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

H2 + O2 H2O

Nitrogen + Hydrogen Ammonia

N2 + H2 NH3

(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

H2 + O2 H2O

__ H atoms + __ O atoms 2 H atoms and 1 O atom

need to balance.

e.g. Nitrogen + Hydrogen Ammonia

N2 + H2 NH3

__ 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:

H2 + O2 H2O (H is OK, now look at O)

H2 + O2 2H2O (O is fixed, but H is now unbalanced.)

Cannot fix H2O by changing it to H2O2 because you are changing the _______________(subscript).

___________ multiplies everything after it. (__________)

2H2 + O2 2H2O (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?

Al2(SO4)3 + Ca(OH)2 Al(OH)3 + CaSO4

Other symbols used in Chemical equations:

(s) _______

(t) _______

(u) _______

(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 AXe.g. 2H2(g) + O2(g) 2H2O(g)

e.g. Mg(s) + Cl2(g) MgCl2(s)

Most metals react with O & S to produce Oxides & Sulphides.

e.g. 2Mg(s) + O2(g) 2MgO(s)

e.g. 8Ba(s) + S8(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 + Xe.g. 2H2O(l) electricity 2H2(g) + O2(g) (Electrolysis)

e.g. 2HgO(s) or Heat 2Hg(l) + O2(g)

e.g. H2O2(l) KMnO4

H2(g) + O2(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 + Bm m nm m nm m

Or

Y + BX BY + Xnm m nm m nm nm

Rns usually need to take place in _______ (aqueous)

Examples:

Mg(s) + AgNO3(aq)

Cl2(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 (H2O)]

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(NO3)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 ____________ (CXHY) or an Alcohol (CXHYOH) then the products are CO2 and H2O which we will focus on mostly.

e.g.

C2H6(g) + O2(g) Ethane

C2H5OH(l) + O2(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. AgNO3 (aq) +NaCl(aq) AgCl(s) + NaNO3(aq)

The ions that do not form precipitates (solids) are called spectators The spectator ions are…. Na+ and NO3

-

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 AgNO3 (aq) +NaCl(aq) AgCl(s) + NaNO3(aq)

Non Metals react with Oxygen to produce Oxides

e.g. S8(s) + 8O2(g) 8SO2(g) Sulphur DiOxide

e.g. C(s) + O2(g) CO2(g) Carbon DiOxide

e.g. 2H2(g) + O2(g) 2H2O(g) Hydrogen Oxide(?) i.e. Water

Active metal oxides react with H2O to produce metal hydroxides.

e.g. Na2O(s) + H2O(l) 2NaOH(s) or (aq)?

Special Decomposition Rns:

a)Metal Carbonates Metal Oxides + CO2 gas

MgCO3(s) MgO(s) + CO2(g)

b) Metal Hydroxides Metal Oxides + Water

Ba(OH)2(s) BaO(s) + H2O(l)

c) Metal Chlorates Metal Chlorides + O gas

2NaClO3(s) 2NaCl(s) + O2(g)

d) Acids Non Metal Oxide + Water

H2SO3(aq) SO2(g) + H2O(l)

e.g.2

N2 + H2 NH3 (balance the N)

N2 + H2 2NH3 (now, balance the H)

N2 + 3H2 2NH3 (double check)

# N atoms (2) before = # N atoms (2) after

# H atoms (6) before = # H atoms (6) after

Balanced Example: CH4 + O2 CO2 + H2O

(see p.100 Figure 3-9)

Example: C3H8 + O2 CO2 + H2O