Sect.11.1 “Chemical Reactions” Chemical Rn: process by which one or more substances are changed into one or more different substances. Reminder: Substances before reaction = _________ Substances after reaction = _________

Signs that a chemical rn has occurred: 1.) Heat: Does rn produce or absorb heat? 2.) Light: Does rn produce light? Note: Heat/Light is not proof positive that a rxn has occurred. 3.) Gas is produced: Bubbles form in the rn. 4.) Precipitate is formed: when 2 liquid solns are combined and a solid is produced – solid is known as a precipitate.

Characteristics of a Chemical Eqn. 1.) Eqn needs to show all that is involved. 2.) Need correct formulas for _________ and __________. 3.) Law of Conservation of Mass holds. i.e. total mass before = total mass after or more importantly: total # atoms before = total # atoms after

Word Eqns It’s easier to write an eqn by first writing it in words – word eqn. It is ________ because no numbers are used. e.g. Hydrogen + Oxygen  Water Nitrogen + Hydrogen  Ammonia where “+” means “reacts with” and is read as “and” “  ” means “______” or “produces” and is read as such.

Next step: Convert the words into formulas, called a “___________________”… need to know how they exist… e.g.Hydrogen + Oxygen  Water H 2 + O 2  H 2 O Nitrogen + Hydrogen  Ammonia N 2 + H 2  NH 3 (other examples?)

Last step: Balance the equation to create a “_______________ chemical equation” i.e. the # of each type of element before = after e.g.Hydrogen + Oxygen  Water H 2 + O 2  H 2 O __ H atoms + __ O atoms  2 H atoms and 1 O atom  need to balance. e.g.Nitrogen + Hydrogen  Ammonia N 2 + H 2  NH 3 __ N atoms + __ H atoms  1 N atom and 3 H atoms  need to balance.

Rule: Balance each element individually by using coefficients. e.g.1: H 2 + O 2  H 2 O(H is OK, now look at O) H 2 + O 2  2H 2 O (O is fixed, but H is now unbalanced.) Cannot fix H 2 O by changing it to H 2 O 2 because you are changing the _______________(subscript). ___________ multiplies everything after it. (__________) 2H 2 + O 2  2H 2 O (H and O are now balanced) i.e. # H before (4) = # H after (4) # O before (2) = # O after (2)

Balancing tricks / hints: 1) Balance the elements ______ at a time 2) First balance _____ ______of the elements that are combined and that only appear once on each side of the equation 3) Treat ______________that appear on both sides of the equation, as single units 4) Balance the H & O atoms _______ ! Examples? Al 2 (SO 4 ) 3 + Ca(OH) 2  Al(OH) 3 + CaSO 4

Other symbols used in Chemical equations: (s) _______ (l) _______ (g) _______ (aq) aqueous – dissolved in water  precipitate (solid) is formed  gas is formed (bubbles) ( other symbols ) (do examples with above symbols)

Types of Reactions: 1) Synthesis Rn: (aka Composition rns) - aka direct combination rn. Where 2 or more substances ________ to form a new compound. Generalized by: A + X  AX e.g.2H 2(g) + O 2(g)  2H 2 O (g) e.g.Mg (s) + Cl 2(g)  MgCl 2(s) Most metals react with O & S to produce Oxides & Sulphides. e.g.2Mg (s) + O 2(g)  2MgO (s) e.g.8Ba (s) + S 8(s)  8BaS (s)

2) DeComposition Reactions: -a single compound __________ (breaks down) into 2 or more simpler substances. Usually requires an input of Energy. (Heat, electricity..) Generalized by: AX  A + X e.g.2H 2 O (l) electricity 2H 2(g) + O 2(g) (Electrolysis) e.g.2HgO (s)  or Heat 2Hg (l) + O 2(g) e.g.H 2 O 2(l) KMnO 4 H 2(g) + O 2(g)

3) Single Replacement reactions: - where 1 element _________ a similar element in another compound, metals replace metals, and non-metals replace non-metals - Generalized by: A + BX  AX + B mm nm m nm m Or Y + BX  BY + X nm m nm m nm nm Rns usually need to take place in _______ (aqueous)

Examples: Mg (s) + AgNO 3(aq)  Cl 2(g) + NaBr (aq)  Note: Just because you can write a chemical rn, it doesn’t mean that it will happen. We need to consult the Activity Series - a list of elements organized according to how reactive they are. If A is higher than B on the list, then rn will occur, if not, then no rn. Same goes if Y is higher than X on the list. ( Do examples )

4) Double Replacement Rns: - where ions of 2 compounds _________ ______in an aqueous solution to form 2 new compounds. Generalized by: AX (aq) + BY (aq)  AY + BX In order for a double replacement rn to occur, one of the ____________ must be a: a) Precipitate (solid), or b) Gas, or c) Water [remember: H + + OH -  HOH (H 2 O)] If products do not contain one of the above 3, then rn will not occur.

Examples: Precipitate: (need to look at Table A-12 p.900) KI (aq) + Pb(NO 3 ) 2(aq)  Gas: FeS (s) + HCl (aq)  Water: HCl (aq) + NaOH (aq) 

5.) Combustion Rns: - Is when a substance * combines with _________ (in the air) releasing a large amount of energy in the form of light, heat and sometimes sound. * If the substance is a ____________ (C X H Y ) or an Alcohol (C X H Y OH) then the products are CO 2 and H 2 O which we will focus on mostly. e.g. C 2 H 6(g) + O 2(g)  Ethane C 2 H 5 OH (l) + O 2(g)  Ethanol

Aqueous solutions Most important reactions _______ _____ _______ (which makes up most of the world and our bodies!) That means that most ______ form ions before reacting Ie. AgNO 3 (aq) +NaCl (aq)  AgCl (s) + NaNO 3(aq)

The ions that do not form precipitates (solids) are called spectators The spectator ions are…. Na + and NO 3 - They can be removed to simplify the equations and when they are it is called a NET IONIC EQUATION Ag + Cl  AgCl

Solubility rules tell you what forms a precipitate Based on the chart you can identify which compounds will form solids They are precipitates, those that do not form solids are the spectator ions… they are dissolved in solution the whole time and do not actually combine Ex  AgNO 3 (aq) +NaCl (aq)  AgCl (s) + NaNO 3(aq)

Non Metals react with Oxygen to produce Oxides e.g.S 8(s) + 8O 2(g)  8SO 2(g) Sulphur DiOxide e.g.C (s) + O 2(g)  CO 2(g) Carbon DiOxide e.g.2H 2(g) + O 2(g)  2H 2 O (g) Hydrogen Oxide(?) i.e. Water Active metal oxides react with H 2 O to produce metal hydroxides. e.g.Na 2 O (s) + H 2 O (l)  2NaOH (s) or (aq)?

Special Decomposition Rns: a)Metal Carbonates  Metal Oxides + CO 2 gas MgCO 3(s)  MgO (s) + CO 2(g) b) Metal Hydroxides  Metal Oxides + Water Ba(OH) 2(s)  BaO (s) + H 2 O (l) c) Metal Chlorates  Metal Chlorides + O gas 2NaClO 3(s)  2NaCl (s) + O 2(g) d) Acids  Non Metal Oxide + Water H 2 SO 3(aq)  SO 2(g) + H 2 O (l)

e.g.2 N 2 + H 2  NH 3 (balance the N) N 2 + H 2  2NH 3 (now, balance the H) N 2 + 3H 2  2NH 3 (double check) # N atoms (2) before = # N atoms (2) after # H atoms (6) before = # H atoms (6) after  Balanced  Example:CH 4 + O 2  CO 2 + H 2 O (see p.100 Figure 3-9) Example: C 3 H 8 + O 2  CO 2 + H 2 O