1. Chapter 7 – Chemical Reactions

2. 7.1 – Describing Reactions

3. What is a chemical change?
Chemical Change: occurs when a substance reacts and forms one or more new substances
EXAMPLES: baking a cake, leaves changing color in the fall, food digesting in your stomach, paper burning

4. Evidence of a Chemical Change
Some clues that a chemical change has taken place:
change in color
production of gas
formation of precipitate 
Click HERE for video (production of a gas)

5. Chemical Reactions
When a substance undergoes a chemical change, a chemical reaction is said to have taken place.  
In order to understand chemical reactions, you must be able to describe them!

6. Describing Chemical Reactions
Identify what is present before and after the change
Reactants: the substances that undergo change
Products: the new substances formed as a result of that change 
During a chemical change, the reactants change into products
Reactants  Products 

7. Chemical Reaction

8. Chemical Reaction Example: Burning charcoal
Word Equation: carbon + oxygen  carbon dioxide
Chemical Equation: C + O2  CO2

9. Chemical Equation: C + O2  CO2
Chemical Equation: a representation of a chemical reaction in which the reactants and products are expressed as formulas
Read aloud as: carbon and oxygen react to form carbon dioxide, or the reaction of carbon and oxygen yields carbon dioxide 

10. Mass is Conserved!
As the charcoal burns, what happens to the mass of the piece of charcoal? Where does it go? It does not just disappear!
Would the mass of the charcoal before you burn it equal the mass after it is burned?

11. Mass is Conserved
If you were to measure the mass of the charcoal before the reactions and the mass of the CO2 (carbon dioxide gas) what would you notice?
THEY WOULD BE EQUAL!!! 
During a chemical reaction, the mass of the products is ALWAYS equal to the mass of the reactants.
This is known as the Law of Conservation of Mass- mass is neither created nor destroyed in a chemical reaction. 

12. Showing Mass is Conserved
In order to ensure that the Law of Conservation of Mass is being obeyed, in other words, that mass is being conserved, you must write a balanced chemical equation.
This means that the number of atoms of an element in the reactants MUST EQUAL the number of atoms of that element in the products  

13. Rules to Balancing a Chemical Equation
Count the number of atoms of each element on each side of the equation
Example: N2H4 + O2  N2 + H2O
N = 2 H = 4 O = 2  N=2 H=2 O=1
Change one or more coefficients until the equation is balanced
Coefficient- the numbers that appear before the formula
NEVER change the subscripts! Only the coefficients!
Example: ____N2H4 + ____O2  ____N2 + ____ H2O

14. Example of Balancing ____N2H4 + ____O2  ____N2 + ____ H2O
How would you correctly balance the problem above?

15. Correctly Balanced
____N2H4 + ____O2  ____N2 + 2 H2O

16. 7.3 – Energy Changes in Reactions

17. Chemical Bonds & Energy
Some reactions yield more than just the products (elements). 
For example, when propane burns in the presence of oxygen, it creates carbon dioxide (CO2) and water (H2O) – but what else??? 
HEAT AND LIGHT! 
The heat that is released in the reaction comes from the reactants. 

18. Chemical Energy
Chemical Energy: the energy stored in the chemical bonds of a substance.

19. Chemical Energy Each of the bonds in propane has stored energy.
Energy changes in chemical reactions are determined by changes that occur in chemical bonding.
In other words… 
Chemical reactions involve the breaking of chemical bonds in the reactants and the formation of chemical bonds in the products. 

20. Chemical Energy Breaking chemical bonds REQUIRES ENERGY.
That’s why a grill requires an igniter to produce a spark -- to give enough energy for the bonds in reactants to break, starting the reaction. 
Forming bonds RELEASES ENERGY.
The heat and light that are given off by propane stoves are the result of the formation of new chemical bonds.
The bonds are formed as carbon, hydrogen, and oxygen atoms rearrange. 

21. Energy Exchange
Remember how physical changes deal with an exchange of energy?
Example of endothermic phase change: melting.
Example of exothermic phase change: freezing. 

22. Endothermic
Exothermic

23. Energy Exchange in Chemical Reactions
Energy also flows into and out of chemical changes. 
During a chemical reaction energy is either released or absorbed. 
Endothermic Reaction (feels cool)
Exothermic Reaction (feels warm)

24. Exothermic Reaction
Exothermic reaction: chemical reaction that releases energy to its surrounding
Feels WARM
Energy released, as the products form is greater than the energy required to break the bonds in the reactants.
Example: combustion reactions (burning)

26. Endothermic Reaction
Endothermic Reaction: a chemical reaction that absorbs energy from its surroundings
Feels COLD
More energy is required to break the bonds in the reactants than is released by the formation of the products
In other words, the energy of the products is greater than the energy of the reactants.
Example: decomposition reactions (breaking down)

27. Decomposition Reaction

29. No matter the reaction… energy is conserved!
In both types of reactions, the amount of energy from the reactants side must equal the amount of energy from the products side.
Energy cannot be created nor destroyed!
You cannot end up with more energy than was put into the reaction, and vice versa.
This is called the Law of Conservation of Energy.

30. 7.4 Reaction Rates

31. Reactions over Time
Progress of a chemical reaction is measured over time and expressed as a rate.
Reaction Rate: rate at which reactants change into products over time
Reaction rates tell you how fast a reaction is going

32. Factors Affecting Reaction Rates
Factors that affect reaction rates include:
Temperature
Surface Area
Concentration
Stirring
Catalysts.

33. Temperature An increase in temperature will increase the reaction rate
A decrease in temperature will decrease the reaction rate
Cooking on stove top: turn up heat faster the food cooks
Store milk in refrigerator to slow down reaction that causes milk to spoil
Reason: Increased temperature = Increased particle motion = Higher number of particle collisions = Increased reaction rate

34. Surface Area
The smaller the particle size of a given mass, the larger is its surface area
Example: Using newspaper to cover the floor of a room
Keeping all the sections folded together versus separating all the pages. If you separate the newspaper into open pages you can cover a much larger are with the same mass of paper
Increase in surface area increases the exposure of reactants to one another
Reason: Increased surface area = Greater exposure = More collisions = More particles reacting = Increased reaction rate

35. Surface Area LESS SURFACE AREA GREATER SURFACE AREA LESS SURFACE AREA

36. Stirring
By stirring you increase the exposure of reactants to each other
Example: Washing your clothes in the washer
Leave clothes to just soak in tub of water and detergent reaction proceeds more slowly
Placing clothes and detergent in washer – reaction sped up by stirring the contents back and forth
Reason: Stirring =Collisions more likely to occur =Increase in reaction rate

37. Concentration Concentration: number of particles in a given volume
More reacting particles = Greater chance for particles to collide = Faster reaction rate

38. Catalyst
Catalyst: a substance that affects the reaction rate without being used up in the reaction
Use a catalyst to:
Speed up a RXN or
Enable a RXN to occur at a lower temperature
Lower the amount of energy required for a reaction to occur
Catalyst is neither a reactant or a product, so it is written over the arrow of the chemical equation (it is NOT used up)
Platinum is a catalyst for breaking hydrogen peroxide into water and oxygen
Example: 2H2O H2O + O2 Pt

39. Catalyst