# Unit 4 Part 3 Chemical Reactions.

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Unit 4 Part 3 Chemical Reactions

The Importance of Chemical Reactions:
Many chemical reactions occur naturally, although some take place so slowly that we do not notice them.

Biochemical Reactions
The natural chemical reactions that we observe occurring in living things are known as biochemical reactions. 2 important chemical reactions are shown below: Respiration: C6H12O6 + 6O2  6CO2 + 6H2O + energy Photosynthesis: 12H2O + 6CO2 + energy  C6H12O6 + 6H2O + 6O2

Manufacture Important Chemicals
Many important chemicals are created through chemical reactions. Examples: Fertilizers, cement, plastics, synthetic textiles, and pharmaceuticals.

Harmful Chemical Reactions
There are also chemical reactions that harm living organisms and harm the environment. (See RAFT assignment)

Review Chemical Reactions:
Indicator that a chemical change has occurred: Other signs of a chemical change: Change in electrical conductivity Change in melting point or boiling point Change in density Change in taste

Types of Chemical Reactions
A reaction occurs when two or more molecules interact and a chemical change occurs. A chemical change MUST occur in order for a chemical reaction to occur. Chemical reactions start with the reactants and go to the products.

Synthesis/ Combination:
A synthesis reaction is when two or more simple compounds combine to form a more complicated one. These reactions come in the general form of: A + B ---> AB

Synthesis/ Combination:
One example of a synthesis reaction is the combination of iron and sulfur to form iron (II) sulfide: 8 Fe + S8 --->

Decomposition: A decomposition reaction is the opposite of a synthesis reaction - a complex molecule breaks down to make simpler ones. These reactions come in the general form: AB ---> A + B

Decomposition: One example of a decomposition reaction is the electrolysis of water to make oxygen and hydrogen gas: 2 H2O--->

Single displacement: This is when one element trades places with another element in a compound. These reactions come in the general form of: A + BC ---> AC + B

Single displacement: One example of a single displacement reaction is when magnesium replaces hydrogen in water to make magnesium hydroxide and hydrogen gas:

Single displacement: The Activity Series:
The activity series of an element is used to determine whether or not a single displacement reaction will occur.

Single displacement: Each metal will displace any metal ion that appears below it in the series. The Activity Series lithium These metals displace hydrogen from water potassium barium calcium sodium magnesium These metals displace hydrogen from acids aluminum zinc iron nickel tin lead hydrogen copper These metals do not react with acids or pure water mercury silver gold

Single displacement: Examples: Cu(s) + FeSO4(aq) Zn(s) + 2 HCl(aq)
2 Na(s) H2O(l)

The Halogen Displacement Series
Single displacement: The Halogen Displacement Series The Halogen Displacement Series Each halogen will displace any halide ion that appears below it fluorine chlorine bromine iodine

Single displacement: Examples: determine whether a reaction will occur, if so, what are the products? Br2(aq) NaCl(aq)  Zn(s) + NiCl2(aq)  I2(aq) KF(aq)  F2(aq) KI(aq)  Cu(s) AgNO3(aq) 

Double displacement: This is when the anions and cations of two different molecules switch places, forming two entirely different compounds. These reactions are in the general form: AB + CD ---> AD + CB

Double Displacement One example of a double displacement reaction is the reaction of lead (II) nitrate with potassium iodide to form lead (II) iodide and potassium nitrate:

Double Displacement How can we predict when a double displacement reaction will occur? Such reactions usually result in the formation of a precipitate, a gas, or water.

Acid Base HA + BOH ---> H2O + BA
This is a special kind of double displacement reaction that takes place when an acid and base react with each other. The H+ ion in the acid reacts with the OH- ion in the base, causing the formation of water. Generally, the product of this reaction is some ionic salt and water: HA + BOH ---> H2O + BA

Acid Base One example of an acid-base reaction is the reaction of hydrobromic acid (HBr) with sodium hydroxide:

Funny Joke: The base is under a salt: NaCl NaOH

Combustion: A combustion reaction is when oxygen combines with another compound to form water and carbon dioxide. These reactions are exothermic, meaning they produce heat. An example of this kind of reaction is the burning of napthalene: C10H O2 --->

Steps to determine types of reactions:
Follow this series of questions. When you can answer "yes" to a question, then stop! 1)  Does your reaction have oxygen as one of its reactants and carbon dioxide and water as products? If yes, then it's a combustion reaction 2)  Does your reaction have two (or more) chemicals combining to form one chemical? If yes, then it's a synthesis reaction 3)  Does your reaction have one large molecule falling apart to make several small ones? If yes, then it's a decomposition reaction 4)  Does your reaction have any molecules that contain only one element? If yes, then it's a single displacement reaction 5)  Does your reaction have water as one of the products? If yes, then it's an acid-base reaction 6)  If you haven't answered "yes" to any of the questions above, then you've got a double displacement reaction

Assignment See types of reactions assignment (Pg 24)
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