Writing Chemical Equations

1. Writing Chemical Equations Ever wonder how to translate a chemical reaction from a written statement in regular English to equation form?

2. Introduction • Chemical reactions occur when bonds between the outermost parts of atoms are formed or broken. • Chemical reactions involve the making of new materials with different properties. • Symbols represent elements, formulas describe compounds, chemical equations describe a chemical reactions. • Chemical equations show the conversion of reactants (the molecules shown on the left of the arrow) into products (the molecules shown on the right of the arrow).

3. Subscripts vs. Coefficients Chemical equations show the kind of atoms and molecules and their relative amounts in a reaction. 4Al + 3O2 ---> 2Al2O3 • The subscripts tell you how many atoms of a particular element are in a compound. • The coefficient tells you about the quantity, or number, of molecules of the compound. This equation means: • four aluminium atoms plus three oxygen molecules produces two molecules of aluminium oxide AND/OR • 4 moles of Al + 3 moles of O2 ---> 2 moles of Al2O3

4. Writing Chemical equations-- Step 1:Write the skeletal (unbalanced) equation by translating the written names into chemical formulas. Step 2: Balance the equation. Step 3:  Figure out the states of matter each of the chemicals in the equation Step 4:  Sticking all the other relevant symbols in here somewhere.

5. Here’s how using an example Example:When calcium hydroxide reacts with hydrochloric acid in water, dissolved calcium chloride and water are formed.  This reaction gives off heat. Step 1:  Write the skeletal 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 equation: Ca(OH)2 + HCl --> CaCl2 + H2O

6. Balance Step 2:  Balance the equation • You need to balance the equation to ensure that the chemical reaction follows the law of conservation of mass • We will cover this topic in the next contract. For this contract you will skip this step. • For this example, the balanced equation looks like this: Ca(OH)2 + 2 HCl --> CaCl2 + 2 H2O

7. States of Matter 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 symbols 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. 

8. How can you tell ? Here are some guidelines that might help you. For example, the equation might tell you-- • if something is "dissolved in water", you know it's aqueous; If something is a "powder", this indicates that it's a solid; If something is a “vapor" these 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 usually a liquid, except at very low temperatures (when it is solid ice) or very high temperatures (as gaseous steam).  • If you're not 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 told otherwise, assume that covalent compounds are liquids.  Generally, covalent compounds have fairly low melting and boiling points. Still, this is just a vague rule of thumb, and won't always work because there are a lot of exceptions to this rule, but it's better than nothing.  • 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.

9. Using our example • So, let's take a look at our equation:  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)

10. Symbols Step 4:  Sticking all the other relevant (important) symbols in here somewhere • The last thing we need to do is to stick a bunch of other symbols around here to indicate other important things about the reaction.  • These 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 warm) or endothermic (absorbs heat and feels cool) • If there is any 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.

11. Important Symbols • A plus sign (+) separates molecules on the same side of the equation • The arrow is read as “yields” • Example: C + O2 CO2 The charcoal used in a grill is basically carbon. The carbon reacts with oxygen to yield carbon dioxide. • The chemical equation for this reaction, C + O2 CO2, contains the same information as the sentence but has quantitative (number or measurement) meaning as well.

12. Symbols you may need:

13. 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!