STOICHIOMETRY REACTION

APPARATUS AND MATERIALS:

Sodium hydrogen carbonate, hydrochloric acid(0.05M), thymol blue indicator, distilled water, electrical balance, test tubes, test tube rack, Bunsen burner, volumetric flask(250), burette
(50), retort stand, conical flask(250), pipette(25).

EXPERIMENTAL PROCEDURES:

Part 1: Thermal Decomposition of NaHCO3

1. Using the 4-decimal place balance, mass of a clean, dry test tube was recorded, and 2.68g of NaHCO3 was added and reweighed. The masses were recorded and then the mass of reactant NaHCO3 added to the test tube was calculated.
2. With the aid of a test tube holder, the tube and contents were gently heated over a hot flame for not longer than 5 minutes.
3. The tube and the contents were allowed to cool to room temperature in a test tube rack or beaker and reweighed on the same balance.
4. These masses were recorded and the amount of solid product (Na2CO3 + any unreacted NaHCO3) was calculated.

Part 2: Titration of Na2CO3 with Hydrochloric Acid

1. A few  of distilled water were added to the test tube, it was heated gently and shaken to dissolve the Na2CO3.
2. The solution was transferred to a 250 volumetric flask using a funnel, a few more  of distilled water was added to the test tube and this was added to the volumetric flask. (it is essential to transfer 100% of the solid into the volumetric flask)
3. The washing was repeated two or three times, and the funnel was washed in too with distilled water, so that all of the Na2CO3  transferred to the volumetric flask.
4. The solution was made up to the 250 mark, capped and the flask was inverted six or seven times to homogenize the solution.
5. The burette was rinsed with a little of the standardized 0.05M HCl solution that was supplied. The burette was filled with the standardized HCl, clamped vertically, a little solution was run out and the volume was recorded to the nearest 0.05(by interpolation). The concentration of the standardized HCl was recorded to 4 decimals.
6. Three 250conical flasks were rinsed out with distilled water and 25.00 aliquots of the Na2CO3 solution was pipetted into each.
7. Two or three drops of thymol blue indicator were added to each flask and the Na2CO3 solution was titrated with the HCl until the end point was reached (a grey-green color). The HCl was added drop-wise towards the end of titration.
8. The volume to the nearest 0.5 was read off; the titration was repeated with the other Na2CO3 aliquots. The readings were recorded in a table. The reading should be consistent to within 0.10. If not, the process was repeated until consistency has been obtained.

DATA/RESULTS:

Mass of the empty test tube:    30.4472g

Mass of NaHCO3 + test tube:   33.1272g

Mass of NaHCO3:   33.1272g – 30.4472g =   2.6800g

   

Mass of NaHCO3 after the thermal decomposition + test tube:  32.1275g

Mass of Na2CO3 + any unheated NaHCO3:   32.1275g – 30.4472g =  1.6803g

                                                            Table 1

DISCUSSION:

An indicator is a solution that changes the color of the reacting solution as because it reacts with an acid or a base. Indicators are always weak acids or weak bases. Indicator changes solution’s color at a particular range of pH values. Thymol blue is a brownish-green or reddish-brown crystalline powder that is used as an pH indicator. It is insoluble in water but soluble in alcohol and dilute alkali solutions. It transitions from red to yellow at pH 1.2–2.8 and from yellow to blue from at pH 8.0–9.6. In this experiment, thymol blue indicator was used to observe the color changes of the Na2CO3 from dark blue to grey-green after adding the thymol blue, where it shows the end point of the titration. If the HCl is pipette in excess, the color of the solution will change to a yellow.

If more HCl was added to this solution at the thymol blue end point, it will react. The products of this new reaction would be an acid. This is because the solution might get below the pH level of 8.0, acidic state. Adding more HCL means it is added in excess. Therefore, according to observation the color of the solution will be yellow. Yellow shows acidity of the solution. Which means the product will be acidic for this new reaction.

A diprotic acid is an  that happens to contain within its molecular structure two  atoms capable of dissociating (i.e. ionizable) in water. The complete dissociation of diprotic acids is of the same form as :

H2SO4 → H+ (aq) + HSO4− (aq) Ka = 1 × 103

HSO4− → H+ (aq) + SO42− (aq) Ka = 1 × 10−2

The dissociation does not happen all at once due to the two stages of dissociation having different Ka values. The first dissociation will, in the case of sulfuric acid, occur completely, but the second one will not. Diprotic acids are of particular note in regards to  experiments, where a pH versus  will clearly show two  for the acid. This occurs because the two ionization capable hydrogen atoms on the acid molecule do not leave the acid at the same time.

Two methods were used in Part 1 and Part 2 of this experiment to determine the amount of Na2CO3 formed from the thermal decomposition of NaHCO3.Thermal decomposition, also called thermolysis is defined as a chemical reaction in which a chemical substance breaks up into at least two chemical substance when heated. If thermal decomposition of a substance is significantly exothermic, then the substance is thermodynamically unstable. If initiated, its decomposition forms a positive feedback loop and undergoes thermal runaway, possibly up to the point of causing an explosion. Titration is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of a known reactant. Because volume measurements play a key role in titration, it is also known as volumetric analysis. In the classic strong acid-strong base titration, the endpoint of a titration is the point at which the pH of the reactant is just about equal to 7, and often when the solution permanently changes color due to an indicator. There are however many different types of titrations .Many methods can be used to indicate the endpoint of a reaction; titrations often use visual indicators (the reactant mixture changes color).    

It is important to transfer all of the Na2CO3 from the test tube to the volumetric flask in the titration section at the experiment Part II so that there will be no production of false result because there will not be any of the solution left in the test tube and to ensure that the final reading or end point is more accurate.

Hess's law is a relationship from  named for , a -born   and . The law is based on the principle of conservation of energy and the path independence of energy changes. Hess's law can be used to predict energy changes that are not easily measured.

The law states that the  change for any chemical or physical process is independent of the pathway or number of steps required to complete the process. In other words, an energy change is path independent, only the initial and final states being of importance. This path independence is true for all .

Hess's law allows the  change (ΔH) for a reaction to be calculated even when it cannot be measured directly. This is accomplished by performing arithmetic operations on  and known ΔH values. Chemical equations may be multiplied (or divided) by a whole number. When an equation is multiplied by a constant, its ΔH must be multiplied by the same number as well. If an equation is reversed, ΔH for the reaction must also be reversed (eg. -ΔH).

Hess's Law says that enthalpy changes are additive. Thus the ΔH for a single reaction can be calculated from the difference between the heats of formation of the  minus the heat of formation of the .

CONCLUSION:

Stoichiometry rests upon the , the  (i.e., the ) and the . Stoichiometry (sometimes called reaction stoichiometry to distinguish it from composition stoichiometry) is the  of  (measurable) relationships of the  and  in a balanced  (). It can be used to calculate quantities such as the amount of products that can be produced with the given reactants and percent yield. In general, chemical reactions combine in definite ratios of chemicals. Since chemical reactions can neither create nor destroy matter, nor  one element into another, the amount of each element must be the same throughout the overall reaction. For example, the amount of elements x on the reactant side must equal the amount of element X on the product side. Reaction stoichiometry allows us to determine the amount of substance that is consumed or produced by a reaction.

REFERENCES:

1.  ( date accessed: 29th June 2009)
2.  ( date accessed: 29th June 2009)
3.  ( date accessed: 29th June 2009)