1. Chemical Reactions
Chapter 5

2. 5.1 The Nature of Reactions
Chemical reactions change substances
Production of gas,change of color,smoke, flame are signs of chemical reactions
Chemical reactions rearrange atoms

3. Reactant- a substance that undergoes a chemical change
Product- a substance that is the result of a chemical change
Na Cl-  NaCl
Reactant Reactant Product

4. Energy and Reactions Energy must be added to break bonds
Forming bonds releases energy
Energy is conserved in chemical reactions

5. Chemical energy- the energy stored within atoms and molecules that can be released when a substance reacts
Exothermic- a reaction that transfers energy from the reactants to the surroundings usually as heat.
Reactions that release energy are exothermic

6. Endothermic reaction a reaction in which energy is usually transferred to the reactants usually as heat from the surroundings
Reactions that absorb energy are endothermic

7. 5.2 Reaction Types A+B--------->AB
Synthetic Reaction- a reaction of at least two substances that forms a new, more complex compound
Synthesis reactions combine substances
A+B >AB
2Na + Cl2  2NaCl

8. Decomposition Reaction- a reaction in which one compound breaks into at least two products
AB > A+B
2H2O  2H2 +O2
Electrolysis- the decomposition of a compound by an electric current

9. Combustion Reaction- a reaction in which a compound and oxygen burn.
Combustion reactions use oxygen as a reactant.
The products of a hydrocarbon and oxygen are carbon dioxide and water.
2CH  2CO2 + 4H2O

10. Single-displacement reaction- a reaction in which atoms of one element take the place of atoms of another element in a compound
In single-displacement reactions, elements trade places
XA + B  BA + X
3CuCl2 + 2Al  2AlCl Cu

11. AX + BY--------->AY + BX
Double-displacement Reaction- reaction in which a gas, a solid precipitate, or a molecular compound is formed from the apparent exchange of ions between two compounds
AX + BY >AY + BX
Pb(NO3)2 + K2CrO4  PbCrO4 + 2KNO3

12. Reduction/oxidation Reaction- a reaction that occurs when electrons are transferred from one reactant to another
Radicals- the fragments of molecules that have at least one electron available for bonding
Radicals have electrons available for bonding

13. 5.3 Balancing Chemical Equations
Describing Reactions:
Methane + Oxygen > Carbon Dioxide + Water
Chemical equations summarize reactions
Chemical Equation- an equation that uses chemical formulas and symbols to show the reactants and products in a chemical reaction
Balanced chemical equations account for the conservation of mass

14. Balancing Chemical Equations
Balanced equation- each side of the equation has the same number of atoms of each element.
Remember the “Law of Conservation of Mass”– Same number of atoms on each side of the equation.

15. 2H2 + O2 ---------------> 2H2O
Coefficients-small whole numbers that are placed in front of the formulas in an equation in order to balance it.
2H O > H2O
Coefficient Coefficient
To write a balanced chemical equation, first write the equation. Then use coefficients to balance the equation so that it obeys the law of conservation of mass.

16. Example. H2 + O2 ---------------> H2O
Reactants Products
2-hydrogen atoms hydrogen atoms
2-oxygen atoms oxygen atom
This is not a balanced chemical equation!! To balance this equation we will use coefficients.

17. 2H2 + O2 ---------------> 2H2O
Reactants Products
4-hydrogen atoms hydrogen atoms
2-oxygen atoms oxygen atoms
This is a balanced equation by using the coefficient 2 in front of hydrogen giving us 4 hydrogen atoms and 2 oxygen atoms on the reactants side . The coefficient 2 on the products side gives us 4 hydrogen atoms and 2 oxygen atoms on the product side.

18. How to balance equations:
CH4 + O > CO2 +H2O

19. Information From a Balanced Equations
Determining Mole Ratios
Magnesium + Oxygen-----> Magnesium Oxide
2Mg + O >2MgO
Balanced equations indicate particles and moles
Balanced equations show the conservation of mass

20. The law of definite properties:
A compound always contains the same elements in the same proportions, regardlessof how the compound is made or how much of the compound is formed.
Mole Ratio- the smallest relative number of moles of the substances involved in a reaction
Mole ratios can be derived from the balanced equations