General Chemistry – Unit 7
Chemical Equations
Chapter 7
What is a chemical rxn?
A process where one or more substances
changes to one or more other substances.
Exchange of electrons!!!
Parts of a chemical reaction
Reactants the original substances
Products the resulting substances
ASK YOURSELF:
Which are the products and which are the
reactants in these equations?
Na+ + Cl- NaCl
2H2O 2H2 + O2
Quick Demo
Magnesium
A word equation
Magnesium metal reacts with oxygen gas
to form white magnesium oxide solid.
We must learn how to translate these!
Balancing equations Tips
__Fe2O3 + __H2SO4 __Fe2(SO4)3 + __H2O
If possible treat polyatomic ions as 1 unit
Balance hydrogen 2nd last
Balance oxygen last
If the last element does not balance double
everything
Ways to Represent Reactions
Chemical equation: symbols that describe a
chemical reaction and show what atoms (and
how many, relatively speaking) are involved.
Word equation: the names of each
product/reactant are written out. Doesn’t
show how many of each
Write out the word equation
Magnesium metal reacts with oxygen gas
to form magnesium oxide solid.
Mg + O MgO
Remember BrINClOFH
Br2 – a liquid
I2 - a solid
N2 - a gas
Cl2 - a gas
O2 - a gas
F2 - a gas
H2 - a gas
Check for BrINClOFH
Any time you see 1 of the 7 diatomic elements
alone, write them with a 2 subscript
Mg + O2 MgO
Balance the charges of ionic comp
In our reaction we have already done this,
but make sure to use proper formulas (for
molecular comp too!):
MgO = Mg2+ O2-
Mg + O2 MgO
Balance number of atoms/ions on
each side
In ordinary chemical changes, atoms just
rearrange
This is the LAW OF CONSERVATION OF
MATTER
We do this with coefficients
2Mg + O2 2MgO
Example
If you take Ca(NO3)2 and change it to
2Ca(NO3)2, how many of each element do
you have?
Put in physical states
Symbols used in chemical equations:
(s) solid
(l) liquid
(g) gas
(aq) aqueous solution (dissolved in H2O)
Put in physical states
I will give you physical states in word
problems:
Magnesium metal reacts with oxygen gas
to form magnesium oxide solid
2Mg(s) + O2(g) 2MgO(s)
Word Equation example
Hydrogen gas and Oxygen gas combine
to form liquid water.
2H2(g) + O2(g) 2H2O(l)
Example:
Sodium solid reacts with chlorine gas to
form sodium chloride solid.
2Na(s) + Cl2(g) 2NaCl(s)
Try this one:
Dicarbon dihydride reacts with oxygen gas
to produce carbon dioxide gas and water
Evidence of rxns
All rxns involve chemical change
Look for evidence of those changes:
Light, heat, gas, color change, precipitate
Other Symbols
precipitate forms
gas forms
“produces” or “yields”
products can reform in to reactants –
end result is a mix of the two.
Δ – means that heat was added
Chemistry – Chemical
Equations
Part II
Experiment 17
Please read over steps 1-8 carefully
Get the proper safety gear, and the proper
equipment
Do steps 1-8, then set the solid aside to dry
(step 9)
Return to your seat – while the solid dries,
you can work on A7
At the end of class – record the mass of your
beaker and solid.
Balanced equations show
RATIOS
The coefficients of a balanced equation
reveal the RATIO of each reactant to each
product:
2H2 + O2 2H2O means there is a 2:1:2 ratio
of H2 to O2 to H2O.
This is the same as a 4:2:4 ratio, etc. etc.
The activity series
Writing an equation doesn’t mean it will
actually happen
How do we know it will happen?
Single replacement reactions are
predicted with the activity series
This activity series has been created
through experiment.
The activity series
Things on the list will displace anything
below but not above other things on the
list
There are two lists – one for elements that
become positive ions, and one for
elements that become negative ions.
Element Reactivity Element Reactivity
Li React with cold H2O and acids, F2 Listed from most reactive to
Rb replacing hydrogen Cl2 least reactive
K Br2
Ba I2
Ca
Na
Mg React with acids or steam but
Al usually not liquid water, to replace
Mn hydrogen
Zn
Cr
Fe
Ni All react with acids but not water, to
Sn replace hydrogen
Pb
H2 All react with oxygen to form
Cu oxides
Hg
Ag Mostly unreactive
Pt
Au
? When a rxn releases energy,
where does it come from?
•You have to put energy
in to break bonds
•If bonding is stronger in
the products they will be
more stable
•More stable = lower
energy
•Lower energy means
energy must be
Is this an example of an ENDOthermic RELEASED
or EXOthermic reaction?
If bonding is
weaker in
the products,
they will be
less stable,
have more
energy, and
will therefore
absorb it.
Is this an endo or exothermic rxn?