Demonstrate a knowledge of balanced chemical equations
A chemical equation describes what happens in a chemical reaction.
The equation identifies the reactants (starting materials) and products
(resulting substance), the formulas of the participants, the phases of the
participants (solid, liquid, gas), and the amount of each substance. Balancing
a chemical equation refers to establishing the mathematical relationship
between the quantity of reactants and products. The quantities are expressed as
grams or moles.
It takes practice to be able to write balanced equations. There
are essentially three steps to the process:
1. Write the unbalanced equation.
* Chemical formulas of reactants are listed on the left hand side
of the equation.
* Products are listed on the right hand side of the equation.
* Reactants and products are separated by putting an arrow between
them to show the direction of the reaction. Reactions at equilibrium will have
arrows facing both directions.
Note: Hydrogen, Nitrogen, Oxygen, Fluorine, Chlorine, Bromine
and Iodine are all diatomic. That is they exist as pairs of atoms rather than
as single atoms (ex. H2, N2, O2, F2, Cl2, Br2, I2). Remember this when you write chemical equations.
2. Balance the equation.
* Apply the Law of Conservation of Mass to get the same number of
atoms of every element on each side of the equation. Tip: Start by balancing an
element that appears in only one reactant and product.
* Once one element is balanced, proceed to balance another, and
another, until all elements are balanced.
* Balance chemical formulas by placing coefficients in front of
them. Do not add subscripts, because this will change the formulas.
3. Indicate the states of matter of the reactants and products.
* Use (g) for gaseous substances. * Use (s) for solids.
* Use (l) for liquids. *
Use (aq) for species in solution in water.
* Write the state of matter immediately following the formula of
the substance it describes.
Worked Example Problem
Tin oxide is heated with hydrogen gas to form tin metal and water
vapor. Write the balanced equation that describes this reaction.
1. Write the
unbalanced equation.
SnO2 + H2 -->
Sn + H2O
2. Balance the
equation.
Look at the equation and see which elements are not balanced. In this
case, there are two oxygen atoms on the left hand side of the equation and only
one on the right hand side. Correct this by putting a coefficient of 2 in front
of water:
SnO2 + H2 --> Sn + 2 H2O
This puts the hydrogen atoms out of balance. Now there are two hydrogen
atoms on the left and four hydrogen atoms on the right. To get four hydrogen
atoms on the right, add a coefficient of 2 for the hydrogen gas. Remember,
coefficients are multipliers, so if we write 2 H2O it denotes 2x2=4 hydrogen
atoms and 2x1=2 oxygen atoms.
SnO2 + 2 H2 --> Sn + 2 H2O
The equation is now balanced. Be sure to double-check your math! Each
side of the equation has 1 atom of Sn, 2 atoms of O, and 4 atoms of H.
3. Indicate
the physical states of the reactants and products.
To do this, you need to be familiar with the properties of various
compounds or you need to be told what the phases are for the chemicals in the
reaction. Oxides are solids, hydrogen forms a diatomic gas, tin is a solid, and
the term 'water vapor' indicates that water is in the gas phase:
SnO2(s) + 2 H2(g) --> Sn(s) + 2 H2O(g)
This is the balanced equation for the reaction.
Balancing Chemical Equations (said again, in another way)
You may remember that the law of conservation of mass says that
matter is neither created nor destroyed during a chemical reaction. This means
that all chemical reactions must be balanced—the number of atoms, moles,
and ultimately the total mass must be conserved during a chemical process. Here
are the rules to follow when balancing equations:
1. Determine
the correct formulas for all the reactants and products in the reaction.
2. Begin
balancing with the most complicated-looking group. A polyatomic ion that
appears unchanged on both sides of the equation can be counted as a single
unit.
3. Save the
elemental (single elements) reactant and products for last, especially if it is
hydrogen or oxygen. Keep your eye out for diatomic molecules such as oxygen,
hydrogen, and the halogens.
4. If you get
stuck, double the most complicated-looking group and try again.
5. Finally,
make sure that all coefficients are in the lowest-possible ratio.
6. Know when
to quit! None of the reactions you will encounter will be that difficult. If
the coefficients are getting wild, double-check what you've done since you may
have a simple mistake.
When balancing reactions, keep your hands off the subscripts! Use
only coefficients to balance chemical equations. Now let's try an example. When
you solve it yourself, make sure to follow the steps!
Example
Write the balanced equation for the reaction between chlorine and
sodium bromide, which produces bromine and sodium chloride.
Explanation
First write the chemical formulas—be on the lookout for the
diatomic elements (such as Cl2):
Cl2 + NaBr -->Br2 + NaCl
Next, find the reagent with the scariest subscripts. In this case,
start with Cl2. You need a coefficient of 2 in front of NaCl, which then
requires a coefficient of 2 in front of NaBr. The balanced equation becomes
Cl2 + 2NaBr --> Br2 + 2NaCl
Finally, count up everything to make sure you balanced the
equation correctly. You have two chlorine atoms, two sodium atoms, and two
bromines on the reactants side and two bromines, two sodiums, and two chlorines
on the products side. You-->re done.
Example
Write the balanced equation for the reaction between aluminum
sulfate and calcium chloride, which produces aluminum chloride and calcium
sulfate.
Explanation
Write the chemical formulas on their correct sides:
Al2(SO4)3 + CaCl2 --> AlCl3 + CaSO4
In this reaction, the aluminum sulfate looks the most complicated,
so start there. Look at what happens with sulfate—since it remains
sulfate on the right side of the reaction, treat it as a unit. You have three
on the left side and only one on the right side, so place a coefficient of 3 in
front of calcium sulfate. Now deal with the aluminum. You have two on the left
and one on the right, so place a coefficient of 2 in front of aluminum
chloride. Last, you must place a coefficient of 3 in front of calcium chloride.
Al2(SO4)3 + 3CaCl2 --> 2AlCl3 + 3CaSO4
Count the atoms on both sides of the reaction and you'll see that
you're done.