Transcript Chemical Equations

Chemical Equations
In this lesson, you will go from chemical
formulas to chemical equations, a required step
in stoichiometry.
If you have trouble writing chemical formulas,
please review that concept before continuing
here.
Chemical Equations
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What happens to matter when it undergoes
chemical changes?
The law of conservation of mass says that
atoms are neither created nor destroyed,
during any chemical reaction.
Thus, the same collection of atoms is present
after a reaction as before the reaction. The
changes that occur during a reaction just involve
the rearrangement of atoms.
A chemical equation shows how the atoms are
rearranged.
Chemical Equations
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Chemical reactions are represented on paper by chemical
equations. For example, hydrogen gas (H2) can react
(burn) with oxygen gas (O2) to form water (H20). The
chemical equation for this reaction is written as:
2H2 + O2  2H2O
The '+' is read as 'reacts with' and the arrow '' means
'produces'. The chemical formulas on the left represent the
starting substances, called reactants. The substances
produced by the reaction are shown on the right, and are
called products. The numbers in front of the formulas are
called coefficients (the number '1' is usually omitted).
Chemical Equations
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Below are the parts of a
chemical equation.
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The reactants are listed on
the left side and show what
went into the reaction.
The products are on the right
side and show what was
produced by the reaction.
The arrow is like an = sign,
but shows the direction of the
reaction.
The coefficients show the
ratio in which the substances
reacted/were produced. When
no coefficient is present, a 1
is implied.
Chemical Equations
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Because atoms are neither created nor destroyed in a reaction, a
chemical equation must have an equal number of atoms of
each element on each side of the arrow (i.e. the equation is
said to be 'balanced').
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Steps involved in writing a 'balanced' equation for a chemical
reaction:
1. Write the chemical formula for each of the named substances.
Don’t forget that the diatomic elements always have a subscript of
two if not combined with another element.
2. Write the skeleton equation using formulas of reactants and
products. Don’t forget the arrow between reactants and products.
3. Balance the equation by determining coefficients that provide
equal numbers of each type of atom on each side of the equation
(generally, whole number values).
Note! Subscripts should never be changed when trying to
balance a chemical equation. Changing a subscript changes
the actual identity of a product or reactant. Balancing a
chemical equation only involves changing the relative
amounts of each product or reactant using coefficients.
Chemical Equations
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Consider the reaction of burning the gas methane (CH4) in
air. We know experimentally that this reaction consumes
oxygen (O2) and produces water (H2O) and carbon dioxide
(CO2). Thus, we have accomplished step #1 above. We now
write the unbalanced chemical equation (step #2):
 CH4 + O2  H2O + CO2
Let’s see if the two sides of the equation are balanced:
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Carbon – 1
Hydrogen – 4
Oxygen – 2
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Carbon – 1
hydrogen – 2
oxygen – 3
The equation is not balanced! What can we do? We can
add coefficients to balance it.
Chemical Equations
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CH4 + O2  2H2O + CO2
By adding the coefficient 2 to the water, we double the
number of hydrogens and the number of oxygens in that
substance. We now have 4 hydrogens on each side of the
equation. That’s one problem solved.
The oxygens, however, are still unbalanced. Of I look at
everything on the products side, I have 4 oxygens. The
reactant side only has two oxygens. How can I fix this
imbalance?
CH4 + 2O2  2H2O + CO2
A coefficient of 2 on the oxygen gas fixed it!
Chemical Equations
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We now have fulfilled step #3, we have a balanced
chemical equation for the reaction of methane with oxygen.
Thus, one molecule of methane reacts with two molecules
of oxygen to produce one molecule of carbon dioxide and
two molecules of water.
 CH4
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+ 2O2  2H2O + CO2
The physical state of each chemical can be indicated by
using the symbols (g), (l), and (s) (for gas, liquid and solid,
respectively):
 CH4(g)
+ 2O2(g)  2H2O(l) + CO2 (g)
Chemical Equations
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Try this:
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Ammonium chloride reacts with sodium
carbonate to produce sodium chloride and
ammonium carbonate.
1. Start by figuring out the formula for each substance.
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Ammonium chloride is
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Sodium carbonate is
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Sodium chloride is
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Ammonium carbonate is
Chemical Equations
NH4Cl + Na2CO3  NaCl + (NH4)2CO3
Because the carbonate had a charge of -2, the only way to
combine sodium and carbonate into a neutral compound is
to use two sodiums for each carbonate.
Note how the ammonium carbonate required parentheses, as
two ammoniums were needed to balance the charge of the
carbonate. Subscripts are used within a chemical formula to
show the number of atoms in a single unit.
Now see of you can balance the equation with coefficients.
Chemical Equations
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NH4Cl + Na2CO3  NaCl + (NH4)2CO3
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The ammonium’s are not balanced! What shall we do?
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2NH4Cl + Na2CO3  NaCl + (NH4)2CO3
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The ammoniums are fixed, but that changed the chlorines.
We now have 2 chlorines on the product side and one on the reactant
side.
What coefficient do we add next?
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2NH4Cl + Na2CO3  2NaCl + (NH4)2CO3
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That fixed the sodiums AND the chlorines.
Anything else?
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Check the carbonates, as that’s all we have left.
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The carbonates are balanced. The equation is all balanced by just
adding those two coefficients, hence:
2NH4Cl + Na2CO3  2NaCl + (NH4)2CO3
This equation tells us that two formula units of ammonium
chloride react with one formula unit of sodium carbonate to
produce two formula units of sodium chloride and one unit
of ammonium carbonate.
Those same ratios apply to moles of the substances.
Two moles of ammonium chloride react with one mole of
sodium carbonate to produce two moles of sodium chloride
and one mole of ammonium carbonate.
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2NH4Cl + Na2CO3  2NaCl + (NH4)2CO3
Those coefficients give us the ratios of all substances
involved:
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There are two ammonium chlorides for each sodium
carbonate. The ratio of ammonium chloride to sodium
carbonate is 2:1.
The ratio of ammonium chlorides to sodium chlorides is 1:1.
Why?
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The ratios are always reduced to their lowest form.
2:2 reduces to 1:1.