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Aluminum and Copper Chloride Lab: Limiting Reagents Edition

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Helena Jensen

on 27 January 2015

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Transcript of Aluminum and Copper Chloride Lab: Limiting Reagents Edition

Introduction
Aluminum and Copper Chloride Lab: Limiting Reagent Edition
The reaction that is going to occur is going to be between Aluminum (foil) and a Copper Chloride solution. The experimenter will simply fill a beaker with a measured quantity of water, then take a teaspoon of Copper Chloride, mass it, and add it to the water to make the solution. The dissolving of the CuCl2 is a physical reaction that occurs in chemistry.
Forming the Solution
The reaction will occur when the Aluminum is added to the newly formed Copper Chloride. First though, the experimenter must mass the Aluminum. It is important to note that this quantity is crucial and will determine the products of the reaction. Given that we will be trying to make one of the reactants the limiting reagent it is important to note some important ratios.
The Solute
When the aluminum is added to the solution, the experimenter will notice bubbling and a sudden appearance of a red substance. This red substance is a precipitate of the reaction as the aluminum replaced the less active metal Copper in the aqueous solution. No longer is the solution made up of copper chloride ions, it is made up of aluminum chloride ions. The aluminum substance has been replaced by copper, which is the newly formed substance.
Reaction
We already noticed that a red precipitate formed, but it is also important to find evidence for the aluminum ionizing in the solution. If you watch the solutions color change during the reaction, you will notice that it has lost it's light blue hue that we noted it was characterized by initially. This drastic color change is evidence of a chemical change.
Qualitative Observations:
In this reaction aluminum goes from being a neutral atom to being an ion in a chloride compound. It goes from having zero to having a positive charge so it loses electrons and thus Al is oxidized. Copper goes from being a positively charged ion in copper chloride to a neutral atom (all atoms are neutral in charge). Thus it is reduced. Chloride remains a negatively charged ion the whole time- it is never reduced nor oxidized. Chloride is a spectator ion.
Oxidation and Reduction
After the reaction has completed- usually this happens overnight. One must run the solution and the products through a filter. Filtration is a method for separating a mixture and very effective in separating solids from liquids.
Then, one must let the filter paper, the precipitates and (potential) excess reactant dry out. After that, assuming that there is excess aluminum as their was in our reaction- one must separate the aluminum and the copper which is easy to using tweezers because they are very different colors and have different qualities.
Post Reaction
In this lab we will be using an experiment to learn how to find the limiting reagent in a chemical reaction. The experiment that we will be performing is a single replacement reaction which will take place in an aqueous solution.
Dissolving? Explain....
Copper Chloride is a binary Ionic compound, thus when it is added to water it is able to form an aqueous solution. This is possible because it is an ionic compound, and thus it forms ions in a solution. This is only possible with ionic compounds and arrheneous acids.
Copper Chloride Solution Equation
Often in an experiment there are multiple reactions, and thus multiple balanced equations that can describe the experiment.

CuCl2 + H2O -> CuCl2 (aq)

Ratios and Balanced Equation
When dealing with any sort of reaction, having a balanced equation is a necessary resource for proper measurements and data analysis. This is because of the law of conservation of mass: which states that in any reaction the mass of the reactants must be equal to the mass of the products.

The balanced equation for this reaction is:

2Al(s) + 3CuCl2(aq) -> 3Cu (s) + 2AlCl3(aq)
This equation is a single replacement reaction in which an active metal (aluminum) replaces a less active metal (copper).
Ratios.....
The molar ratios can be broken down:

2M of Al (s) is needed to produce 3M of Cu (s) -> 2:3 ratio of Al:Cu

We can convert this to mass as well->
53.96g Al : 190.638gCu

This is important for finding the limiting reagant, because anytime you are performing this reaction, the products will combine in this ratio. This too is because of the law of conservation of mass.

Nomenclature:
Copper Chloride
Copper Chloride is an Ionic Compound made up of the metal Copper and the non-metal Chlorine. Thus it is custom to name the compound by stating the Metal first and the Non-Metal second, in its ion name (ie Chloride not Chlorine)
Qualitative Observation:
Copper Chloride
It is important to note any physical characteristics of the reactants so that after the reaction, the experimenter can note any changes in color or properties. Those physical changes can be indicative of a chemical reaction. Copper Chloride has a distinctly blue hue, and once it is added to the water, the aqueous solution thus has a turquoise hue as well.
Qualitative Observation:
When the reaction first occurs, there is a lot of bubbling that will appear. This is not because a hydrogen gas is being released as a result of the reaction as often bubble indicate. In fact in this reaction it is because as soon as the aluminum comes in contact with the copper chloride the solution starts to boil.
Thermochemistry:
Boiling is also indicative of the release of heat, meaning that this reaction is an exothermic reaction.
Units:
Mass will always be measured in grams (or kg) in labs, which can be easily done using a gram scale.

The amount of substance though will be measure in moles. Moles are indicated by coefficients in a balanced equation.
Qualitative Observation: Density
When the aluminum is placed in the solution, it is clear that it does not sink. This means that aluminum must be less dense than the copper chloride solution. However once the copper precipitate starts to form, the copper sinks to the bottom- evidence that copper is more dense than the aluminum chloride solution and of aluminum.
Limiting Reagent
1)Note any known measured or calculated mass of reactants
2)Multiply mass by 1 mol/formula mass:-> this will give you the moles for the
measured quantity
3) Divide those moles by the coefficients of the balanced equation (2 for Al and 3 for CuCL2)
4) Compare those two numbers and which ever is the smaller number will be the limiting reagent

What is a Limiting Reagent?
A limiting reagent is the reactant in an reaction that you have the least "batches" of. If one thought of an experiment like a recipe, the limiting reagent is like the ingredient that runs out the quickest. If not for that ingredient, a larger amount of product could be made, however the limiting reagent is keeping the reaction from having a greater product outcome. In an experiment with a limiting reagent there will always be an excess reactant, and thus the reaction will never be fully complete.
Products Formed:
Once the aluminum replaced the copper in the aqueous solution, a copper solid precipitate was formed. To find the mass of this solid, one must filter the solution and let the solid dry out. Then separate any excess aluminum, in order to have an accurate measurement of the copper.
Quantitative Observation
ALUMINUM USED: 1.730 g



COPPER CHLORIDE USED: 2.255 g
Difference between accuracy and precision
Accuracy is how close a measurement is to the actual value. To find out whether your measurements are accurate, one can find the theoretical yield of a specific product.

Precision is how close measurements are to each other. If a person was performing this experiment multiple times, they would be able to compare data results to each other to determine the precision of measurements.
Determining the Theoretical Yield of Copper and Aluminum Chloride
Al moles used= AlCl3 moles produced (0.011)
CuCl2 moles used= Cu moles produced (0.017)


Multiply Mass of AlCl3 and Cu by the indicated moles
AlCl3 : 011 x (133.5)= 1.469 grams of AlCl3
Mass of Cu= .017 x (63.5)= 1.080 grams of Cu

To find the total mass of the products, one must also calculate the mass of the

To find the quantity of excess reactant , multiply the used molar amount of the limiting reagent by the ratio between it and the other reactant.
.017 mol x 2/3= 0.011 mol Al
Subtract X from original molar amount to discover excess reactant.
.064 mol - .011 mol= .053 mol Al
Multiply this molar amount by formula mass of reactant. The moles will be canceled out and mass of excess reactant will be left.
.053 mol x 27 gram/mol = 1.431 grams

Finding Excess Reactant
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