Chapter 7

Chemical Reactions

7.1 Describing Chemical Reactions

What is a chemical reaction? Demos

Chemical Reaction: is when a substance undergoes a chemical change to produce a new substance or substances

7 different types of chemical reactions

Chemical equations: are used to represent a chemical change/reaction Ex. 2 H2 + O2 2 H2O

Reactants: are the substances that undergo chemical change (bonds broken) (ingredients)

Products: are the new substances produced by the change (new bonds made) (cookies)

Reading Chemical equations

+ sign = “reacts with” sign = “to produce”, yields

Mole is an amount of a substance that contains 6.02X1023 particles. So rather than using atoms, molecules or ions we use the MOLE

Coefficient = # of moles

Mole continued

Shoes come in pairs, Eggs in a dozen. Particles to chemists come in MOLES Particles can be atoms, molecules, or

ions

Law of conservation of mass: states that mass is neither created nor destroyed in a chemical reaction

By ordinary means, we can’t make or destroy matter

ONLY REARRANGE IT!!!!!!!!

Balancing Equations

When writing chemical equations, the law of conservation of mass must be followed.

We must have the same amount of each element on both sides of the equation

Coefficients: are used to show proportions of reactants and products)

PERFECT PRACTICE, MAKES PERFECT!!!!!!

2H2(g)+O2(g) 2H2O(l)

Coefficients represent the number of units of each substance in the reaction

Subscripts represent the number of atoms in the molecule

Symbols show the state of reactants (s) solid, (aq) aqueous), (g) gas, (l) liquid

Discussion question

Why are coefficients important? Chemists need to know how much of

a reactant will produce a certain amount of a product.

7.2 Types of Reactions

7 types of reactions

Synthesis: is a reaction in which two or more substances react to form a more complex single substance

A+BC

Ex. 2 Na + Cl2 2 NaCl

Decomposition Reaction: is a reaction in which a compound breaks down into two or more simpler substances

ABA+B Ex. 2 H20 2 H2 + O2 Compost pile, digesting food,

electrolysis (breaking down H2O with electricity)

Combustion Reaction: is reaction where a substance reacts rapidly with oxygen.

Flammability or explosiveness Ex. CH4 + 2 O2 CO2 + 2 H2O Ex. Burning any type of fuel Usually produces CO2 (greenhouse gas)

Oxidation Reaction: are reactions where a substance reacts slowly with the oxygen in air or water

Happens with metals “rust” or “tarnish”

Ex. 2 Ca + O2 2 CaO

Single Replacement Reaction: is a reaction where one element takes the place of another element

A+BC AC+B or D+BC BD +C

Cu + 2 AgNO3 2 Ag + Cu(NO3)2

Double Replacement: is a reaction where 2 elements replace each other

AB+CDAD+CB

CaCO3 + 2 HCl CaCl2 + H2CO3

Discussion Question:

What two Chemical reactions are “opposites” of each other and why?

Synthesis and decomposition; in synthesis multiple substances combine to form a new one, while decomposition a single substance breaks apart into multiple simpler substances

Exothermic Reactions: are reactions that release energy into their surroundings

Give off heat (exergonic) Ex. Combustion Reactions

Ex. Burning fossil fuels

Endothermic Reaction: is a reaction where heat energy is absorbed by its surroundings

Ice absorbs heat to melt into water Gets colder (endergonic) Ex. Ice pack and decomposition of

mercury

Increasing Reaction Rate: 1. Temperature 2. Surface area 3. Stirring 4. Concentration of Reactants 5. Catalyst: is a substance that

affects reaction rate without being used up.

Discussion Question:

How are chemical bonds involved in energy exchanges?

Breaking bonds requires energy; forming bonds releases energy.