logo imageChapter 3 Notes
Dr. Floyd Beckford

CHAPTER 3
CHEMICAL EQUATIONS AND REACTION STOICHIOMETRY

Stoichiometry describes the quantitative relationships among elements in compounds (composition-) and among substances as they undergo chemical changes (reaction stoichiometry).

CHEMICAL EQUATIONS
A chemical equation is a shorthand expression for a chemical change (reaction). The chemical substances that start the chemical reaction are called reactants. The substances that result from the reaction are called products.

2Al(s) + 3Br2(l) ---> Al2Br3(s)

A balanced chemical equation shows the relative amounts of reactants and products. The numbers preceding each substance in the chemical equation tell us how many molecules or moles of each reactant are needed to produce a given number of molecules or moles of the product. The physical states of the reactants and products are indicated by the symbol (s) for solid, (g) for gas, (l) for liquid and (aq) for substances dissolved in water. The equation above says two atoms (moles) of aluminum solid react with three molecules of bromine liquid to produce a single molecule of solid aluminum bromide.

The law of the conservation of matter demands that all atoms of the reactants in a chemical reaction remain in the system after the reaction. This is reflected in the need for balanced equations. For a chemical equation to be balanced, it must have the same number of atoms for each element on both sides of the equation.

There are a number of guidelines that have been formulated to aid in balancing equations.
1. The correct formulas for the reactants and products must be written as they exist. Thus oxygen is written as O2 instead of O.
2. a) Count and compare the number of atoms of each element on both sides of the equation to determine those that are unbalanced.
b) Balance each element one at a time, by placing small numbers (coefficients) in front of the formulas of the substances containing unbalanced elements. Remember that a number placed in front of a substance multiplies each element in the substance by that number e.g. 2H2SO4 means 2 molecules (moles) of sulfuric acid and also that we have 4 H atoms, 2 S atoms and 8 O atoms.
c) Check all elements after each individual element is balanced to see if, in balancing one element, others have become unbalanced.
d) The numbers used to balance the equation should be in the smallest possible ratio.

		4HgO ---> 4Hg + 2O2		(Incorrect)

		2HgO ---> 2Hg + O2		(Correct)


	2 KClO3 + H2SO4 ---> 2 HClO3  + K2SO4
	CrCl3 + 3 NH3 + 3 H2O ---> Cr(OH)3 + 3 NH4Cl
	3 Cu + 8 HNO3 ---> 3 Cu(NO3)2 + 2 NO + 4 H2O
	P4(s) + 5 O2(g)  ---> P4O10(s) 
	C3H8(g) + 5 O2(g)  ---> 3 CO2(g) + 4 H2O(l)

CALCULATIONS BASED ON CHEMICAL EQUATIONS
Mole-ratio method
A mole ratio (mole factor) is a ratio between the number of moles of any two involved species in a chemical reaction.

Moles of DS = Moles of SS x Mole ratio
The above relationships can be extended to masses and formula weights.
Mass of desired substance = (moles of DS) x (formula weight)

THE LIMITING REACTANT CONCEPT
In a reaction between two or more substances, the amount of product formed depends on the amounts of each reactant available. The quantity of product also depends on the stoichiometry of the reaction. In most reactions, one reactant is in excess. The reactant that is NOT in excess controls the outcome of the reaction. This reactant is called the limiting reactant or limiting reagent. 

	2CH3OH(l)    +      3O2(g)    --->    2CO2(g) + 4H2O(g)
	 1.1 mol            3.8x1019 mol
We need only 1.5 mol of O2 for each mole of methanol. We oxygen in virtually unlimited excess. The outcome of the reaction is controlled by the amount of methanol and so methanol is the limiting reactant.

It should be noted that the limiting reactant is not always the one with the least amount of moles. The reaction ratio also plays a part.

Consider the reaction given below.

Zn(s) + 2HCl(aq) ---> ZnCl2(aq) + H2(g)

We carry out three experiments in which we have the same amount of HCl (0.100 mol) but different amounts of Zn.
(a) Reaction 1: 7 g of Zn
(b) Reaction 2: 3.27 g of Zn
Reaction 3: 1.31 g of Zn
For a reaction that consumes all the reactants, the required ratio of mol HCl/mol Zn is 2 (as seen from the balanced equation).

* In Reaction 1 the ratio of HCl to Zn is 0.93 (0.100 mol/0.107 mol), so not enough HCl is available to consume all the Zn. Zn remains after the reaction ceases and so HCl is the limiting reactant.
* In Reaction 2 the ratio of HCl to Zn is 2 (0.100 mol/0.050 mol). The reactants match perfectly and are completely consumed.
* In Reaction 3 the ratio of HCl to Zn is 5 (0.100 mol/0.0.020 mol). More than enough HCl is available to consume all the Zn and leave HCl in solution. Zn is the limiting reactant.

Determining limiting reactants:
1. Calculate the number of moles of each substance used.
2. Compare the ratio of the moles calculated (available ratio) with the mole ratio of the two substances in the balanced equation (required ratio).
If available ratio < required ratio, then numerator is limiting
If available ratio > required ratio, then denominator is limiting

After calculating the number of moles of each substance (step 1) you can also determine the number of moles of the product that can be formed from each substance. The smallest answer is always right.

The yield or amount of product that is expected or calculated from the chemical reaction is called the theoretical yield. The actual yield is the amount of product that we obtain from an actual reaction.

CONCENTRATION OF SOLUTIONS
A solution is a homogeneous mixture of two or more substance. Typically,

Solution = solute + solvent
Generally the solute is the dissolved substance. The medium in which it dissolves is the solvent. Water is the most important (and common) solvent and solutions in water are called aqueous solutions.

The concentration of a solution describes in relative terms the amount of solute present. The term dilute describes a solution that contains a relatively small amount of dissolved solute. Conversely, a concentrated solution contains a relatively large amount of dissolved solute.

Concentration of solutions are expressed as
· The amount of solute present in a given mass or volume of solution.
· The amount of solute dissolved in a given mass or volume of solvent.
The major expression used for concentrations is molarity or molar concentration. Molarity (which has the symbol M) is defined as the moles of solute dissolved in 1 liter of solution or “moles per liter”. Therefore, a 1.2 M solution of KMnO4 contains 1.2 mole of KMnO4 per liter of solution.

Molarity = mol/L or mol/dm3
The following version of the relationship is worth committing to memory:

Moles of solute = Volume of solution (L) x Concentration (M, mol/L)
To make solutions we usually put a weighed amount of solute into a volumetric flask and then fill the flask with the desired amount of solvent.

Dilution
Suppose we need to make a dilute solution in which the amount of solute needed is too small to be weighed out accurately.? In such situations we use the dilution process.
Dilution is the process by which we lower the concentration of a solution. We make a concentrated solution and then dilute it to the concentration we want.

Titration
A titration is the process in which a solution of one reactant called the titrant is added to a solution of another reactant and the volume of the titrant required for complete reaction is measured. The titrant is usually a solution of known concentration (standard solutions). In titrations an indicator is used to signal the end of the reaction. An indicator is a substance that can exist in different forms depending on the pH of the solution. The indicator will change color at the completion of the reaction. The completion of the reaction is when equivalent amounts of the acid and base have reacted. This is known as the equivalence point. This is theoretically equal to the end point, the point at which the indicator changes color and the titration is stopped.