Chemical Equations

2H2 + O2 --> 2H2O 2H2O --> 2H2 + O2 We must first become familiar with the “look” of chemical equations and how they may change during a reaction: 1.  A synthesis reaction involves two or more substances combining to make a more complex substance.  The reactants may be elements or compounds, and the product will always be a compound.  Formation of Water 2H2 + O2 --> 2H2O 2.  In a decomposition reaction, one substance is broken down into two or more, simpler substances.  This type of reaction is the opposite of a synthesis reaction. Electrolysis of Water 2H2O --> 2H2 + O2 3. In a Single Displacement type of reaction, a neutral element becomes an ion as it replaces another ion in a compound.  The general form of this equation can be written as: positive ion being replaced:  A + BC ----> B + AC or a negative ion being replaced:  A + BC ----> C + BA EXAMPLE: Zn + H2SO4 ---> ZnSO4 + H2

AgNO3 + NaCl ----> AgCl + NaNO3 4. In a Double Displacement reaction (like dancing couples) the compounds in this type of reaction exchange partners.  The basic form for this type of reaction is shown below; AB + CD ----> CB + AD Example: AgNO3 + NaCl ----> AgCl + NaNO3 5. Combustion - When organic compounds like propane are burned, they react with the oxygen in the air to form carbon dioxide and water.  The reason why these combustion reactions will stop when all available oxygen is used up is because oxygen is one of the reactants.  CH4 + 2O2 ----> 2H2O + CO2 You may have noticed something strange within some of these formulas...large numbers in front of some molecules...????

When atoms of elements exists as ions, we must include additional information as a superscript of the symbol. (Remember, a number in the subscript lets us know if there is more than one atom in the molecule) A plus or minus sign simply tells us if it is a positive ion (cation) or a negative ion (anion). The number lets us know how many electrons are gained of lost to form the ion. 1. How many protons does this silicon ion have? Electrons? 2. How many protons does this oxygen atom have? Electrons? 3. What type of bond could form between these two?

Balancing Chemical Equations: WHY BOTHER? Because of The Law of Conservation of Mass we know that atoms are not lost or gained during a chemical reaction – just rearranged. Therefore, we have to make just that the reactants always add up to the product. A chemical equation is balanced when the number of atoms of each type on each side of the equation is the same. Which means if you have 12 hydrogens on the left hand side of the equation, you must have 12 hydrogens on the right hand side, if there are 4 oxygens on the left, there must 4 oxygens on the right, and so on. ! EXAMPLE: Unbalanced : C3H8 + O2 ---> H2O + CO2 Balanced : C3H8 + 5O2 ---> 4H2O + 3CO2 Confusing? Luckily, the only math operation needed to balance an equation is multiplication – and you can manage that!

One step at a time..... H2 + O2 H2O L R 2H2 + O2 2H2O 1. Realize that the reactants (before) are on the left separated by an arrow from the products (after) on the right – and both sides contain the very same elements. H2 + O2           H2O 2. Make an “inventory” table of the elements involved and the number of atoms on each side: L R Hydrogen (H) 2 2 Oxygen (O) 2 1 3. You must now make the #’s of each element equal by multiplying somewhere. You may be temped to multiply the oxygen on the right by 2 to strike an easy balance...But wait...The product is already a bound molecule. If you alter the number of only the oxygen atoms, this molecule will no longer be water! Instead: 2H2 + O2        2H2O ***By putting a 2 before the water molecule in this case, we have doubled the amount of any atoms at a fixed rate. This number multiplies the atoms in the entire molecule, whether the molecule consists of 1 single atom, or many amounts of different atoms.

A shortcut for more complicated balancing: This is the equation for the burning of aluminum in oxygen. If we convert each of the chemical names into the appropriate symbols, we get the following: Al + O2      Al2O3 We must balance the amount of atoms to be equal between both sides. However, like in the previous example, we can’t adjust the aluminum or oxygen atoms separetly on the left side because it is now a bound product -Aluminum Oxide. Balance this equation!! HINT: Also, as in the previous example, we can only gain flexibility with the atom numbers on the right by changing the number of full molecules. If we double the amount of molecules, it will result in doubling the atoms in that molecule. Remember, two water molecules would mean 4 hydrogens and 2 oxygens. Tripling the molecules would triple the atoms... Three water molecules would obviously include 6 hydrogens and 3 oxygens...and so on. Most of this is no different than finding a least common denominator in math. For example, what is the lowest common demoninator between 6 and 9? 18, right? (3) 6 = (2) 9 They meet at an equal value by multiplying each a specific number of times. Solution: 4Al + 3O2 ----> 2Al2O3