Writing Word Equations



Ever wonder how to translate a chemical reaction from a written statement in regular English (or Serbo-Croatian) to equation form?  Here's how, using the example below:

Example:  When calcium hydroxide reacts with hydrochloric acid in water, dissolved calcium chloride and water are formed.  This reaction gives off heat.


How to solve a problem like this:
 

Step 1:  Write the unbalanced equation by translating the written names into chemical formulas

In this case, the formulas you need to know are those for calcium hydroxide, hydrochloric acid, calcium chloride, and water.  When you translate these into their formulas, you should get the unbalanced equation:

Ca(OH)2 + HCl --> CaCl2 + H2O

If you've forgotten how to write formulas, visit here for more info about writing formulas for ionic compounds and here for writing formulas for covalent compounds.


Step 2:  Balance the equation

You need to balance the equation to ensure that the chemical reaction follows the law of conservation of mass, which says that you've got to have the same number of atoms of each element on both sides of the equation.  I'll assume for the purposes of this activitiy that you know how to balance equations.  If you don't, try visiting here for help.

For this reaction, the equation, when balanced, looks like this:

Ca(OH)2 + 2 HCl --> CaCl2 + 2 H2O


Step 3:  Figure out the states of each of the chemicals in the equation

"States" refers to the form in which you can find a chemical.  The states you need to worry about are solid, liquid, gas, and aqueous.  Solid, liquid, and gas are probably familiar to you, and "aqueous" is just a fancy word for "dissolved in water".  The symbol for a solid is (s), liquid is (l), gas is (g), and aqueous is (aq).  You need to make sure you write these in the parentheses, and that you write them right after the formulas in the same place and size that you put the subscripts in the formulas.  Check out the example below to see what I mean.

Now, the big question is this:  How can you tell if something is a solid, liquid, gas, or aqueous?  Here are some guidelines that might help you:

  • The equation might tell you.  For example, if something is "dissolved in water", you know it's aqueous.  If something is a "powder", this indicates that it's a solid.  "Vapors" are gases.

  • Some chemicals are so common that you should be able to figure it out.  You should know that carbon dioxide is a gas because you learned that you breathe it out of your lungs after you breathe in oxygen.  Likewise, you should have a pretty good idea that water is generally a liquid, except at very low temperatures (when it is solid ice) or very high temperatures (as gaseous steam). 

  • If you're not explicity told otherwise, assume that ionic compounds are solids. That's because ionic compounds have very high melting and boiling points, so they usually are.  If they're dissolved in water or in some other form, the equation should tell you.

  • If you're not explicity told otherwise, assume that covalent compounds are liquids.  This is actually not that great an assumption because there are a lot of exceptions to this rule, but it's better than nothing.  Generally, covalent compounds have fairly low melting and boiling points, and many organic compounds are liquids at room temperature.  Still, this is just a vague rule of thumb, and won't always work.

  • All metallic elements but mercury are solids.  Mercury is a liquid.

  • All nonmetallic elements are solids, except for the following:  Bromine is a liquid;  The noble gases, chlorine, fluorine, nitrogen, oxygen, and hydrogen are gases.

So, let's take a look at our equation:  (In case you forgot what it was, it was 

Ca(OH)2 + 2 HCl --> CaCl2 + 2 H2O

  • Calcium hydroxide is an ionic compound, so we'll assume it's a solid.

  • Hydrochloric acid is a covalent compound, so we'll assume it's a liquid.

  • The equation tells us that calcium chloride is dissolved in water, so it's aqueous.

  • Water is a liquid, because nothing about the statement told us that the reaction took place at anything but room temperature.

Putting all this stuff together, we get the following equation:

Ca(OH)2(s) + 2 HCl(l) --> CaCl2(aq) + 2 H2O(l)


Step 4:  Sticking all the other relevant symbols in here somewhere

The last thing we need to do is to stick a bunch of other symbols around here to indicate other relevant things about the reaction.  These relevant things may include reaction conditions (things you need to do to make the reaction take place) or indications about whether the reaction is exothermic (gives off heat and feels hot) or endothermic (absorbs heat and feels cold).  Here's a list of the symbols you may need to use:

Symbol

What it means

D

The reaction requires heat or added energy to occur.  This symbol is typically written over the arrow.

500 C

The reaction takes place at the temperature indicated, in this case 50 degrees Celsius.  This symbol is written over the arrow

50 kPa

The unit "kPa" stands for kilopascals, which is a unit of pressure.  This symbol is also written above the arrow and indicates the pressure at which the reaction should take place.  Other pressure units are "atmospheres", "Torr", and "mm Hg".

other stuff around
the arrow

Do whatever the instructions tell you to do.  There are actually quite a few other symbols which are commonly used, but memorizing them all can be tough.

DH symbols
written at the end
of the equation

These symbols tell you how much energy is absorbed or given off during the reaction.  If energy is absorbed, the reaction is endothermic and DH has a positive sign.  If energy is given off, the reaction is exothermic and DH gas a negative sign.  If a number is given here, this indicates how exo or endothermic the reaction is.  Common units for DH are "kJ/mol" or "kcal/mol".

In the reaction we were given, nothing much was said except that the reaction gives off heat, meaning that it's an exothermic reaction.  As a result, the only symbol we really need is a DH symbol.  Since the amount of heat given off wasn't specified, all we can do is say that DH is negative.  As a result, our equation looks like this:

Ca(OH)2(s) + 2 HCl(l) --> CaCl2(aq) + 2 H2O(l)          DH =  -

And that's all you need to do!



Questions?  Comments?  Email them to me at misterguch@chemfiesta.com


 
 

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