Chapter 6: Chemical Reactions

Chemical Equation represents a chemical change or reaction Reactants  Products Reactants – chemicals before the reaction Products – substance formed in the reaction

5 signs of a chemical reaction 1. Color Change

5 signs of a chemical reaction 2. Precipitation of a Solid

5 signs of a chemical reaction 3. Energy Change – heat or light absorbed or released

5 signs of a chemical reaction 4. Odor Changes

5 signs of a chemical reaction 5. Gas Release

Reactions can be described several ways: 1. In a sentence Copper reacts with chlorine to form copper (II) chloride. 2. In a word equation Copper + chlorine  copper (II) chloride 3.Using formulas Cu + Cl 2  CuCl 2

Symbols used in Reactions SymbolExplanation +Used to separate two reactants or two products  Yields, separates reactants and products (s), (l), (g)State of matter; Fe (s) (aq)Aqueous solution – dissolved in water Heat supplied to the rxn Catalyst used (speeds up the rxn time)

Assignment 7 Write the word equations below as chemical equations and balance: 1. Zinc and lead (II) nitrate react to form zinc nitrate and lead.

2. Aluminum bromide and chlorine gas react to form aluminum chloride and bromine gas.

Diatomic Elements

3. Sodium phosphate and calcium chloride react to form calcium phosphate and sodium chloride.

In a chemical reaction Law of Conservation of Matter states that matter isn’t created or destroyed. The way atoms are joined is changed

Because of the LAW OF CONSERVATION OF MATTER, we have to BALANCE EQUATIONS!!! To BALANCE AN EQUATION means to make sure that the NUMBER of atoms of each element is the same in the reactants and in the products!

Balancing Rules: 1.You may change the coefficients to any number you want, but… 2.You may NEVER change a subscript. A coefficient is a number written in front of the compound

Balancing example #1 C + O  CO 2

Balancing example #2 Zn + HCl  H 2 + ZnCl 2

Balancing example #3 CuO + C  Cu+ CO 2

Balancing example #4 O 3  O 2

Balancing example #5 Al + CuCl 2  Cu + AlCl 3

Balancing example #6 C + H 2  C 3 H 8

Balancing example #7 CF 4 + Br 2  CBr 4 + F 2

Balancing example #8 Hg 2 I 2 + O 2  Hg 2 O + I 2

Balancing example #9 N 2 + H 2  NH 3

Balancing example #10 KClO 3  KCl + O 2

Balancing example #11 H 2 + O 2  H 2 O

Balancing example #12 AlBr 3 + K 2 SO 4  KBr + Al 2 (SO 4 ) 3

5 Types of Chemical Reactions 1.Synthesis 2.Decomposition 3.Single Replacement (Displacement) 4.Double Replacement (Displacement) 5.Combustion

Synthesis Reaction Synthesis reaction – 2 substances are combined to form a single product A + B  AB + 

Synthesis Examples 2Mg + O 2  2MgO Magnesium and oxygen combine to form magnesium oxide. 2H 2 + O 2  2H 2 O Hydrogen and oxygen combine to form dihydrogen monoxide (water)

Synthesis Demo Ignite magnesium

Decomposition Reaction Decomposition reaction – A single compound is broken down into 2 or more products. AB  A + B  +

Decomposition Examples 2NaCl(s)  2Na(s) + Cl 2 (g) Sodium chloride (table salt) decomposes into sodium and chlorine gas. CaCO 3  CaO + CO 2 Limestone (CaCO 3 ) decomposes into lime (CaO) and carbon dioxide.

Decomposition Demo Decomposition of Hydrogen Peroxide with MnO 2 as a catalyst H 2 O 2  O 2 + H 2 O

Single Replacement Reaction Single Replacement reaction – A single element takes the place of an element in a compound. A + BC  B + AC +  +

Single Replacement Examples Mg + BeO  Be + MgO Magnesium replaces beryllium in beryllium oxide to form magnesium oxide and beryllium. 2Na + 2HCl  H 2 + 2NaCl Sodium replaces hydrogen in hydrochloric acid to yield hydrogen and sodium chloride.

Single Replacement Demo Magnesium reacting with Hydrochloric acid Compared to Copper reacting with HCl

Double Replacement Reaction Double Replacement reaction – Elements in 2 compounds switch places to form 2 new compounds. AB + CD  AD + CB +  +

Double Replacement Examples MgO + BeS  MgS + BeO Oxygen and sulfur switch places to form magnesium sulfide and beryllium oxide. Na 2 S + Zn(NO 3 ) 2  2Na(NO 3 ) + ZnS Sulfur and nitrate switch places to form sodium nitrate and zinc sulfide.

Double Replacement Demo A double replacement between barium chloride and potassium sulfate Potassium Iodide and lead (II) nitrate produces potassium nitrate and lead (II) iodide

Combustion Reaction Combustion reaction – hydrocarbon (H and C) burn in oxygen to produce water and carbon dioxide. Heat is given off as energy. C x H y + O 2  H 2 O + CO 2

Combustion Examples CH 4 + 2O 2  2H 2 O + CO 2 Methane burns in oxygen to produce water and carbon dioxide. C 12 H 22 O O 2  11H 2 O + 12CO 2 A sugar molecule burns in oxygen to produce water and carbon dioxide

Combustion Demo Burn sugar Burn gasoline

Factors that affect reaction rates Temperature – Higher temperature faster reaction Concentration – Changing the amounts of reactants – High concentration means more particles per volume (3% H 2 O 2 vs. 30% H 2 O 2 )

Surface Area – Reactions occur faster when there is more surface area Sugar cube vs. sugar packet dissolving Catalyst – Used to speed up a reaction without being used up – Enzymes are examples of catalyst

Can you slow a reaction down? Inhibitor – a substance added to slow reactions down – Added to food products

Limiting Reactant The reactant that is used up first Making a bicycle requires 1 frame + 2 wheels  1 bicycle If you have 100 frames and 120 wheels, how many bicycles can you manufacture? What is the limiting reactant?