in the format of a chemical equation;
Example: Cu(s) + Cl2(g) → CuCl2(aq) *

7. Objectives:
The student will verify the law of conservation of mass by balancing chemical equations
Assignment: None
Focus:
What happens in a chemical reaction?
a. Atoms are destroyed.
b. Molecules are created.
c. Atoms are created.
d. Atoms are rearranged.
A substance that undergoes a change in a chemical reaction is
a. a product. c. a reactant.
b. a chemical. d. an enzyme.

8. Coefficients show the law of conservation of mass in chemical reactions
Sometimes chemical reactions need to be balanced so that the law of conservation of mass is communicated in the chemical equation
Changing the subscripts in the chemical formulas would change the identity of the chemicals involved in the reaction, so this is a no-no!
Instead, coefficients are added to the front of chemical formulas within the equation until the number and type of each element in the chemical reaction is the same on both sides of the chemical equation.

9. Examples of balancing a chemical equation with coefficients

10. Examples of balancing a chemical equation with coefficients

11. Examples of balancing a chemical equation with coefficients

12. Objectives:
The student will apply the law of conservation of energy to energy transfers during chemical reactions.
The student will describe the differences between exothermic and endothermic reactions.
Assignment: None
Focus:
When bread rises, this is a sign that a chemical reaction is producing __________________.
a. light c. gas
b. heat d. a precipitate
In an equation, the symbol for a substance in water solution is followed by
a. (1). c. (aq).
b. (g). d. (s).

13. Energy in Chemical Reactions
Some reactions are Spontaneous Reactions - Reactions that proceed immediately when two substances are mixed together without any energy input.  But not all reactions proceed spontaneously. 
Sometimes energy is required to break chemical bonds. These reactions require: Activation Energy – the amount of energy that is required to start a chemical reaction.
Once the correct amount activation energy is reached the reaction continues until you run out of material to react.  

14. Reaction Energy = Chemical Energy
Chemical Energy - The energy released when a chemical compound reacts to produce new compounds.
Breaking chemical bonds requires energy be absorbed.
Forming chemical bonds releases energy.
But chemical reactions also follow the law of conservation of energy.

15. Law of Conservation of Energy in Chemical Reactions
The total energy of the reactants before the reaction must be equal to the total energy of the products and their surroundings after the reaction.
If there is an imbalance in the amount of energy absorbed to break the bonds of the reactants and the amount of energy released in forming the bonds of the products, energy will either be released into the surroundings of the products or absorbed from the surroundings of the products.

16. Reaction Energy
Exothermic - A chemical reaction that releases heat into the surroundings.
If more energy is released as the products form than is absorbed to break the bonds in the reactants, the temperature of the surroundings rises because energy is released.
The energy released comes from the chemical energy of the reactants.
Endothermic - A chemical reaction that needs to absorb heat from the surroundings to proceed.
If more energy is required to break the bonds of the reactants than is released by forming the bonds in the products, the temperature of the surroundings fall as energy is absorbed from them.

17. Endothermic Reactions cont..
If enough heat cannot be absorbed from the surroundings of an endothermic reaction to make it takes place, additional heat or light energy may be needed for the reaction to take place
Plants use light energy from the sun to fuel the endothermic reaction we call photosynthesis

18. What is a catalyst?
A substance that speeds up a reaction without being changed by the reaction.
Enzymes are biological or protein catalysts. *

19. This graph shows that the amount of free energy needed for the reaction to progress is much greater without a catalyst than it is with a catalyst.