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 H 2 + O 2  2 H 2 O  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 6.02X10 23 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!!!!!!

 2H 2(g) +O 2(g)  2H 2 O (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+B  C  Ex. 2 Na + Cl 2  2 NaCl

 Decomposition Reaction: is a reaction in which a compound breaks down into two or more simpler substances  AB  A+B  Ex. 2 H 2 0  2 H 2 + O 2  Compost pile, digesting food, electrolysis (breaking down H 2 O with electricity)

 Combustion Reaction: is reaction where a substance reacts rapidly with oxygen.  Flammability or explosiveness  Ex. CH O 2  CO H 2 O  Ex. Burning any type of fuel  Usually produces CO 2 (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 + O 2  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 AgNO 3  2 Ag + Cu(NO 3 ) 2

 Double Replacement: is a reaction where 2 elements replace each other  AB+CD  AD+CB  CaCO HCl  CaCl 2 + H 2 CO 3

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.