Prentice Hall Physical Science s1

Prentice Hall Physical Science Chapter 7 – Chemical Reactions

7.1 Describing Reactions - Chemical Equations - chemical equations are the written form of a chemical reaction - the original substance that are being changed are called the reactants (usually on the left side) - the new substances that are formed are called the products (usually on the left side)

EX: C + O2  CO2

C + O2 are the reactants and CO2 is the product - The law of conservation of mass says that mass is neither created or destroyed in a chemical reaction SO you must have EXACTLY the same number of atoms when a reaction is finished as you did when you started. - To show this we must balance chemical equations so we have the same number of atom at the end as at the beginning - We do this by changing the coefficients in front of the chemical formulas.

EX: 1) Na + H2O  NaOH + H2

2Na + 2 H2O  2NaOH + H2

2) HCl + CaCO3  CaCl2 +CO2 + H2O

2HCl + CaCO3  CaCl2 +CO2 + H2O

3) Al + Cl2  AlCl3

2Al + 3Cl2  2AlCl3 Do p. 224 #28 Prentice Hall Physical Science Chapter 7 – Chemical Reactions

7.2 Types of Chemical Reactions A. Synthesis Reactions - when two or more substances react to form a single substance - general equation:A + B  AB - always has two or more reactants but only one product

EX: Mg + O2  MgO B. Decomposition Reactions - when one compound breaks down into two or more simpler substances - general equation:AB  A + B - always has one reactant but two or more products

EX: CaCO3  CaO + CO2 C. Single Replacement Reactions - when one element (by itself) takes the place of another element that is in a compound - general equation:A + BC  B + AC - always has a single element and a compound as reactant and product

EX: Cu + AgNO3  Ag + Cu(NO3)2 D. Double Replacement Reactions - when two different compounds exchange positive ions and form two new compounds - general equation:AB + CD  AC + BD - always has two compounds as reactants and products

EX: Pb(NO3)2 + KI  PbI2 + KNO3 E. Combustion - when a substance reacts rapidly with oxygen to form carbon dioxide, water, heat, and light

- EX: CH4 + O2  CO2 + H2O

** Electrons can also be transferred during a chemical reaction. - If an element loses electrons during a chemical reaction it is called oxidation - If an element gains electrons during a chemical reaction it is called reduction Prentice Hall Physical Science Chapter 7 – Chemical Reactions

7.3 Energy Changes in Reactions A. Chemical Bonds and Energy - chemical energy is the energy stored in the chemical bonds of a substance - chemical reactions are when the chemical bonds in the reactants are broken and new bonds are formed in the reactants B. Exothermic and Endothermic Reactions - exothermic reactions release (give off) energy - the energy of the reactants is greater than the energy of the products - figure 18 A, p. 208 - endothermic reactions absorb energy; energy must be added in order for the reaction to “go” - the energy of the products is greater than the energy of the reactants - figure 18 B, p. 208 Prentice Hall Physical Science Chapter 7 – Chemical Reactions

7.4 Reaction Rate - reaction rate is how fast a reaction is going - there are several factor that affect reaction rates A. Temperature - increasing T increases reaction rate because the particles move faster and are more likely to collide and then react B. Surface Area - increasing the surface area of the reactants (by making the particles as small as possible) increases the reaction rate because there are more surfaces for collisions and the more collisions, the faster the reaction happens C. Stirring - stirring increases reaction rates because stirring increases the exposure of the reactants to each other D. Concentration - concentration is how much reactant you have - increasing the concentration of the reactants increases the reaction rate because there are more reactants to collide and therefore react E. Catalysts - a catalyst is a substance that speeds up a reaction without being used up by the reaction because it can allow the reaction to happen at a lower temperature - figure 23, p. 215