3.6 Determination of a Chemical Formula by Titration
The purpose of this lab is to learn what happens when active Group 1 or Group 2 metals react with water and to introduce the use of an acid-base titration to determine the chemical formula of the product formed when calcium reacts with water.
In a single replacement reaction, one element in a compound is replaced by another. For example, zinc reacts with hydrochloric acid to produce hydrogen gas and zinc chloride:
Zn(s) + HCl(aq) −→ H2(g) + ZnCl2(aq) (3.1)
Hydrogen is a lighter-than-air flammable gas. At one time it was used in rigid hydrogenfilled luxury airships that made regular Atlantic crossings. One of them, the German dirigible Hindenburg, caught fire and burned as it was landing in New Jersey in 1937. World War II ended the era of the rigid airship.
In the chemical reaction shown in Equation 3.1, the element hydrogen has been replaced by zinc. What if we put zinc in water instead of an HCl solution? We would see that over the course of a few minutes or even an hour, very little reaction would take place. But if we put a more active metal, like sodium, in water, we would observe a violent reaction that produces hydrogen gas and a strong base, sodium hydroxide:
2Na(s) + 2H2O(l) −→ H2(g) + 2NaOH(aq) (3.2)
By boiling the solution to evaporate the water, we could recover the sodium hydroxide as a white, translucent solid. It is used in the manufacture of paper and soaps and in petroleum refining. It is extremely corrosive to the skin and other tissues and is sometimes called caustic soda or lye.
The heavier Group 2 metals, such as calcium, strontium, and barium, also react readily with water at room temperature to form hydrogen and the metal hydroxide. (Magnesium does not react rapidly with water at room temperature, but will react with high-temperature steam.)
The reaction of calcium with water is also a single replacement reaction in which calcium replaces the hydrogen in water:
calcium + water −→ hydrogen gas + calcium hydroxide (3.3)
We did not write the chemical formulas for this reaction because the point of this experiment is to determine through experiment the chemical formula for the calcium hydroxide produced when calcium reacts with water. We don’t want to give away the end of the story, but your first clue has already been revealed in Equation 3.2.
Like sodium hydroxide, calcium hydroxide is corrosive to the skin. When calcium hydroxide is heated to drive off water, calcium oxide (often called lime) is produced. Lime is an important ingredient in making the mortar used in plastering walls.
Let’s look at the reactions of active metals with water in greater detail. They involve changes in the charges on the metal atom and on the oxygen (and hydrogen) atoms in water, so they could also be classified as oxidation-reduction reactions. In the reaction of sodium with water, a sodium atom loses an electron to a water molecule. In the process it becomes a positively-charged sodium ion with an electron configuration like that of neon, a noble gas.
The water molecule with the extra electron it received from the sodium atom becomes unstable. It splits up into a neutral hydrogen atom and a hydroxyl group with a negative charge, called a hydroxide ion. So the product of the reaction is ionic, and solid NaOH is best characterized as containing sodium ions, Na+, and hydroxide ions, OH−.
The neutral hydrogen atoms, each with one electron, wander around until they find each other. When they do, they combine to form molecular hydrogen, H2. This “wander and combine” process occurs in a fraction of a second.
The net result is that when an active Group 1 or Group 2 metal reacts with water, an ionic metal hydroxide is formed along with molecular hydrogen.
Here is your second clue: If a calcium atom tends to give up electrons until it has the same number of electrons as the noble gas nearest to it on the periodic table, what would be the charge on the calcium ion that is left after the electrons have been removed?
Now for the third and last clue: If every substance that you can put in a bottle must be electrically neutral, what would you predict for the formula of calcium hydroxide, keeping in mind that each hydroxide ion has a 1− charge?
To test this prediction, we need an experimental method. We know that if we react a known amount of metal with water, we will obtain the metal hydroxide, which is a base. If we then react the base with a known amount of acid, we can determine how many moles of the base were present. Knowing the moles of base produced and the mass of the metal and its atomic mass, we can easily calculate how many moles of base were produced for every mole of metal. This allows us to write the correct formula for the metal hydroxide produced in the reaction.
The procedure we will use for determining the amount of hydroxide ion produced from a given amount of metal is called an acid-base titration. The base is contained in a reaction flask. To the base we add a known concentration of acid dispensed from a buret. Knowing the volume and concentration of the acid, we can calculate the moles of acid added, which will be equal to the moles of hydroxide, because they react one-to-one as illustrated by the reaction of sodium hydroxide with hydrochloric acid:
NaOH(aq) + HCl(aq) −→ NaCl(aq) + H2O(l) (3.4)
This is an acid-base reaction, driven to completion by the formation of the very stable compound water.
The reactants and products shown in Equation 3.4 are all colorless, so we cannot see anything happening as the reaction proceeds. We need something to indicate when we have added exactly enough hydrochloric acid to react with the hydroxide. This is called the endpoint of the titration.
To detect the end point we use a substance called an acid-base indicator. It has the property of changing color when the solution goes from slightly basic to slightly acidic (or vice versa). See Appendix D for the pH range of common acid-base indicators.
The Reaction of Zinc and Calcium with HCl and Water
Place 5 mL of 6 M HCl in a 15 × 125 mm test tube. Weigh out 1 piece of mossy zinc on a piece of glazed weighing paper, and place the zinc in the test tube containing the 6 M HCl. Observe what happens and record your observations.
While the reaction is proceeding briskly, invert over the reaction tube, a second 15 × 125 mm test tube, holding it so that the two tubes are mouth-to-mouth with no gap between them. After about 30 s, ask a neighbor to light a match for you. Quickly move the mouth of the inverted test tube close to the flame of the match (Figure 3.9). Be prepared to hear a “woof” or sharp “bark” if the hydrogen ignites. Describe what happens. If you neither see nor hear anything, try the procedure again.
Figure 3.9: Testing for hydrogen gas.
Now put 5 mL 6 M HCl in a 18 × 150 mm test tube. Using forceps to handle the calcium, put one or two calcium metal turnings (approximately 0.1-0.2 g) on a creased piece of glazed weighing paper. Add the calcium turnings to the 6 M HCl in the test tube. Observe what happens and record your observations. Repeat the procedure for collecting the hydrogen in an inverted test tube and igniting it. Again, record your observations.
Now weigh out on creased weighing paper fresh samples of zinc and calcium of the same size as before. Record the weight and then add each metal by itself to separate 18 × 150 mm test tubes half filled with deionized water. Observe what happens and record your observations. Did both the zinc and calcium react with water? Do your observations confirm the notion that active metals react more rapidly with acid solutions than with water? What is the insoluble white compound that forms when calcium reacts with water?
Add two drops of phenolphthalein indicator to the water in the test tubes. Describe the color of the indicator. Phenolphthalein is colorless in acidic solutions and red or pink in basic solutions. Does the color indicate that the solution containing calcium and water is acidic or basic? What about the solution containing zinc and water?
Discard the solutions in the test tubes into the proper waste container.
Titration of the Calcium-Water Reaction Product
NOTE: You should review Section 1.8, Titration Technique, in the Introduction on page 21. Additionally, titration is a component of Experiment 3.7, which is the evaluative lab report. Take time to hone your titration skills!
Perform a total of 3 titrations, minimum. Obtain a piece of calcium metal weighing between 0.8 and 1.0 g, weighing it to the nearest milligram. Record the mass of the calcium metal sample. Weigh additional samples of calcium metal, recording the masses as before. It will save time if you weigh out all of your samples and carry out the reactions at the same time, as described in the following paragraph.
Add the previously weighed calcium metal sample to a clean 250-mL Erlenmeyer flask. Mark the flask with your initials. Add about 150 mL of water to the flask. It may help to speed the reaction if you swirl the flask gently every few minutes. If the calcium turnings are fresh, the calcium should react vigorously and completely with the water. If they are not fresh, the reaction may be slow, and it may be necessary to heat the flask containing the calcium and water until the calcium has completely reacted. Note that it’s O.K. if some dark specks remain.
Condition the buret with the standardized 6.00 M HCl. If you don’t know how to condition your buret, ask your instructor! After properly conditioning your buret, fill it with the standardized 6.00 M HCl. Record the initial buret reading, estimating to the nearest 0.01 mL.
When the calcium metal in the Erlenmeyer flask has completely reacted, add 10 drops of 0.1% thymol blue indicator. With the tip of the buret slightly below the mouth of the flask, begin adding the 6.00 M HCl a drop at a time, swirling the flask as you add the HCl solution.
As you near the endpoint of the titration, you will notice that the blue color of the indicator turns yellow at the point where the acid enters the solution. If you approach the endpoint carefully, the solution will change within 1 to 2 drops from a blue color to a yellow color that remains for at least a full minute when the solution is swirled (swirling is very important here). Read and record the final volume. Also record the exact molarity of the HCl titrant used, which may be different from the nominal 6.00 M value.
If you add the HCl solution too rapidly near the endpoint, you may overrun the endpoint, spoiling the measurement. Take care to add the titrant dropwise near the endpoint because if you add a few drops excess, the solution will have the same yellow color as if the correct volume had been added. A pink or red color indicates that you have grossly overrun the endpoint. If you overrun the endpoint by more than 2 to 3 drops, the entire procedure should be repeated with a fresh sample of calcium metal.
If you are are carrying out additional titrations, repeat the procedure with the rest of your samples.
When you have finished all of your titrations, discard the solutions in the proper waste container. Rinse the buret with water. Use deionized water for the final rinse.
Results and Calculations
Summarize your observations and the reactions that occurred when zinc and calcium are added to 6 M hydrochloric acid. What is the reaction when zinc and calcium are added to water? Compare the reactions in the first part of this lab and comment on the gas that is produced in the reactions.
Calculate the formula for the calcium hydroxide by using the molar mass and exact sample mass of calcium to determine the number of moles used in your titration. Next calculate the moles of HCl that was used to titrate each sample. Use this information to calculate the mole ratio between HCl and Ca for each sample. Remember from Equation 3.4 that one mole of HCl reacts with one mole of hydroxide; therefore, how many moles of hydroxide must have been produced for every mole of calcium that reacted? From your experimental result, what must be the formula for the calcium hydroxide produced in the reaction of calcium with water? Remember that the point is to find the experimental formula for the calcium hydroxide. Your calculations must be done as if you don’t know the actual formula!
Write an appropriate discussion for this experiment. Include a discussion about the similarities and differences between the reactions when early representative metals (Group 1, Group 2) or zinc react with water and HCl. How does the activity series for metals related to your results in this experiment? Why is it possible to use the acid-base titration to determine the chemical formula of the product formed when calcium reacts with water?
Write an appropriate conclusion for this experiment.
- Complete and balance the following sequence of chemical reactions that takes place when the calcium reacts with water and when the calcium reaction products are titrated with HCl solution:
Ca(s) + H2O(l) −→
Ca(OH)2(aq) + 2HCl(aq) −→
The two reactions may be combined (added together) to get the net chemical reaction that shows the initial reactants and the final reaction products after the titration is completed.
- If some of the calcium metal remains unreacted when the titration is begun, the following reaction would occur as 6.00 M HCl is added:
Ca(s) + HCl(aq) −→ CaCl2(aq) + H2(g)
Would this cause an error in the titration? If so, would more or less HCl be required?