Writing Writing Chemical EquationsChemical Equations

Ever wonder how to translate a Ever wonder how to translate a chemical reaction from a written chemical reaction from a written statement in regular English to statement in regular English to

equation form?equation form?

IntroductionIntroduction

Chemical reactions occur when bonds between Chemical reactions occur when bonds between the outermost parts of atoms are formed or the outermost parts of atoms are formed or broken.broken.

Chemical reactions involve the making of new Chemical reactions involve the making of new materials with different properties.materials with different properties.

Symbols represent elements, formulas describe Symbols represent elements, formulas describe compounds, chemical equations describe a compounds, chemical equations describe a chemical reactions.chemical reactions.

Chemical equations show the conversion of Chemical equations show the conversion of reactants (the molecules shown on the left of reactants (the molecules shown on the left of the arrow) into products (the molecules shown the arrow) into products (the molecules shown on the right of the arrow).on the right of the arrow).

Chemical equations show the kind of atoms and molecules and Chemical equations show the kind of atoms and molecules and their relative amounts in a reaction.their relative amounts in a reaction.

4Al + 3O4Al + 3O22 ---> 2Al ---> 2Al22OO33

The subscripts tell you how many atoms of a particular element The subscripts tell you how many atoms of a particular element are in a compound.are in a compound.

The coefficient tells you about the quantity, or number, of The coefficient tells you about the quantity, or number, of molecules of the compound.molecules of the compound.

This equation means: This equation means: four aluminium atoms plus three oxygen molecules produces four aluminium atoms plus three oxygen molecules produces

two molecules of aluminium oxidetwo molecules of aluminium oxide

AND/ORAND/OR

4 moles of Al + 3 moles of O4 moles of Al + 3 moles of O22 ---> 2 moles of Al ---> 2 moles of Al22OO33

Subscripts vs. CoefficientsSubscripts vs. Coefficients

Writing Chemical equations-- Writing Chemical equations--

Step 1:Step 1:    WriteWrite the skeletal (unbalanced) the skeletal (unbalanced)

equation by translating the written equation by translating the written

names into names into chemical formulaschemical formulas..

Step 2: Step 2:  BalanceBalance the equation. the equation.

Step 3: Step 3:  Figure out the Figure out the states of matterstates of matter each of the each of the

chemicals in the equationchemicals in the equation

Step 4:Step 4:  Sticking all the   Sticking all the otherother relevant relevant symbolssymbols in in

here somewhere.here somewhere.

Here’s how using an exampleHere’s how using an example

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

Step 1:  Write the skeletal equation by Step 1:  Write the skeletal equation by translating the written names into chemical translating the written names into chemical formulasformulas In this case, the formulas you need to know are those In this case, the formulas you need to know are those

for calcium hydroxide, hydrochloric acid, calcium for calcium hydroxide, hydrochloric acid, calcium chloride, and water.  chloride, and water. 

When you translate these into their formulas, you When you translate these into their formulas, you should get the equation:should get the equation:

Ca(OH)Ca(OH)22 + HCl --> CaCl + HCl --> CaCl22 + H + H22OO

BalanceBalance

Step 2:  Balance the equationStep 2:  Balance the equation You need to balance the equation to ensure that You need to balance the equation to ensure that

the chemical reaction follows the law of the chemical reaction follows the law of conservation of massconservation of mass

We will cover this topic in the next contract. We will cover this topic in the next contract. For this contract you will skip this step.

For this example, the balanced equation looks For this example, the balanced equation looks like this: like this:

Ca(OH)Ca(OH)22 + 2 HCl --> CaCl + 2 HCl --> CaCl22 + 2 H + 2 H22OO

States of MatterStates of Matter

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

"States" refers to the form in which you can find a "States" refers to the form in which you can find a chemical.  chemical. 

The states you need to worry about are solid, liquid, gas, The states you need to worry about are solid, liquid, gas, and aqueous. and aqueous. 

Solid, liquid, and gas are probably familiar to you, and Solid, liquid, and gas are probably familiar to you, and "aqueous" is just a fancy word for "dissolved in water".  "aqueous" is just a fancy word for "dissolved in water". 

The symbol for a solid is (s), liquid is (l), gas is (g), and The symbol for a solid is (s), liquid is (l), gas is (g), and aqueous is (aq).  aqueous is (aq). 

You need to make sure you write these symbols in the You need to make sure you write these symbols in the parentheses, and that you write them right after the parentheses, and that you write them right after the formulas in the same place and size that you put the formulas in the same place and size that you put the subscripts in the formulas. subscripts in the formulas. 

How can you tell ?How can you tell ?

Here are some guidelines that might help you. For example, the equation might tell Here are some guidelines that might help you. For example, the equation might tell you-- you--

if something is "dissolved in water", you know it's aqueous; If something is a 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 "powder", this indicates that it's a solid; If something is a “vapor" these are gases. gases.

Some chemicals are so common that you should be able to figure it out.  You 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 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 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 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).  (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 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 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 usually are.  If they're dissolved in water or in some other form, the equation should tell you. should tell you.

If you're not told otherwise, assume that covalent compounds are liquids.  If you're not told otherwise, assume that covalent compounds are liquids.  Generally, covalent compounds have fairly low melting and boiling points. Still, 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 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.  of exceptions to this rule, but it's better than nothing. 

All metallic elements but mercury are solids.  Mercury is a liquid. All metallic elements but mercury are solids.  Mercury is a liquid. All nonmetallic elements are solids, except for the following:  Bromine is a All nonmetallic elements are solids, except for the following:  Bromine is a

liquid;  The noble gases, chlorine, fluorine, nitrogen, oxygen, and hydrogen are liquid;  The noble gases, chlorine, fluorine, nitrogen, oxygen, and hydrogen are gases. gases.

Using our exampleUsing our example So, let's take a look at our equation:  So, let's take a look at our equation: 

Ca(OH)Ca(OH)22 + 2 HCl --> CaCl + 2 HCl --> CaCl22 + 2 H + 2 H22OO

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

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

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

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

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

Ca(OH)Ca(OH)2(s)2(s) + 2 HCl + 2 HCl(l)(l) --> CaCl --> CaCl2(aq)2(aq) + 2 H + 2 H22OO(l)(l)

SymbolsSymbolsStep 4:  Sticking all the other relevant Step 4:  Sticking all the other relevant

(important) symbols in here somewhere(important) symbols in here somewhere The last thing we need to do is to stick a bunch of other The last thing we need to do is to stick a bunch of other

symbols around here to indicate other important things symbols around here to indicate other important things about the reaction.  about the reaction. 

These things may include reaction conditions (things you These things may include reaction conditions (things you need to do to make the reaction take place) or indications need to do to make the reaction take place) or indications about whether the reaction is exothermic (gives off heat about whether the reaction is exothermic (gives off heat and feels warm) or endothermic (absorbs heat and feels and feels warm) or endothermic (absorbs heat and feels cool)cool)

If there is any other stuff around the arrow, do whatever If there is any other stuff around the arrow, do whatever the instructions tell you to do. the instructions tell you to do. 

There are actually quite a few other symbols which are There are actually quite a few other symbols which are commonly used.commonly used.

Important Symbols Important Symbols

A plus sign (+) separates molecules on the same side of the A plus sign (+) separates molecules on the same side of the equationequation

The arrow is read as “yields”The arrow is read as “yields” Example:Example: C + OC + O22 CO CO22

The charcoal used in a grill is basically carbon. The The charcoal used in a grill is basically carbon. The carbon reacts with oxygen to yield carbon dioxide. carbon reacts with oxygen to yield carbon dioxide.

The chemical equation for this reaction, C + OThe chemical equation for this reaction, C + O22 CO CO22, , contains the same information as the sentence but has contains the same information as the sentence but has quantitative (number or measurement) meaning as well.quantitative (number or measurement) meaning as well.

Symbols you may need:Symbols you may need:

SymbolSymbol What it meansWhat it means

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

5050ooCC The reaction takes place at the temperature indicated, in this case 50 degrees The reaction takes place at the temperature indicated, in this case 50 degrees Celsius. This symbol is written over the arrow Celsius. This symbol is written over the arrow

75 kPa75 kPa The unit "kPa" stands for kilopascals, which is a unit of pressure.  This symbol is 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 also written above the arrow and indicates the pressure at which the reaction should take place.  Other pressure units are "atmospheres", "Torr", and "mm should take place.  Other pressure units are "atmospheres", "Torr", and "mm Hg". Hg".

H H symbolsymbol written at the written at the endend of the of the equationequation

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

(()) Gases are escaping from the reactionGases are escaping from the reaction

In the reaction we were given, In the reaction we were given, nothing much was said except --nothing much was said except --

that the reaction gives off heat, meaning that it's an that the reaction gives off heat, meaning that it's an exothermic reaction.  exothermic reaction. 

As a result, the only symbol we really need is a As a result, the only symbol we really need is a H symbol.  H symbol.  Since the amount of heat given off wasn't specified, all we can Since the amount of heat given off wasn't specified, all we can do is say that do is say that H is negative.  H is negative. 

As a result, our equation looks like this: As a result, our equation looks like this:

Ca(OH)Ca(OH)2(s)2(s) + 2 HCl + 2 HCl(l)(l) --> CaCl --> CaCl2(aq)2(aq) + 2 H + 2 H22OO(l) (l)     H =  -H =  -

And that's all you need to do!And that's all you need to do!