Finding the Percentage Yield of a Double Displacement Reaction

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Abstract

The purpose of this lab was to find the percentage yield of the double displacement reaction between sodium carbonate, Na2CO3, and copper chloride, CuCl2. A solution of aqueous sodium carbonate was mixed with a solution of aqueous copper chloride, and the precipitate formed was measured. Using the measured masses, the percentage yield was calculated to be 67%. The results of this lab show that the effects of reaction procedures and laboratory techniques will have an effect on percentage yield. However, because there is always loss of product and human error in any chemical reaction, these results are completely reasonable and valid.

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Introduction

In chemistry, it is essential to be able to make accurate and useful predictions when performing experiments. In order to understand the relationship between reactants and products, chemists use stoichiometry. Stoichiometry is the study of the quantitative relationships amongst the amounts of reactants and the amounts of products formed in a chemical reaction1.

The history of stoichiometry extends all the way back to 1792, when the German chemist Jeremiah Richter introduced the term as the “art of chemical measurement, which has to deal with the laws according to which substances unite to form chemical compounds” 2.

Fittingly, the term “stoichiometry” is derived from the Greek words stoicheion and metron, which mean “element” and “measure” respectively3. Using stoichiometry, chemists are able to calculate the amount of product formed in a reaction, the limiting/excess reactants, theoretical yields, and other functional values.

There are several quantity units used in stoichiometry. Some of them, like grams and milliliters are easy to convert to one another but others, like moles, can be hard to keep track of when performing stoichiometric calculations.

For this reason, a useful strategy to use is dimensional analysis. Dimensional analysis is a way chemists and other scientists convert units of measurement4. With a known conversion factor, such as the molar mass of a element, dimensional analysis is a quick and easy way to do stoichiometry.

When a reaction has completed, it is important to know the accuracy of the results in comparison to the predicted value so that experimental errors can be accounted for and fixed the next time around. In order to find the accuracy of the results, the percentage yield of a reaction is calculated.

Percentage yield refers to the percent ratio of actual yield to the theoretical yield5. If the actual yield and the theoretical yield were to be the same, the percentage yield would be 100%. However, percentage yield is often lower than 100% for a variety of reasons. For example, the actual yield might be affected by a competing reaction. Because the competing reaction uses the same reactants as the principal reaction, the principal reaction will have a lower yield1. Another reason could be the reaction rate, purity of the reactants and laboratory techniques. If the reaction is not complete, the reactants are impure, or the laboratory techniques cause product loss, the percentage yield will be affected negatively1.

Safety

Used safety goggles throughout the lab.
Tied back loose clothing and hair.
Made sure lab station was clear before starting.
Wore proper footwear1.

Materials & Experimental Method

Refer to pages 326-327 of the McGraw-Hill Ryerson text Chemistry 11 by Clancy et. al. with the following modifications:

Used 1.00 g of copper (II) chloride instead of calcium chloride.
Excluded distilled water, grease marker, stirring rod.
Did not do steps 9 and 10.

Results and Observations

Table 1. Masses of lab equipment, reactants and products

Name	Mass (g)
Empty labelled beaker 1	56.85
Empty labelled beaker 2	55.75
Clean watch glass	41.84
Beaker 1 with sodium carbonate	58.87
Beaker 2 with copper (II) chloride	56.76
Watch glass with copper (II) carbonate	42.46

Calculations

Calculating masses of reactants and products:

MNa2CO3 = MBeaker 1 with sodium carbonate - MEmptylabelled beaker 1

= 58.75g - 56.85g

= 2.02g

MCuCl2 = MBeaker 2 with copper(II) chloride - MEmptylabelled beaker 2

= 56.76g - 55.75g

= 1.01g

MCuCO3 = MWatch glass with copper(II) carbonate - MClean watch glass

=42.46g - 41.84g

= 0.62g

Determining the limiting reactant and calculating the theoretical yield:

Balanced chemical equation- Na2CO3(aq) + CuCl2(aq) → 2 NaCl(aq) + CuCO3(s)

Molar Masses- Na2CO3

= 2(22.98977) + 12.011 + 3(15.9994)

= 105.98874 g/mol

CuCl2

= 62.546 + 2(35.453)

=134.452 g/mol

CuCO3

= 63.546 + 12.011 + 3(15.9994)

=123.5552 g/mol

2.02g Na2CO3 ( )()() = 2.35g CuCO3

1.01g CuCl2 ( )()() = 0.928g CuCO3

∴The limiting reactant is the copper(II) chloride and there is a theoretical yield of 0.928g.

Calculating percentage yield:

% yield = × 100%

= × 100%

= 67%

Table 2. Qualitative observations from the reaction between sodium carbonate and calcium chloride

Observations

The copper (II) carbonate precipitate had a blue colour.

After the copper (II) carbonate dried, there was some white solid along the edges of the filter paper.

Discussion and Experimental Error

The basis of this lab was to determine the percentage yield of a double displacement reaction. More specifically, the double displacement reaction between sodium carbonate and copper chloride. The percentage yield proved to be a reasonable value given the amount of experimental errors that could have occurred. The theoretical yield was 0.928g and the actual yield was 0.62g producing a percentage yield of 67%.

In general, chemists consider yields of 90% or better as excellent while 20% or less are poor6. Therefore, a percentage yield of 67% is fairly acceptable. In fact, it is quite logical given the factors that were present in this lab that could affect product yield. As stated before, the main factors that generally affect product yield are competing reactions, reaction rate, purity of the reactants and laboratory techniques1. Reaction rate and laboratory techniques are the main factors that were present in this lab.

After the sodium carbonate solution was mixed with the calcium chloride solution, there was approximately a two minute wait before the product was filtered. Despite the majority of the reaction only taking two or so minutes, there could have been unreacted solution that did not have time to react. Because the reaction was incomplete, the product yield would be lower than what was predicted.

In this lab, the product was filtered using filter paper to separate the precipitate from the dissolved materials. Since this method was used, some of the precipitate might have accidentally gone through the filter paper. This would have caused the product yield to be lower, as mass of the product would be lost. Another laboratory technique that was used was stirring the reactants when adding water to them, as some of the reactants may have gotten attached to the stirring rod. If this had occurred, the percentage yield would consequently decrease, as mass of the reactants would be lost, resulting in less product. Lastly, when pouring the solution of aqueous sodium carbonate into the solution of aqueous copper (II) chloride, there would have been drops of aqueous sodium carbonate left in the beaker that were not accounted for. Like the stirring rod and filter paper, this would result in less product.

According to the law of conservation of mass, mass cannot be created nor destroyed. Therefore, the difference in mass from the actual yield to the theoretical yield must be due to the factors listed above, as well as experimental error. There are several experimental errors that could have occured in this lab. For example, the measurement of the distilled water that was added to the reactants might not have been completely accurate due to parallax when measuring. Another error that could have occurred would be the scale adjusting to someone walking by or movement which would skew the measurements.

While some of these errors are systematic and preventable, others like the movement near scale are caused by unpredictable changes in the environment8. There would be no way for the exact measurement to be known in a school classroom environment and it would be a error that could not have been prevented.

The results obtained from conducting this investigation are important because they show the significance of being precise with your experimental method, eliminating possible sources of experimental error beforehand, and knowing what factors to look out for so that the percentage yield can be as high as possible