Reaction Types.doc

Reaction Types

Now that you are familiar with atoms and molecules and how they make up the solids, liquids, and gases we see around us, let’s discuss what happens when two molecules meet: namely, chemical reactions. A chemical reaction is defined as a process by which one or more substances are changed into one or more new substances. Chemical reactions are usually portrayed in this way:

reactantproduct

Theis read as “yields” or “produces.” You will often see the states of matter in parentheses as subscripts after the chemical formulas of the reactants and products. The symbols for the states of matter and some other chemical reaction symbols you should be familiar with are given in the table below.

Balancing Chemical Equations

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, especiallyif 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 + NaBrBr2 + 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 + 2NaBrBr2 + 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 + CaCl2AlCl3 + 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 three 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 + 3CaCl22AlCl3 + 3CaSO4

Count the atoms on both sides of the reaction and you’ll see that you’re done.

Types of Chemical Reactions

It is important that you know the basic types of chemical reactions for the SAT II Chemistry test since the test often refers to reactions as being of one type or another. Here’s a list of the different types of reactions, with examples of each type included.

Synthesis reaction: This is a reaction in which two or more elements or compounds combine to form a single product. This type of reaction follows the general equation

A + BC

where A and B may be either elements or compounds.

Here are some examples:

2Na(s) + Cl2(g)2NaCl(s)

MgO(s) + H2O(l)Mg(OH)2(aq)

SO2(g) + H2O(l)H2SO3(aq)

Decomposition reaction: In this type of reaction, a single reactant, a compound, breaks into two or more parts. Often these are the most difficult to predict. Here is the general equation:

ABA + B

where A and B may be either elements or compounds.

Here are some examples of decomposition reactions:

2H2O(l)2H2(g) + O2(g)

H2CO3(aq)H2O(l) + CO2(g)

CaCO3(s)CaO(s) + CO2(g)

2KClO3(s)2KCl(s) + 3O2(g)

Single replacement or displacement reaction: In this type of reaction, a more active element replaces a less active element in a compound. Among the halogens, F2is the most active halogen, and the activity of the halogens decreases as you go down the group. For the metals, you will need to be given an activity series. General equation:

A + BCAC + B

where A is a metal.

Here is an example of a displacement reaction in which a metal is involved:

Cu(s) + 2AgNO3(aq)2Ag(s) + Cu(NO3)2(aq)

...