GENERAL CHEMISTRY LABORATORY PRACTICE 7

PRACTICE 7.
STOICHIOMETRY: JOB METHOD
PURPOSE

At the end of this session, the student will be able to determine the stoichiometric relationship in which they combine.
the reactants of a chemical reaction. In addition, it will be able to calculate the yield of the reaction.

THEORETICAL FRAMEWORK

Chemical reactions

A reaction is a process by which one or more substances combine to produce new substances.
The reaction is represented by a chemical equation, which symbolizes the type of substances involved in the
process and its stoichiometric proportions.

For example, in the combustion reaction of propane gas with oxygen, carbon dioxide and water are formed as
products. This process is represented by the chemical equation:

C3H8(g) + O2(g) → CO2(g) + H2O(g)

However, the equation must be balanced to accurately represent the chemical quantities of each.
substances that participate in the reaction:

C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g)

The stoichiometric coefficients denote the proportion, as chemical quantities, in which they combine the
reactants appear and the products. Therefore, for 1 mole of C3H8that reacts, 5 moles of O are required2and they are formed
3 moles of CO24 moles of H2O.

Proportions can also be expressed in mass:

44 g + 160 g = 132 g + 72 g

204 g = 204 g

Limit reagent

When a reaction occurs, one or more of the reactants may be in excess. This means that there is a
reactant that determines the exact stoichiometric proportion in which the chemical change will occur. Said reactant,
limiting reagent, is completely consumed if the process is irreversible. All the
Stoichiometric calculations should be performed using the limiting reactant as a reference.

UNIVERSITY OF ANTIOQUIA
DEPARTMENT OF CHEMICAL ENGINEERING
GENERAL CHEMISTRY LABORATORY PRACTICE 7
Example:

Hydrazine, N2H4(g) has been used as fuel for rockets combined with hydrogen peroxide,
H2O2The chemical equation that represents the process is:

N2H4(g) + 2 H2O2(g) → N2(g) + 4 H2O(g)

If 1.60 g of hydrazine is combined with 10.0 g of peroxide, what mass of nitrogen is obtained?

One of the methods to determine which is the limiting reactant is to select one of them as a reference and
calculate how much is required of the other:

m H2O21.60 g N2H4x 2 x 34.0 g H2O23.62 g H2O2

32 g N2H4

Therefore, 3.62 g of H is required.2O2(g) to react with hydrazine. This means that the peroxide is present
in excess, since there are 10.0 g of it. The limiting reagent is N2H4and peroxide left over: 10.0 g - 3.62 g = 6.38 g
H2O2(g)

Yield of a reaction

The amount of product that, according to calculations, is formed when all the limiting reactant reacts is known as
theoretical yield. The amount of product that is actually obtained in a reaction is known as actual yield.
The actual yield is generally less than (and never greater than) the theoretical yield. The percentage yield
The efficiency of a reaction relates the actual yield to the theoretical one:

Percentage yield = (actual yield / theoretical yield) * 100%

Job Method or Method of Continuous Variation

This method was devised to experimentally determine the stoichiometric ratio in which they combine.
reactants of a reaction. It is based on carrying out a series of reactions using different amounts of each
reactive but keeping the total amount of both constant. A variable of the system can then be measured,
related to mass, and represented graphically against the quantities of reactants used. The variable can be
the weight of the precipitate or its height, or the amount of heat released.

Example:

The PbI2(s) can also be obtained by reaction between Pb(NO3)2(ac) and the KI(ac):

aPb(NO3)2(ac) + bKI(ac) → cPbI2(s) + dKNO3(ac)

a, b, c, d are the stoichiometric coefficients of the equation.

University of Antioquia
DEPARTMENT OF CHEMICAL ENGINEERING
GENERAL CHEMISTRY LABORATORY PRACTICE 7

In a series of experiments, the masses of the reagents are varied while maintaining a constant value of the total mass.
Both. The precipitate is filtered, washed, dried, and the following results are obtained:

Table 1: Job's Method for the reaction between Pb(NO3)2(ac) and the KI(ac)

Experiment m Pb(NO3)2(ac), g m Kl(ac), g m PbI2(s), g

1 0.50 4.50 0.75

2 1.00 4.00 1.39

3 1.50 3.50 1.98

4 3.00 2.00 2.78

5 4.00 1.00 1.45

Figure 1 shows the graph of precipitate mass versus mass of Pb(NO3)2(ac)ard of KI(ac).

Figure 1. Mass of PbI2(s) versus masses of Pb(NO)3)2(ac) and KI(ac)

From the graph, it can be concluded that the reactants are exactly in stoichiometric proportion when:

This relationship, expressed in chemical quantities, is transformed into:

and the maximum amount of PbI2The obtained (s) is 3.50 g (see the graph).

UNIVERSITY OF ANTIOQUIA
DEPARTMENT OF CHEMICAL ENGINEERING
GENERAL CHEMISTRY LABORATORY PRACTICE 7
Stoichiometrically, the results obtained from the graph can be verified. Indeed, the chemical equation
balanced (according to the literature) shows that a = 1 and b = 2

In the region of the graph before the maximum, the limiting reagent is always Pb(NO3)2(ac); and in the region to the right, the
The limiting reagent is always KI(ac). How would you demonstrate that this conclusion is true?

In the Job method, other parameters related to stoichiometry can also be measured.
the reaction: height of a precipitate, amount of heat released or volume of a gas.

MATERIALS AND EQUIPMENT

Aqueous solutions: sodium hydroxide (NaOH(aq)) 2.0 M; hydrochloric acid (HCl(aq)) 2.0 M

Test tubes, porcelain capsule, watch glass

Acid-base indicators: methyl orange and phenolphthalein

NOTE: EACH TEAM MUST BRING A SHEET OF GRAPH PAPER AND A RULER TO
CREATE THE GRAPH.

BASIC PROCEDURE

A. Job Method

Have nine (9) clean and dry test tubes in a rack for tubes. Label them with a number from 1 to
9.

Add 1, 2, 3, 4, 5, 6, 7, 8, and 9 mL of NaOH(ac) 2.0 M to each tube, in order. Measure the temperature in each tube.
essay. Then, add 9, 8, 7, 6, 5, 4, 3, 2, 1 mL of HCl(ac) 2.0 in the same order, avoiding the reactant.
my walls. Measure the temperature at each tube every time the acid is added

After recording the temperatures, take test tubes 1 to 3 and add a drop of methyl orange.
In tubes 7 to 9, add a drop of phenolphthalein. Record your observations.

Take tube 4 and divide it into two approximately equal portions, in one of them add a drop of phenolphthalein and
In the other, a drop of methyl orange. Repeat the procedure for tube 6. Note your observations.

Take a 5.0 mL aliquot from tube 5 and set it aside for the determination of reaction efficiency. The rest of the
The contents of tube 5 are divided into two portions, and to one of them, a drop of phenolphthalein is added, and to the other, a drop...
of methyl orange. Note your observations.

B. Graph

UNIVERSITY OF ANTIOQUIA
DEPARTMENT OF CHEMICAL ENGINEERING
GENERAL CHEMISTRY LABORATORY PRACTICE 7
Create a graph of ∆T versus the volume of each reactant. Indicate on the graph which compound is the limiting reactant.
in the region before and after the pipe with the greatest ∆T.

1. Write the chemical equation for the neutralization reaction with the stoichiometric coefficients obtained from the graph.
(show all calculations)
2. Do you think that based on the results of numeral 1 it is possible to balance the equation? Explain
3. Starting from the behavior of the acid indicators-Based on the test tubes, what is the limiting reagent and what is the reagent?
excessively in each of them? Justify
4. Calculate the yield of the reaction in tube #5. Show all calculations.

UNIVERSITY OF ANTIOQUIA
DEPARTMENT OF CHEMICAL ENGINEERING