Chemistry All-in-One For Dummies Cheat Sheet

Converting metric units

Because chemists must be able to communicate their measurements to other chemists all over the world, they need to speak the same measurement language. This language is the SI system of measurement, related to the metric system.

Below is a chart of the most commonly used metric prefixes, their abbreviations, and how they relate to a metric base unit. Remember, the metric system is based on 10s, so each time you are converting from one unit to another you can just count the number of jumps between prefixes to have an idea of what your conversion should be.

For example, if you want to convert 500 ml to liters, all you have to do is multiply that by the conversion shown in the chart for milli as shown below. Be sure to cancel out your units!

Writing the formulas of ionic compounds

A quick way to determine the formula of an ionic compound (remember ionic compounds are metals bonded to nonmetals) is to use the crisscross rule. The crisscross rule uses the ionic charges of the ions to predict the formula of the ionic compound. It doesn’t work every time but it is a very useful method for writing many ionic formulas. Check it out as shown here:

Take the numerical value of the metal ion’s superscript (forget about the charge symbol) and move it to the bottom right-hand side of the nonmetal’s symbol — as a subscript. Then take the numerical value of the nonmetal’s superscript and make it the subscript of the metal. (Note that if the numerical value is 1, it’s just understood and not shown.)

So, in this example, you make magnesium’s 2 a subscript of bromine and make bromine’s 1 a subscript of magnesium (but because it’s 1, you don’t show it), and you get the formula MgBr₂.

Naming covalent compounds

Covalent compounds are nonmetals to nonmetals. These differ from ionic compounds in that their names clearly specify how many of each type of atom participate in the compound. The table below shows you the prefixes used when naming covalent compounds.

You can attach the prefixes in the table above to any of the elements in a covalent compound, as exemplified by SO₃ (sulfur trioxide) and N₂O (dinitrogen monoxide). The second element in each compound receives the –ide suffix, as in ionic compounds (which we discuss earlier in this chapter).

In the case of covalent compounds, where cations or anions aren’t involved, the more electronegative element (in other words, the element that’s closer to the upper right-hand corner of the periodic table) tends to be named second.

Finally, if the compound only has 1 atom for the first element you can leave off mono as the prefix. For example, you don’t call CO₂ monocarbon dioxide. Instead, you just call it carbon dioxide.

Balancing reactions

Balancing chemical reactions can be a real challenge for some people. Sometimes just getting started is a real challenge. The example below shows how to balance a combustion reaction. There is a good chance that if you are here reading this you might be stuck on a combustion reaction, so make sure to read on. Even if not, this example is going to give you a solid idea of how to go about balancing any reaction you encounter.

1. Start with the unbalanced reaction written out.

   C₄H₁₀(g) + O₂(g) ®CO₂(g) + H₂O(g)

2. Balance the carbon atoms first.

   Remember: You want to wait until the end to balance hydrogen and oxygen atoms. You have 4 carbon atoms on the left and 1 carbon atom on the right, so put a coefficient of 4 in front of the carbon dioxide:

   C₄H₁₀(g) + O₂(g) ® 4 CO₂(g) + H₂O(g)

3. When all non-hydrogen and non-oxygen atoms are balanced, balance the hydrogen atoms.

   Carbon is the only other atom in this example, so you can move on to hydrogen now. You have 10 hydrogen atoms on the left and 2 hydrogen atoms on the right, so use a coefficient of 5 in front of the water on the right:C₄H₁₀(g) + O₂(g) ®4 CO₂(g) + 5 H₂O(g)

4. Balance the oxygen atoms.

   You have 2 oxygen atoms on the left and a total of 13 oxygen atoms on the right [(4 × 2) + (5 × 1) = 13]. What can you multiply 2 with in order for it to equal 13? How about 6.5?

   C₄H₁₀(g) + 6.5 O₂(g) ®4 CO₂(g) + 5 H₂O(g)

5. Multiply all coefficients in the equation to get the lowest whole-number ratio of coefficients.

   For this example, multiply the entire equation by 2 (just the coefficients, please) in order to generate whole numbers:

   [C₄H₁₀(g) + 6.5 O₂(g®4 CO₂(g) + 5 H₂O(g)] ×2

   Multiplying every coefficient by 2 (don’t touch the subscripts!) gives you

   2 C₄H₁₀(g) + 13 O₂(g) ®8 CO₂(g) + 10 H₂O(g)

6. Check the atom count on both sides of the equation to ensure that the equation is balanced and the coefficients are in the lowest whole-number ratio.

This is a pretty complex example and hopefully it helps you out. Sometimes, though, all you might need to do is add a single coefficient of 2 or 3 in front of one compound. Just stay organized and keep things simple and you’ll be balancing reactions in no time!

Extensive and intensive properties

There are all kinds of ways you can classify matter. One is to look at the different physical properties that that matter has. There are two major types of physical properties:

• Extensive Properties: properties that depend on the amount of matter present

  Examples: mass, volume, length, width or anything else that depends entirely on how much of the substance you have

• Intensive Properties: properties that don’t depend on the amount of matter present

  Examples: density, malleability, ductileness, hardness, melting point

A helpful way to remember this is that intensive properties rely on what is “in”-side the matter, with the “in” signifying intensive and how it doesn’t matter how much of the matter is present.

Extensive properties rely on the “ext”-erior of the matter, with the “ext” signifying extensive and the fact that now, the amount of what you see on the exterior of the matter does impact the property.

Some physical properties are extensive properties, properties that depend on the amount of matter present. Mass and volume are extensive properties. A large chunk of gold has a larger mass and volume than a smaller chunk.

Intensive properties, however, don’t depend on the amount of matter present. Hardness is an intensive property. A large chunk of gold, for example, has the same hardness as a small chunk of gold. The mass and volume of these two chunks are different (extensive properties), but the hardness is the same.

Intensive properties are especially useful to chemists because they can use intensive properties to identify a substance.