Quantitative chemical analysis, which is commonly referred to as stoichiometry, is the quantitative relationship between the reactants and the products in a balanced chemical equation. The term stoichiometry is a combination of two words derived from the Greek language: stoicheion (meaning “element”) and metron (meaning “measure”). Stoichiometric calculations are dependent upon the law of conservation of mass which states that all matter cannot be created nor destroyed, thus in any chemical reaction that occurs in a closed system the mass of the products is equivalent to the mass of the reactants. Due to such laws of nature, a chemical equation must be balanced in order for the amounts to remain equivalent following the reaction (“Chemical Stoichiometry”). The coefficients in a balanced chemical equation represent the ratio between the particles in a perfect reaction, where all the particles in a chemical experiment will react. These ratios are also classified as the stoichiometric or molar ratio, which can be compared between any of the compounds in a reaction, which includes both reactants and products. These ratios can be used interchangeably for any particle in stoichiometric calculations because moles simply represent a specific amount of particles, thus the molar ratio within the equation is the proportional relation of each element or compound to one another. Stoichiometry is also useful when calculating mass ratios because if the mass of any substance in a reaction is known, the mass of any other substance can be calculated (“Reaction Stoichiometry”). Stoichiometric calculations are very important in real life applications because having the accurate proportions of any item is important when limited amounts of a certain reactant is present, which is useful to reduce cost and waste. Additionally, stoichiometry is significant in the field of chemistry as chemical calculations can be used to prevent overdose since many chemicals may be toxic in inadequate amounts.
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An example of an everyday stoichiometric calculation can be displayed through the act of making a s’more. Two crackers, one marshmallow, and three chocolate squares must be used to formulate an entire s’more (as show in figure 1). One can only be formed if exact amounts of each ingredient is used. However if only 1 cracker is available, a s’more could no longer be formed.
Figure 1.
Stoichiometry is most commonly used when one reactant completely reacts with the other in a chemical reaction. These absolute amounts are called theoretical yield. On the other hand, when performing a lab, the reactants will not be in perfect stoichiometric amounts because of potential errors that occur during an experiment. Therefore the actual yield of an experiment may not correspond to the theoretical yield. In order to find the percent yield, the theoretical yield must be divided by the actual yield and multiplied by one hundred. Examples of errors that may result in loss of yield include temperature, surface area, pressure, medium, the purity of the reactants, procedural mistakes, poor technique, lab accidents, or miscalculations (“Theoretical and Percent Yield”). As well, competing reactions can also contribute to the loss of yield. These reactions occur at the same time as the initial reaction, and consequently use the compounds and element in the initial reactions. Due to the many factors can contribute to the amount of yield lost, it is important that their effects are considered once performing the experiment.
In stoichiometric calculations there will most often be leftover reactants causing a short supply, due to the imperfect quantities of each reactant due to the potential errors or an insufficient amount of a reactant. In an equation that is not in a perfect stoichiometric ratio, a limiting and excess reactant will always be present. The limiting reactant is the one that forms the smaller amount of product, thus stopping and limiting the reaction after it is completely consumed. While, the excess reactant is the one that is leftover after the reaction is stopped by the limiting reactant (“Chemical Stoichiometry”). Using the s’more as an example, if an insufficient amount of ingredients are present than a s’more cannot be formed. If there are four crackers, one marshmallow, but only five pieces of chocolate squares, only one s’more will be assembled instead of two. Hence, in this scenario the chocolate squares would be the limiting reactant. Even though the chocolate squares represent the largest number of ingredient, an inadequate amount is present therefore the rest of the ingredients will appear in excess. It is important to always use the limiting reactant to determine the final product. If an excess reactant is used, there would not be enough of the limiting reactant to create the product. In addition, it is not possible to determine the limiting reactant instantly from the masses given, since stoichiometry is in proportions by moles. The mass of each compound cannot be compared because the molecular weight of each compound is different. Nevertheless, compounds can be compared by moles since molecules react on a molecular level which makes the amount consistent throughout the chemical equation. For example, the amount of one mole of hydrogen is equivalent to one mole of carbon, although one mole of hydrogen weighs 1.01 grams while a mole of carbon weighs 12.01g. Therefore, in all standard stoichiometric calculations any measurement must first be converted into moles in order to be compared to another (“MOLS, PERCENTS, and STOICHIOMETRY”).
The purpose of the lab performed is to produce two grams of copper through a single displacement reaction between Copper (II) Chloride Dihydrate and solid Aluminum. When determining whether a single displacement reaction will take place, the activity series (See Apendix _) must be considered. Given that Aluminum is higher on the activity series than copper, meaning that it is more reactive, the Aluminum will begin to bond with the chlorine, thus replacing the Copper in the Copper (II) Chloride compound. This would result in the Copper (II) Chloride compound to break apart, creating solid Copper and Aluminum Chloride solution. Another factor that must be considered when performing a single displacement reaction, is that the compound must be changed into an aqueous solution, in which the element would then be placed. Consequently, Copper (II) Chloride Dihydrate must be dissolved in water when creating an aqueous solution. An exact amount of 4.23 grams of aqueous Copper (II) Chloride and as well as 0.566 grams of Aluminum must be used in order for a perfect reaction to occur, which was established through stoichiometric calculations (See Apendix _). However, this experiment required Aluminum to act as the excess reactant, therefore 1 gram of Aluminum was obtained instead of 0.566 grams. Additionally, since only Copper (II) Chloride Dihydrate is available, and the anhydrous form was used in the balanced chemical equation, the amount of the hydrous form must be found in order to identify how much must be utilized. Through stoichiometric calculations (See Appendix _), 5.36 grams of Copper (II) Chloride Dihydrate would need to be acquired for a perfect reaction to take place. Although, since lab data is not generally accurate due to procedural inaccuracies, the actual yield obtained during the trial may not correspond to the theoretical yield, which was determined using the stoichiometric calculations. Without error exactly two grams would be produced, however an oxidization process will occur which will consequently add on additional weight to the solid copper product. Due to oxidization and as well the possibility of not being able to remove excess aluminum from the product, the estimated yield percent would most likely be over 100%, but may be balanced out if any errors in the process of the trial take place.
WORK CITED
Source: Boundless. “Reaction Stoichiometry.”Boundless Chemistry. Boundless, 14 Nov. 2014. Retrieved 18 Apr. 2015 from https://www.boundless.com/chemistry/textbooks/boundless-chemistry-textbook/chemical-kinetics-13/reaction-rates-98/reaction-stoichiometry-414-3637/
http://www.science.uwaterloo.ca/~cchieh/cact/c120/stoichio.html
http://www.iun.edu/~cpanhd/C101webnotes/quantchem/thtclandpctyld.html
http://www.chemtutor.com/mols.htm
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