Download Chapter 9 Chemical Reactions

Chapter 9 Chemical Reactions
-orThe old element shuffle
Review:
Naming and writing formulas for ionic
compounds.
Ex1: FeCl3
Find the name for each ion using your ion
sheet. Cl has a -1 charge and is chloride.
Fe is iron and has more than 1 possible
ion. Since we have 3 Cl’s and therefore 3
negative charges, the iron must be +3, so
the ion is iron (III).
Put the ion names together, cation first.
iron (III) chloride
Ex2: Barium nitrate
Find the ion symbols: barium = Ba+2
nitrate = NO3Put them together so that the charges cancel:
Ba(NO3)2
Practice:
chromium (II) sulfate
K2S
All chemical reactions are written as:
Reactants → Products
Both reactants and products may be one or more
elements or compounds. A written chemical reaction is a
description of what really happens in nature.
Two laws govern what happens in a chemical reaction:
1. The law of conservation of mass says that matter is
not created or destroyed in a chemical reaction. This
means that only atoms that you start with on the reactant
side must be present on the product side. These atoms
are rearranged to form new substances. This also
means that the total mass of the reactants must equal
the total mass of the products
Ex: Diagramming the reaction of CH4 with O2.
2. The law of conservation of energy says that energy
is not created or destroyed in a chemical reaction.
Energy can only be transferred to or from the
surroundings or transformed into a different form
of energy.
This means that the energy stored in the reactant
chemicals plus any absorbed energy must equal the
energy stored in the product chemicals plus any
released energy.
When considering a chemical reaction, the
surroundings include everything that is not actually
involved in the reaction ie. Solvents, beakers, air, your
fingers.
Ex: Diagram the reaction of CH4 with O2 in terms of
energy.
Energy diagrams:
Exothermic
Endothermic
The amount of energy that is gained or released in a
given chemical reaction can be measured.
The amount of energy change for a reaction is specific
to that reaction and is called the enthalpy of the
reaction. It is given the symbol ΔH
Ex: For the reaction 2H2 + O2 → 2H2O
For every 1 mole of H2 that reacts, 68.15 kcal of
heat are released to the surroundings.
We write this as ΔH = -68.15 kcal/mol
Note that ΔH is negative. This indicates that heat is
removed from the system (exothermic). ΔH for an
endothermic reaction will be positive (heat added to
the system).
Signs of a chemical reaction:
1. Heat (given off or absorbed)
2. Light emitted
3. Color change
4. Gas given off (bubbling, fizzing)
5. Forming a precipitate (solid forms from solution)
There are 2 types of numbers present in chemical equations:
1. Subscripts. These describe the compositions of the
substances in the reaction. They tell how many atoms of
each element are present in a given molecule. These are
fixed for a substance and cannot be changed.
Ex: O2
Na2SO4
Al2O3
2. Coefficients. These are the larger font numbers that
appear in front of a substance in a reaction. They tell how
many of that substance take part in this particular
reaction. These may vary from one reaction to another.
Ex: 2H2 + O2 → 2H2O
CH4 + 2O2 → CO2 + 2H2O
Note that when the coefficient is 1, we don’t write
anything. 1 is always understood.
The coefficient multiplies the element or compound
that comes after it.
Ex: 2H2O means 2 water molecules, (H2O H2O)
this means 4 H atoms and 2 O atoms.
3Na2SO4 + 2Fe → Fe2(SO4)3 + 6Na
How many Na
Na
S
S
O
O
Fe
Fe
Other symbols that you may see in chemical
equations:
Subscripts: (s) = solid eg. Na(s)
(l) = liquid
eg. H2O(l)
(g) = gas eg. Cl2(g)
(aq) = aqueous (dissolved in water)
eg. NaCl(aq)
Ex: 2H2(g) + O2(g) → 2H2O(l)
Sometimes there are symbols or numbers above or
below the arrow. These indicate conditions for
the reaction.
1. Reversible reaction –
2. Heat added –
3. Specific temperature –
4. Specific pressure –
5. Catalyst –
Examples:
Balancing chemical equations:
Since a chemical equation is a description of what
happens in a given reaction, the identities of the
products and reactants cannot change. This means
that you may never change a subscript when
balancing an equation.
To balance an equation you may only insert
coefficients so that the number of each type of
atom is the same on both sides of the equation.
Ex: CH4 + 2O2 → CO2 + 2H2O
1C
4H
4O
1C
4H
4O
(Java – balancing)
How to balance equations:
1. Make an inventory of the number of atoms of
each element on both sides of the arrow.
Ex: C2H4 + O2 → CO2 + H2O
CCHHOO2. Pick an element to balance first. Save an
element that appears by itself until the end.
Ex: Save O until the end. Balance C first.
3. Adjust your inventory to show changes. Work
back and forth between reactants and products
until all elements are balanced.
4. If it will not work out. Start over with a different
element to begin.
5. Make sure that the coefficients are in the lowest
ratio.
Hints:
Polyatomic ions that do not change in a
reaction may be balanced as a group.
Ex: H2SO4 + Cr → Cr2(SO4)3 + H2
6.
Balance with a fraction and then multiply the
whole equation to get rid of the fraction.
Ex: C3H6 + O2 → CO2 + H2O
Practice:
CH4 + Cl2 → CCl4 + HCl
KClO3 → KCl + O2
MgCl2 + FeN → FeCl3 + Mg3N2
Pb(NO3)2 + NaCl → NaNO3 + PbCl2
What a balanced equation can tell you – Charting a reaction
2C2H6
Moles
Molecules
Molar
mass
Mass (g)
Total
mass (g)
+ 7O2
→ 4CO2
+ 6H2O
2BF3
Moles
Molecule
s
Molar
mass
Mass (g)
Total
mass (g)
+ 3Li2SO3 → B2(SO3)3
+ 6LiF
The coefficients can be considered as mole
ratios. They tell you the relative numbers of
moles of all products and reactants in the
reaction.
Types of chemical reaction
1. Synthesis. In these reactions , the reactants are elements
(or very simple compounds) that combine to form a more
complex compound.
Form: A + B → AB
(where A and B are
elements and AB is a
compound formed
from the elements)
Ex:
2H2 + O2 → 2H2O
Predicting the products of a synthesis reaction:
Use your ion sheets to tell what ions A and B will
form.
Put the ions together to form a neutral compound.
Balance the equation.
Ex: Mg + Cl2 → ????
Mg forms a +2 ion
Cl forms a -1 ion.
They would combine as 1 Mg and 2 Cl so the
compound that forms isMgCl2.
Fill in the product and balance.
Mg + Cl2 → MgCl2
Ex2: Al + Br2 →
Al form s a +3, Br forms a -1
so, AlBr3
Al + Br2 → AlBr3 balance!!!!
2. Decomposition. In these reactions , the reactants are
compounds that break down into the elements (or
simpler compounds)
Form: AB → A + B
(just the opposite of
synthesis)
Ex:
2H2O → 2H2 + O2
Predicting the products of a decomposition reaction:
Break up the compound into elements (do not
forget BOFINCH)
Balance the equation.
Ex:
NO2 → N2 + O2
Balance!!!
3. Single displacement. In these reactions you start
with a single element and a compound. The
single element takes the place of (displaces) one
of the elements in the compound.
Form: A + Bx → Ax + B
where A is an element and Bx is a compound. A
takes the place of B with x.
Ex: Al + CuCl2 → AlCl3 + Cu
Predicting the products of a SD reaction:
Find if the lone element on the reactant
side forms a + or – ion. If it forms a + ion
then it takes the place of the + ion in the
compound. (- ion takes – ion’s place).
Put A and x together to form a neutral compound. B
ends up as the elemental form.
Balance.
Ex: Na + ZnCO3 → ???
Na forms a +1 ion. Zn is a +2 and CO3 is a -2.
Na will take the place of Zn.
The compound that forms will be Na2CO3
Zn will be left alone
Ex2:
Cl2 + AlBr3 → ???
Cl forms a -1 ion. Al is +3 and Br is -1
Cl will take the place of Br.
The compound that forms will be AlCl3
Br will be left alone
4. Double displacement. In these reactions you start
with 2 compounds. The + ions switch partners
Form: Ax + By → Ay + Bx
where A and B are + ions and x and y are -. A
and B switch places.
Ex: Na2CrO4 + Pb(NO3)2 → PbCrO4 +
NaNO3
Predicting the products of a SD reaction:
Switch the partners of the + ions.
Figure out how each of the + ions will go
with their new partners to form a neutral
compound.
Balance.
Ex:
Na3PO4 + Co(NO3)3 → ???
Co+3 and Na+ are the cations so they will switch partners.
Co+3 and PO4-3 go together 1:1
CoPO4
Na+ and NO3- go together 1:1 NaNO3
5. Combustion. In these reactions you start with a
CH compound. The compound reacts with O2 to
form CO2 and H2O.
Form: CxHy + O2 → CO2 + H2O
The only thing that will change is the CH
compound. The products are always the same.
Ex: C2H6 + O2 → CO2 + H2O
Predicting the products of a combustion
reaction:
Always the same: carbon dioxide and
water
Balance.
Practice: For the following, give the type of reaction, predict
the products and balance.
HgO →
C6H6 + O2 →
Al + Cl2 →
K + Al2(SO4)3 →
NaBr + AgNO3 →
Net Ionic Equations:
these are equations that are simplified to shown
only the elements or ions that undergo a change
in the reaction.
Ions that do not undergo a change (are the same
on both sides of the equation) are dropped out.
These are called spectator ions.
Writing net ionic equations:
Na2CrO4(aq) + Pb(NO3)2(aq) → PbCrO4(s) + 2NaNO3(aq)
Ionic compounds dissociate into ions when
dissolved so we can write:
2Na+(aq) + CrO4 -2(aq) + Pb+2 (aq) + 2NO3-(aq) →
PbCrO4(s) + 2Na+(aq) + 2NO3-(aq)
Since Na+ and NO3- do not change (spectator ions) they drop
out of the equation:
CrO4 -2(aq) + Pb+2 (aq) → PbCrO4(s)
Practice: Write the net ionic equation for
Na3PO4(aq) + Co(NO3)3(aq) → CoPO4(s) + 3NaNO3(aq)
Activity series is a listing of elements and ions in
order of decreasing reactivity. It is used to
predict whether a reaction with certain reactants
will take place or not.
If element A is higher on
The list than element B,
Then A will replace B in a
Single displacement
Reaction. If A is lower
Than B then no reaction
Practice: Which of the following reactions will take place?
3Ca + 2AlCl3 → 3CaCl2 + 2Al
H2 + ZnCl2 → 2HCl + Zn
2Na + Cu(NO3)2 → 2NaNO3 + Cu