Mr. Matthew Totaro Legacy High School Honors Chemistry Chemical Reactions

2 Chemical Reactions Reactions involve chemical changes in matter resulting in new substances. Reactions involve rearrangement and exchange of atoms to produce new molecules. Elements are not changed into new elements during a reaction. Thermite Reaction

3 Example of a Chemical Reaction Reactants  Products

4 Evidence for a Chemical Reaction Look for evidence of a new substance. Visual clues (permanent). Color change. Precipitate formation.  Solid that forms when liquid solutions are mixed. Gas bubbles. Large energy changes.  Container becomes very hot or cold (transfer of heat).  Emission of light. Other clues. New odor. Whooshing sound from a tube. Permanent new state.

5 Evidence of a Chemical Change Color Change Formation of Solid Precipitate Formation of a Gas Emission of Light Release or Absorption of Heat

6 Evidence of Chemical Change, Continued In order to be ABSOLUTELY SURE that a chemical reaction has taken place, you need to go down to the molecular level and analyze the structures of the molecules at the beginning and at the end. Is boiling water a chemical change?

7 Practice—Decide Whether Each of the Following Involve a Chemical Reaction. Photosynthesis Heating sugar until it turns black Heating ice until it turns liquid Digestion of food Dissolving sugar in water Burning of alcohol in a flambé dessert Yes, CO 2 and H 2 O combine into carbohydrates Yes, sugar decomposing No, molecules still same Yes, food decomposing and combining with stomach acid No, molecules still same Yes, alcohol combining with O 2 to make CO 2 and H 2 O

Chemical Equations Concise representation of a chemical reaction

9 The Combustion of Methane Methane gas burns to produce carbon dioxide gas and gaseous water. Whenever something burns it combines with O 2 (g).

Anatomy of a Chemical Equation CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g)

Anatomy of a Chemical Equation Reactants appear on the left side of the equation. CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g)

Anatomy of a Chemical Equation Products appear on the right side of the equation. CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g)

Anatomy of a Chemical Equation The states of the reactants and products are written in parentheses to the right of each compound. CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g)

Anatomy of a Chemical Equation Coefficients are inserted to balance the equation. CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g)

Subscripts and Coefficients Give Different Information Subscripts tell the number of atoms of each element in a molecule

Subscripts and Coefficients Give Different Information Coefficients tell the number of molecules

17 Symbols Used in Equations Symbols used to indicate state after chemical. (g) = gas; (l) = liquid; (s) = solid. (aq) = aqueous = dissolved in water. Energy symbols used above the arrow for decomposition reactions. ∆ = heat. hv = light. shock = mechanical. elec = electrical.

Law of Conservation of Mass “We may lay it down as an incontestable axiom that, in all the operations of art and nature, nothing is created; an equal amount of matter exists both before and after the experiment. Upon this principle, the whole art of performing chemical experiments depends.” --Antoine Lavoisier, 1789

19 Conservation of Mass in a Reaction In a chemical reaction, matter cannot be created or destroyed. Therefore, the total mass cannot change. And the total mass of the reactants will be the same as the total mass of the products. In a chemical reaction, all the atoms present at the beginning are still present at the end. If all the atoms are still there, then the mass will not change.

20 Balancing Example When magnesium metal burns in air, it produces a white, powdery compound magnesium oxide. Mg (s) + O 2(g) → MgO (s)

21 Another Balancing Example Under appropriate conditions at 1000°C, ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and steam NH 3(g) + O 2(g) → NO (g) + H 2 O (g)

Reaction Types

Synthesis (Combination) Reactions the combination of 2 or more substances to form a compound only one product A + B  AB

Synthesis (Combination) Reactions Examples: N 2 (g) + 3 H 2 (g)  2 NH 3 (g) C 3 H 6 (g) + Br 2 (l)  C 3 H 6 Br 2 (l) 2 Mg (s) + O 2 (g)  2 MgO (s) Two or more substances react to form one product

2 Mg (s) + O 2 (g)  2 MgO (s)

Decomposition Reactions a compound breaks down into 2 or more simpler substances only one reactant AB  A + B

Decomposition Reactions Examples: CaCO 3 (s)  CaO (s) + CO 2 (g) 2 KClO 3 (s)  2 KCl (s) + O 2 (g) 2 NaN 3 (s)  2 Na (s) + 3 N 2 (g) One substance breaks down into two or more substances

Combustion Reactions CH 4(g) + 2O 2(g)  CO 2(g) + 2H 2 O (g) the burning of any substance in O 2 to produce heat A + O 2  B

Combustion Reactions Examples: CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O (g) C 3 H 8 (g) + 5 O 2 (g)  3 CO 2 (g) + 4 H 2 O (g) Rapid reactions that produce a flame Most often involve hydrocarbons reacting with oxygen in the air

Whoosh Bottle Demo is a Combustion Reaction 30

C. Johannesson Single Replacement one element replaces another in a compound metal replaces metal (+) nonmetal replaces nonmetal (-) A + BC  B + AC

C. Johannesson Single Replacement Cu (s) + 2AgNO 3(aq)  Cu(NO 3 ) 2(aq) + 2Ag (s)

C. Johannesson AB + CD  AD + CB Double Replacement ions in two compounds “change partners” cation of one compound combines with anion of the other

C. Johannesson Double Replacement Pb(NO 3 ) 2(aq) + K 2 CrO 4(aq)  PbCrO 4(s) + 2KNO 3(aq)

35 Practice—Classify the Following Reactions as Synthesis, Decomposition, Single Displacement, or Double Displacement. 3 Mg(s) + 2 FeCl 3 (aq)  3 MgCl 2 (aq) + 2 Fe(s) CO 2 (g) + H 2 O(l)  H 2 CO 3 (aq) 3 KOH(aq) + H 3 PO 4 (aq)  K 3 PO 4 (aq) + 3 H 2 O(l)

36 Practice—Classify the Following Reactions as Synthesis, Decomposition, Single Displacement, or Double Displacement, Continued. 3 Mg(s) + 2 FeCl 3 (aq)  3 MgCl 2 (aq) + 2 Fe(s) CO 2 (g) + H 2 O(l)  H 2 CO 3 (aq) 3 KOH(aq) + H 3 PO 4 (aq)  K 3 PO 4 (aq) + 3 H 2 O(l) Single displacement. Double displacement. Decomposition. Synthesis.

Reaction Prediction

Synthesis Reactions A + B  AB Al (s) + O 2 (g)  Two substances combine to form a single compound. 2 elements combine to form a single compound

Decomposition Reactions K 2 CO 3 (s)  ∆  One compound breaks down into multiple substances (usually because of the presence of heat). AB  A + B Metal carbonate + heat → metal oxide + carbon dioxide gas

Combustion Reactions CH 4 (s) + O 2 (g)  One substance reacts with Oxygen to form Carbon Dioxide and Water A + O 2  CO 2 + H 2 O

Single Replacement Reactions Cu (s) + AgNO 3 (aq)  A + BC  B + AC A more active metal replaces a metal in a compound to form a new compound and an element

Double Replacement Reactions AgNO 3 (aq) + KCl (aq)  AB + CD  AD + CB Two compounds exchange partners to form two new compounds