1. Chemical Reactions

2. Chemical Reactions
Chemical Reaction: A process that transforms one set of chemical substances into another
When atoms break existing chemical bonds and form new ones
Evidence of a chemical reaction:
Change in properties (gas or precipitate, color)
Change in energy (temperature or light emission)

3. Chemical Reactions
Reactants are the chemicals present BEFORE the reaction
Products are the chemicals present AFTER the reaction

4. Chemical Equations Ex: CaCO3  CaO + CO2
Chemical Equations symbolize what happens during a reaction
like a math equation…with  instead of =
Reactants  Products
Reactants and products are shown as chemical formulas
Ex: CaCO3  CaO + CO2
When limestone is heated, it changes into calcium oxide and carbon dioxide
Notice that the elements are the same before and after the reaction.

5. CaCO3  CaO + CO2 Mass is Conserved
The same atoms that are present at the start of the reaction are also there at the end.
CaCO3  CaO + CO2
1 calcium, 1 carbon, and 3 oxygen atoms
The atoms are simply rearranged

6. Mass is Conserved
Law of Conservation of Mass: The mass of the products will equal the mass of the reactants
Matter is neither created nor destroyed.
The atoms present in the reactants are the same atoms that form the products…they are just rearranged.

7. Balancing Chemical Equations
The law of conservation of mass is illustrated in chemical equations by ensuring that the equation is “balanced”.
A balanced equation is one in which the number of atoms of each element in the reactants is the same as the number of atoms of each element in the products.

8. Balanced Equations Ex) N2O5 + H2O  2HNO3
Subscripts belong only to the element preceding them.
N2O5 = 2N + 5O
Coefficients are distributed to each element in the compound. (multiply by subscripts)
2HNO3 = 2(HNO3) = 2H + 2N + 6O

9. Balanced Equations Ex) N2O5 + H2O  HNO3 Reactants: Products:
2 nitrogen, 6 oxygen, and 2 hydrogen atoms
Products:
1 hydrogen, 1 nitrogen, and 3 oxygen atoms
This equation is NOT balanced!
Therefore, it violates the law of conservation of mass and is incorrect.

10. Balanced Equations Ex) N2O5 + H2O  2HNO3 Reactants: Products:
2 nitrogen, 6 oxygen, and 2 hydrogen atoms
Products:
2 hydrogen, 2 nitrogen, and 6 oxygen atoms
This equation is balanced!
It shows that matter is neither created nor destroyed.

11. Types of Reactions Synthesis: “building a new compound”
A + B  C
2SO2 + O2 + 2H2O  2H2SO4
Decomposition: “taking a compound apart”
A  B + C
2H2O2  2H2O + O2
Replacement: “rearranging compounds”
AX + BY  AY + BX
CaCO3 + 2HCl  CaCl2 + H2CO3

12. ENERGY Activation Energy: the energy required to begin a reaction
ex) some reactions require heat to begin
Exothermic Reactions: give off energy (they feel hot)
ex) calcium chloride and water give off heat
Endothermic Reactions: absorb energy (they feel cold)
ex) water and ammonium nitrate absorb heat

13. Controlling Reactions
Chemists can control the rate of a reaction by adjusting the temperature.
Increasing the temperature will speed up the reaction
Molecules are moving faster so they collide more frequently
Decreasing the temperature will slow down the reaction
Molecules are moving slower so they collide less frequently

14. Controlling Reactions
Chemists can also control the rate of a reaction by adjusting the:
Concentration –amount of a substance in the reaction
Higher concentration  bigger/faster reaction
Surface Area – amount of surface exposed to the reaction
Greater surface area  faster reaction
Catalysts and Inhibitors – chemicals that speed up or slow down the reaction
Catalysts – speed up the reaction or cause it to happen
Inhibitors – slow down the reaction or cause it to stop