Unit 6

Read Sections 5.1 and 5.2 before viewing the slide show.

Unit 17Chemical EquationsWriting Chemical EquationsBalancing Chemical EquationsVolume Relationships from Chemical Equations

Writing Chemical Equations (5.1)Verbal descriptions of what happens during a chemical reaction can become quite complex, thus a shorthand notation is desirable The use of chemical formulas greatly simplifies the expressions that are used to describe chemical changesPrior to exploring the shorthand notation, a note about the identification of diatomic elements is in order

Diatomic Elements There is an important set of seven diatomic elements that occur in natureDiatomic means that, as elements, they exist as two atoms bonded to each other. The seven diatomic elements are indicated in the periodic chart with a black border notice how they sort of form the number 7.In formula form, the sevenare H2, N2, O2, F2, Cl2, Br2,and I2 These are gases undernormal conditions exceptfor Br2 which is liquid andI2 which is solid.

Writing Chemical Equations (5.1)Consider the following statement:Sodium solid and chlorine gas react to form solid sodium chloride

The statement can be abbreviated somewhat by using chemical formulas in place of words (recall that Cl2 is diatomic):

Na solid and Cl2 gas react to form solid NaCl

To streamline even more, the physical states of the substances involved may be represented by using the symbols (s) for solid, () for liquid, (g) for gas, and (aq) for a water-based solution. Then:

Na(s) and Cl2 (g) react to form NaCl(s)

And, to finally get rid of the words altogether, replace the and by a + and the react to form by .

Na(s) + Cl2 (g) NaCl (s)

This equation is not quite done, but it is now in symbolic form. Notice the number of Cl atoms is different on the left-hand side and the right-hand side atoms are always conserved in a chemical change so that will need to be fixed.

Balancing Chemical Equations (5.1)The equation from the previous slide is:

Na(s) + Cl2 (g) NaCl (s)

Substances to the left of the arrow are called reactants and those to the right productsThe reactant side contains 1 Na atom and 2 Cl atoms, while the product side contains 1 Na atom and 1 Cl atom. Atoms are not created or destroyed during a chemical change, so the equation will have to be balanced to ensure the numbers of atoms of each kind are the same on each side.Balancing is done by placing coefficients in front of the chemical substances as needed the subscripts are never changed.Placing a 2 in front of NaCl will give two Cl atoms, but also gives 2 Na atoms. To get 2 Na atoms on the left, place a two in front of Na as well. The balanced equation becomes:

2 Na(s) + Cl2 (g) 2 NaCl (s)

More Balancing Chemical Equations (5.1)The approach we using for balancing is pretty much trial-and-error. Lets try a few more examples:

___ LiOH (s) + ___CO2 (g) ___Li2CO3 (s) + ___H2O ()

(For space buffs, this is the reaction used to remove carbon dioxide from spacecraft cabins made famous sort of in the movie Apollo 13)Notice the oxygen atoms are distributed through each reactant and product. A key is to not worry about them first as they will be taken care of when everything else is balanced.Notice there is one Li on the left and two on the right. It might be reasonable to start by balancing them. The lines above are drawn to stress that we can only put numbers IN FRONT of the compounds. We cannot mess with the existing subscripts remember you are experts at writing formulas and that process does not depend on what reaction they are involved in.Continued on next page.

More Balancing Chemical Equations (5.1)So, lets put a 2 in front of LiOH.

_2_ LiOH (s) + ___CO2 (g) ___Li2CO3 (s) + ___H2O ()

Now there are 2 Li atoms on the left and 2 Li atoms on the right.Look at H. There are now 2 on the left (2 x LiOH gives 2 H) and 2 on the right. H is now balanced.Look at C. There is 1 carbon on the left and 1 carbon on the right. They are balancedFinally, check the oxygen atoms. There are 4 oxygen atoms on the left (2 x LiOH = 2 oxygen atoms plus 1 x CO2 = 2 oxygen atoms) and 4 oxygen atoms on the right (1 x Li2CO3 = 3 oxygen atoms plus 1 x H2O = 1 oxygen atoms). They are balanced.If you have to work too hard at balancing these, you have probably miscounted somewhere. The equations we consider will typically balance fairly easily.

Simulation Practice for Balancing EquationsIf you would like to get some practice using simulations:

Go to http://phet.colorado.eduClick on Play with simsChoose Chemistry/General ChemistrySelect the Balancing Chemical Equations simulationSelect either the Save or Run Now option

The first tab lets you look at three reactions. You change the coefficient in front of each compound to achieve a balanced equation and will get a big smiley face if you are successful.

The second tab lets you play a balancing game.

Gay-Lussacs Law of Combining Volumes (5.2)In the 1700s Joseph Louis Gay-Lussac proposed the law of combining volumes:When all measurements are made at the same temperature and pressure, the volume of gaseous reactants and products are in a small whole number ratio.As an example, consider the reaction

2 H2 (g) + O2 (g) 2 H2 O (g)

The balanced equation says that 2 molecules of hydrogen react with one molecule of oxygen to form 2 molecules of water. Molecules are pretty small we only see the reaction with massive numbers of hydrogen and oxygen molecules reacting.Gay-Lussacs law says that in this reaction we will always see 2 volumes of hydrogen reacting with volume of oxygen to produce 2 volumes of gaseous water as long as the temperature and pressure remain constant.For example, 2 L of hydrogen gas react with 1 L of oxygen gas to produce 2 L of gaseous water; 10 gallons of hydrogen gas react with 5 gallons of oxygen gas to produce 10 gallons of gaseous water; 200 cups of hydrogen gas react with 100 cups of oxygen gas to produce 200 cups of water, etc.As a side note, if you reacted 2 L of hydrogen gas with 8000 L of oxygen gas, you would still only get 2 L of gaseous water because you would run out of hydrogen gas before oxygen gas.

Avogadros Hypothesis (5.2)Based on Gay-Lussacs law and experimental observations, Amedeo Avogadro in 1811 explained the law of combining volumes by stating that equal volumes of all gases, when measured at the same temperature and pressure, contain the same number of molecules.Further studies by Avogadro led to the next step in understanding the relationships between masses of material and the number of molecules, covered in the next Unit.

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