Chemical Reactions
- Chemical Reactions
- Why do reactions occur
anyway? To lower energy and entropy of the reactants. Many
reactions occur in water = aqueous chemistry.
- There are three main
types of reactions we will look at in the chapter:
- precipitation reactions:
typically look like this: (aq) + (aq) g (s) + (aq) A precipitate solid is formed. (The
cations swap anion partners)
- acid base /
neutralization
reactions: HA(aq) + MOH(aq) g H2O(l) +
MA(aq)
- redox reactions: when
electrons are transferred so that oxidation states change: Cu2+(aq) + Mg(s) g Cu(s) + Mg2+(aq)
- Electrolytes - solution containing lots of ions
- Compare covalent
compounds and ionic compounds in water:
- Sugar dissolves in
water. It is a covalent compound. When it dissolves the
molecules stay intact - they just separate from each other. The
IMF (such as dipole dipole forces) between the
molecules are broken, but NOT the covalent
bonds (that hold the atoms within one molecule). Note that to
determine solubility of covalent compounds in a liquid the general rule
is "like dissolves like" which means polar liquids dissolve
polar compounds and nonpolar liquids dissolve nonpolar compounds.
Sugar water does not conduct electricity since there are no ions - it is
a non-electrolyte.
- Salt
also dissolves in water, BUT it goes one step further. Besides having the NaCl
units separate from each other, they also break apart into the Na+
and Cl- ions. When ionic compounds dissolve in water
they also dissociate = ionize = break into
ions. The ionic bonds
actually break so that the ion-dipole forces can form when each ion is
surrounded by water. If the ion-dipole forces will be more
stabilizing than the original ionic bonds, a salt will dissolve.
Note that to determine solubility of ionic compounds in water we use
solubility rules. Soluble ionic compounds put lots on ions into solution
so they make strong electrolytes. Insoluble ionic compounds only
put a few ions into solution so they make weak electrolytes.
Electrolytes conduct electricity.
- Watch
this movie.
Here's another one if you click on Activities and Dissolution
of NaCl movie.
- Acids break the
generalization. Acids are covalent molecules that actually ionize
as though they were ionic compounds! For example HCl (hydrochloric acid) in water can ionize into H+(aq) and Cl-(aq) ions. Practically all the HCl will ionize, thus putting lots of ions in
solution - strong electrolyte. Weak acids are acids that ionize
just a little bit putting some ions in solution = weak electrolyte. Weak
acids set up equilibrium since the reaction is never over - the rxn goes forwards and backwards forever.
- Summary Table
Strong Electrolytes ionize > 70%
|
Weak Electrolytes ionize < 70%
|
Non Electrolytes ionize 0%
|
soluble
ionic compounds
|
insoluble
ionic compounds
|
most
covalent compounds
|
strong
acids
|
weak
acids
|
|
strong
bases
|
weak
acids
|
|
conducts
well
|
conducts
a little
|
don't
conduct at all
|
- Watch these two
electrolyte movies by clicking on Activities then Electrolytes
movie and Strong
and Weak Electrolytes movie.
- How many ions are in
solution if I have 0.444 moles of calcium carbonate in water? 0.444
moles CaCO3 (2 ions / CaCO3)(6.02
x 1023 / mol) = 5.35 x 1023 ions. Note CaCO3
contains 2 ions: Ca2+ and CO32-.
If the question had asked about nickel(III)
nitrate there would have been 4 ions.
- Ionic and Net Ionic Reactions
- Given this
reaction: copper(II) nitrate plus
potassium carbonate in water. Write and balance the reaction.
Draw pictures of the reactants and products. Use your solubility
rules to determine which of these ionic compounds are ionized in water
and which are solids. Remember precipitation
reactions are swap partner reactions (also called double displacement)
B.
Answer: Cu(NO3)2(aq) + K2CO3(aq)
g CuCO3(s) + 2
KNO3(aq) This is the molecular
reaction. Here are the beakers. Water molecules are omitted for
clarity.
- Notice the molecular
reaction doesn't really reflect reality. All the (aq) species are ions in water - not bonded
together. So we can write ionic
reactions which show all the ionized species as ions: Cu2+(aq) + 2 NO3-(aq) + 2 K+(aq) +
CO32-(aq) g CuCO3(s)
+ 2 NO3-(aq) + 2 K+(aq) This is much closer to reality and matches
the picture above much better. It is just a pain to write it all
out! YOU have to remember that the (aq) state really means dissolved and ionized and
surrounded by water for ionic compounds.
- Now we can write the net ionic reaction where we cross out the ions
that appear on both sides called spectator
ions since they don't do anything and are not part of the
reaction really. What are the spectator ions? See the
potassium and nitrate ions are on both sides - they did nothing.
Cross them out. Cu2+(aq)
+ CO32-(aq)
g
CuCO3(s)
- Precipitation Reactions
- Defined above. Cations
swap anion partners = double displacement. Remember to look at
solubility rules to determine if the ionic compounds are ionized in water
or solid. Now that we have already seen one reaction worked out - here
are the steps to take when writing reactions:
- Will a reaction occur
or is it no reaction (NR)? If nothing changes at all there is no reaction.
- Write the correct
product formulas so that compounds add up to zero. (don't even pay
attention to the reactant formulas - they have nothing to do with it)
Review oxidation states and ionic formulas on the Help page if
needed.
- Write the state:
solid, liquid, gas or aqueous
- last of all balance
(because of the Law of Conservation of Mass)
- Write and balance the
molecular, ionic and net ionic reaction between lithium nitrate and
sodium sulfate. Well hum, Check it
out - we have four kinds of ions in solution before reaction - Li+,
NO3-, Na+, and SO42-.
The minus ions won't react - they repel each other. The positive
ions won't react - they repel each other. Li+ and NO3-
don't react or they would not have dissolved in the first place. Na+
and SO42- don't react or they also would not have
dissolved in the first place. So the only possible products are
lithium sulfate and sodium nitrate - both are soluble and ionized in
water! So we end with the same ions in solution. Nothing
happened. NR.
- Write and balance the
molecular, ionic and net ionic reaction between potassium sulfide and
magnesium iodide.
- First imagine the
products - potassium iodide and magnesium sulfide. One of those is
insoluble so there is a rxn.
- K2S + MgI2
g
KI + MgS (these are the correct
formulas: KI since +1-1=0 and MgS since
+2-2=0)
- K2S(aq) + MgI2(aq)
g
KI(aq) + MgS(s)
from solubility rules
- K2S(aq) + MgI2(aq) g 2 KI(aq) + MgS(s)
balanced!
- Imagine a picture -
draw it yourself! In one beaker there is two K+ and one
S2- ions. In the second beaker there is one Mg2+
and 2 I- ions. Pour them
together and we get MgS
solid sinking to the bottom with two K+ and two I-
ions still ionized in the water. The ionic reaction reflects this.
- Ionic rxn: 2 K+(aq)
+ S2-(aq) + Mg2+(aq) + 2 I-(aq)
g
2 K+(aq) + 2 I-(aq) + MgS(s)
- Net ionic rxn: S2-(aq) + Mg2+(aq)
g
MgS(s) (spectator ions were
potassium and iodide)
- Write the molecular, ionic
and net ionic reactions for adding an aqueous solution of sodium sulfate
with a solution of lead(II) nitrate.
- Figure out the
products - NaNO3 and PbSO4 Are they
soluble? NaNO3 is soluble but PbSO4 is
not.
- Now write the three
versions of the reaction, don't forget to balance them. Draw it if
you want to.
- Na2SO4(aq) + Pb(NO3)2(aq) g 2 NaNO3(aq) + PbSO4(s)
- 2 Na+(aq) + SO42-(aq) + Pb2+(aq)
+ 2 NO3-(aq) g 2 Na+(aq) + 2 NO3-(aq) + PbSO4 (s)
- SO42-(aq) + Pb2+(aq)
g
PbSO4(s)
- Write and balance the
molecular, ionic and net ionic reaction between silver nitrate and sodium
chloride.
- NaCl(aq)
+ AgNO3(aq) g AgCl
(s) + NaNO3(aq)
- Na+(aq) + Cl-(aq) +
Ag+(aq) + NO3-(aq) g AgCl
(s) + Na+(aq) + NO3-(aq)
- Cl-(aq) + Ag+(aq) g AgCl
(s) Note that Na+ and NO3- cancelled
out - they are the spectator ions
- Precipitation
movie.
- Acid Base / Neutralization Reactions
- Acids - species that
ionize and lose H+ ions in water. Generic acid HA g
H+(aq) + A-(aq).
- Strong acids are those
in which almost all of the molecules lose H+ ions in
water. Strong acids are soluble in water and are strong
electrolytes since they ionize almost 100%. These are strong
acids: HCl = hydrochloric acid, HBr = hydrobromic acid, HI
- hydroiodic acid, HNO3 = nitric acid, and H2SO4 = sulfuric
acid. HCl g H+(aq) + Cl-(aq).
- Weak acids ionize much
less than 100% so are weak electrolytes since they make few ions. Acetic
acid in vinegar CH3COOH and HF are weak acids. They set up
equilibrium in water. HF D H+(aq) + F-(aq).
- Please note that H+
does not really exist - it is shorthand for H3O+
because H+ jumps on a water molecule. Remember acids lose H+
by "people that take acids are losers" Watch this acid movie by clicking on
Activities then Intro
to Acids movie.
- What makes something acidic is H+ ions in
water.
- Bases - species that
make OH- ions in water. Most are metal hydroxides. Generic
base MOH g
M+(aq) + OH-(aq).
- Strong bases are
soluble and ionize in water. These are strong bases: NaOH = sodium hydroxide, KOH = potassium hydroxide, LiOH = lithium hydroxide, and Ba(OH)2
= barium hydroxide. Strong bases are strong electrolytes since
they ionize almost 100% KOH g K+(aq) + OH-(aq)
- Weak bases ionize much
less than 100% so they are weak electrolytes since they make just a few
ions. Magnesium hydroxide and ammonia are weak bases. Mg(OH)2 is only slightly soluble in water
(Milk of Magnesia). Ammonia does not even contain the OH-
ion but it reacts with water like this; NH3(aq) + H2O(l) D NH4+(aq) + OH-(aq).
So it creates some OH-ions.
- What makes something basic is hydroxide ions being
present. Watch this base movie by clicking on Activities then Intro
to Bases movie.
- Now the neutralization reaction is when we add an
acid already in water to a base already in water. A typical acid
base reaction is: acid + base g ionic salt +
water. In other words, the H from the acid combines with the OH
from the base to make a water molecule and the other elements combine to
form an ionic salt which is usually soluble. Note that this is a
double displacement reaction.
- Example - write and
balance the molecular, ionic, and net ionic reaction between hydrochloric
acid and sodium hydroxide:
- HCl(aq)
+ NaOH(aq) g NaCl(aq) + H2O(l)
- the ionic is H+(aq) + Cl-(aq)
+ Na+(aq) + OH-(aq) g Na+(aq) + Cl-(aq)
+ H2O(l)
- and finally the net ionic
reaction is H+(aq)
+ OH-(aq) g H2O(l)
and the spectator ions are sodium and chloride.
- Example - write and
balance the chemical, ionic, and net ionic reaction between sulfuric acid
and lithium hydroxide:
- 2 LiOH(aq) + H2SO4(aq) g Li2SO4(aq) + 2 H2O(l) is the molecular
reaction.
- 2 Li+(aq) +2 OH-(aq)
+ 2 H+(aq) + SO42-(aq) g 2 Li+(aq) + SO42- (aq) + 2 H2O(l) is the ionic
reaction.
- OH-(aq) + H+(aq) g H2O(l)
is the net ionic reaction. (the 2 coefficients cancel out)
- Oxidation and Reduction
- Redox reactions are so
very important:
- formation reactions
(element + element g compound)
- combustion reactions (CxHy + O2(g) g CO2(g)
+ H2O(g))
- single replacement
reactions (Sn(s) + Cu(NO3)2(aq) g Sn(NO3)2(aq) + Cu(s))
- corrosion
reactions (4 Fe(s) + 3 O2(g) g 2 Fe2O3(s))
- respiration, bleaching, and
batteries also!
- It's all about charges
(oxidation states) changing! You MUST KNOW THE OXIDATION STATES.
- oxidation is losing
electrons so the charge goes up
- reduction is gaining
electrons so the charge is reduced (goes down)
- You must have both
together - and electron is transferred from what is getting oxidized to
what is getting reduced
- What are the oxidation
states for:
- H2O
(H is +1 and O is -2)
- CO2
(C is +4 and O is -2)
- N2
(N is 0) all elements are zero
- Ni2O3
(O is -2 to that means Ni must be +3)
- Redox Reactions
- Watch this base movie
by clicking on Activities then oxidation
and reduction part I movie.
- Example of a redox
reaction: Co (s) + Pb2+(aq) g Pb
(s) + Co2+(aq) is a single
replacement rxn
- Co goes from zero to
+2
- Pb2+ goes
from +2 to zero
- What is
oxidized? Co (s)
- What is reduced?
Pb2+(aq)
- What is the oxidizing
agent? Pb2+(aq)
- What is the reducing
agent? Co (s)
- This is already a net
ionic reaction by the way. You can't actually have positive
charges without negative charges.
- Example: CH4(g)
+ 2 O2(g) g CO2(g) + 2
H2O(g) a combustion reaction
- C in CH4
goes from -4 to a +4 in CO2
- O in O2
goes from zero to -2 in CO2 and H2O
- What is
oxidized? C in CH4
- What is reduced?
O in O2
- What is the oxidizing
agent? O2
- What is the reducing
agent? CH4
- Note also that the
answer to those four questions is never a product since they are being
formed. There is not ionic rxn here since
there are NO IONS!
- Example: 2 Na (s)
+ 2 HCl (aq) g 2 NaCl(aq) + H2(g) is a single
replacement rxn
- Na goes from zero in
the solid to +1 in NaCl
- H goes from +1 in HCl to zero in hydrogen gas
- What is
oxidized? Na (s)
- What is reduced?
H in HCl(aq)
- What is the oxidizing
agent? HCl(aq)
- What is the reducing
agent? Na (s)
- What is the net ionic
reaction? 2 Na (s) + 2 H+(aq)
g
2 Na+(aq) + H2 (g)
(Cl was just a spectator)
- Activity Series
- How did I know if those
single replacement reactions would occur? Well the activity series
lets you know. The MORE active metal or cation wants to be an ion
in a compound. The LESS active metal or cation wants to be in its
elemental state. Look at example D and B above again and look at the
activity series.
- Practice
these fabulous examples
- Watch the formation
of silver crystals movie here by clicking on Activities. It is a
single replacement rxn
- Write the reaction
between solid copper and lead(II) nitrate.
Actually NR since Cu is less active and want to
be elemental solid copper anyway.
- Write the molecular,
ionic and net ionic reaction between solid zinc and lead(II)
nitrate. Remember the 4 steps?
- Yes there is a
reaction since zinc is more active and wants to be in the compound more
than lead.
- Products would be
solid lead Pb(s) and zinc nitrate Zn(NO3)2(aq) (remember Zn is always +2 charged)
- Zn(s) + Pb(NO3)2(aq)
g
Pb(s) + Zn(NO3)2(aq)
- already balanced
- ionic reactions:
Zn(s) + Pb2+(aq) + 2 NO3-(aq) g Pb(s)
+ Zn2+(aq) + 2 NO3-(aq)
- net ionic: Zn(s)
+ Pb2+(aq) g Pb(s)
+ Zn2+(aq)
(nitrate was a spectator ion)
- What is oxidized, reduced? What are the agents? oxidized is solid Zn as its charge goes up, reduced is
lead 2+ ion as its charge goes down, the ox agent is lead 2+ ions and
the red agent is solid zinc.
- The most active metal
is Li. why is it active anyway? Well because Li WANTS
to be +1 charged so it can be like a noble gas. Na is also active
because it wants to be +1 charged. Remember the metal atoms goal in life
- to be s2p6 and they do that by losing
electrons. The transition metals are not as active since changing
their d electrons is not that important to them.
Done!!! Study this chapter
hard. It is so important.