Challenge of Ratios
Why do some substance have only a single proportion while others vary?
Efforts to optimize formation of substances such as the gases prepared in Experiment V-2 centered on the assumption that ideal proportions and conditions could be found. Most famous was the search for the precise proportions required to create Gold from less costly materials. Following the revised definition of element by Antoine Lavoisier (Paris, 1743-1794) and his claim that weight (now called mass) provides the optimum way to measure compositions, a number of investigators found that set proportions of reagents were consumed in the formation of many substances Excess materials were often left unused. Other investigators instead found substances could be successfully combined in a wide range of proportions. Most, but not all the discrepancy, was eventually understood by distinguishing physical and chemical change. Materials that were merely physically combined in a wide range of possible proportions, such as mixing a solution, could relatively easily be separated again by processes such as distillation. Materials chemically combined in a single precise ratio were difficult if not impossible to reverse to their original separate states.
In 1799 Joseph-Louis Proust (1754-1826, portrait at right→) was the first to have given attention to set ratios. He suggested
a new law of nature, the law of constant proportions, which stated we must recognize an invisible hand which holds the balance in the formation of compounds. A compound is a substance to which Nature assigns fixed ratios. Proust proposed that the metals are permitted only two degrees of oxidation, a minimum and a maximum. (Read Proust's paper on Copper compounds at Carmen Giunta's Classic Chemistry.) His opponent, Claud Louis Berthollet (1748-1822) thought that compounds are always formed in very variable proportions, unless they are determined by special causes, such as crystallization, insolubility, or elasticity.
Between 1801 and 1810 John Dalton (Manchester, 1766-1844, ←portrait at left) proposed that if substances were assumed to be composed of small, indivisible particles called atoms, the experimental evidence of compound proportions could be explained. Dalton suggested that compounds between two elements form so that one atom of one element unites with one, two, three, or more atoms of the other. It follows from Dalton's atomic theory that compounds must therefore be formed in constant proportions. By 1808 the law of constant proportions was accepted by nearly all chemists resulting in Dalton's atomic theory rapidly becoming accepted as a reasonable hypothesis. (Read an excerpt from A New System of Chemical Philosophy by Dalton, 1808.)
Joseph Louis Gay-Lussac (1778-1850) in 1808 read before the Philomathic Society a claim that gaseous substances with each other are always form compounds in very simple ratios, so that representing one of the terms by unity, the other is 1, or 2, or at most 3. These ratios by volume are not observed with solid or liquid substances, nor when we consider weights. Still most chemists viewed the volume ratios as additional evidence for the existence of atoms.
The new definition of element by Lavoisier and the new application by Dalton of the old atom concept provided
Such major paradigm shifts are rare in science. They involve turmoil where the future is largely uncertain. Some people miss the value of potential changes and cling to the old ideas and methods. They may be left behind. Others find most exciting the exchange of new data, strange thoughts and even stranger implications. There are often many new paths to follow, not all of which will turn out to be productive. New vocabulary springs into use and other terms change meaning. New leaders often emerge as others fall by the wayside.
- explanations for a body of observations that previously had escaped explanation,
- new questions beckoning for experimental investigation, and
- a framework for understanding the flood of new experimental results.
There are many chemical reactions that illustrate and support the concepts proposed above. But we wish to use one that can be done with easily obtainable substances and equipment and perhaps increase our understanding general of chemical reactions in . We choose to use a reaction between a reactive metal, Aluminum, and common stomach acid, called muratic acid by the ancients, and still avialable by that name in stores:
2Al + 6HCl ⇒ 2AlCl3 + 3H2
- Note in the equation above describing the chemical reaction
- • each symbol for a kind of atom begins with a capital letter; because there are more than 26 elements, a second letter is often needed but is lower case. (here second letter
l distinguishes Cl and Al from other elements C, Ca, Cd, Ce, Cf, Cm, Cr, Co, Cu and Ac, Ag, Am, Ar, As, At, and Au.)
- • trailing subscripts indicate when additional atoms are a package called a molecule. (If a number is absent, it is assumed to be ONE.)
- • preceeding numbers indicate how many packages (i.e., molecules).
- • if you count all the starting atoms (those before of the arrow) called reactants, they equal the number of atoms after the reaction (those right of the arrow) called products. Chemists say that atoms are conserved, meaning none are created or destroyed in the reaction.
- splash protection eye safety goggles
- Aluminum kitchen foil
- Muratic acid
- two identical food storage bags
- a makeshift balance
- running water and paper towels in case of spills
- microwave oven for evaporating solution
- Put the end of a roll of Aluminum foil on a cutting board. Placing something of uniform width such as a ruler NEAR the edge, Carefully using a sharp knife, cut along BOTH sides to make a uniform-width strip of aluminum. Repeat making 6 or 7 strips with as close as possible identical width and length. In this experiment we wish to make precise measurements. Since the width and thickness are uniform, comparing lengths of aluminum is proportional to comparing the more fundamental masses.
- Obtain two identical plastic food storage bags. Place one strip of Aluminum in one bag with about a table spoon of muratic acid. (Caution: This acid is very corrosive. For sure wear eye protection goggles. Even a tiny drop in an eye requires rinsing with water for 15 minutes followed by medical attention. During the reaction there may be some spray due to popping bubbles. Carefully wipe up any splash or spills and rinse area with plenty of water. Low concentrations of the odor may be annoying. This is the acid our stomachs uses to digest protein. It is available for purchase in hardware stores.) Allow the Aluminum to totally react. Add a little more muratic acid if necessary. Avoid using too much acid because excess will create undesirable fumes in the next step.
- Avoiding measurement errors by making sure no liquid is spilled, place the OPEN bag in a microwave and heat slowly until no liquid remains.
- Make or obtain an equal-arm balance, checking to make sure that when equal-mass objects are suspended at opposite ends, that both (1) they balance, and (2) a small difference (such as adding a short piece of tape to one object) will be noticeably out of balance.
- Hang the bag with dried contents on one side of the balance and the identical bag on the other side. Place several of the identical strips of Aluminum in the empty bag. Add, remove, or shorten one of the strips GRADUALLY until balance is found. Compare the length of the original Aluminum (now reacted) strip with the total length of the balancing unreacted Aluminum strips to obtain the ratio of the Aluminum's mass to the compound's mass.
- Repeat the experiment starting with only half a length of Aluminum. Do you get the same composition ratio?
Finally, record your procedures, measurements, and findings in your journal. If you need course credit, use your observations recorded in your journal to construct a formal report