NAME: Nickisha Williams COLLEGE
PRESIDENT’S
CANDIDATE #:
DATE: 27th/02/2015 Skill: ORR
CAPE CHEMISTRY UNIT 1
PRACTICAL # 7 Title: Investigation of the Factors that Affect Chemical Equilibrium Aim: To investigate the factors that affect chemical equilibrium using universal indicator solution and to what extent a chemical reaction precedes. Theory: Many chemical reactions do not go to completion, but reach a state of equilibrium before the reactants are completely consumed. At the equilibrium state, the concentration of all reactants and products remains constant, yet the reaction continues with the rates of the forward and reverse reactions being equal. The equilibrium can be described by the equilibrium constant, K. Disturbing the equilibrium position causes the equilibrium to shift, towards reactant or products, to counteract or minimize the effect of the disturbance. The is known as Le Chatelier’s principle. There are many factors that can change the position of the equilibrium once the reaction appears to have stopped. In this investigation, you will identify factors that have an effect on the above reaction. As you complete this investigation, you will observe what happens when various substances are added to an equilibrium mixture, describe the chemistry taking place when reagents are added that change the equilibrium system, and identify patterns about stresses placed on equilibrium systems. 1. A Simple Experiment with Universal Indicator Solution
Universal indicator is an acid-base indicator which can react as a proton donor or proton acceptor. The colour of this solution is different in these two possible forms: H2O + HIn ↔ H3O+ + InColour 1
Colour 2
This experiment suggests why neutral litmus solution has a different colour.
Apparatus and Materials:
Test tubes Teat pipettes H2SO4 (dil.) NaOH (dil.) Universal indicator solution
Procedure: •
5cm3 of H2SO4 (dil.) was poured into test tube (A) and 5 drops of neutral universal indicator solution. It was stirred with a glass rod. The colour changed of the solution was observed.
•
5cm3 of NaOH (dil.) was poured into second test tube (B) and 5 drops of neutral universal indicator solution was added. It was then stir with a cleaned rod. The colour change of the solution was observed.
•
5cm3 of water was poured into the third test tube (C) and 5 drops of neutral universal indicator solution was added. It was then stirred with a clean glass rod. The colour change of the solution was observed.
•
Test tubes A and B were arranged so that when looking through the pair of tubes the colour of it will be that due to the solution in both test tubes. The ‘combined colour’ was compared of A and B with that of C.
•
A drop of NaOH (dil.) to test tube C. The colour of the solution with that in test tube B was stirred and compared. Then, 2 drops of H2SO4 (dil.) was added to the mixture. The colour of the solution was stirred then compare with that in test tube A.
Diagram: Burettes containing universal indicator solution
Drop of Solution H2SO4 and BiCl
A
B
C Test Tube A&B align so that the experiment, record the dominant color
5cm3 H2SO4
5cm3 NaOH
5cm3 Water
Table Showing The Amount of Acid Present, The Amount of Base Present also with Observations Test Tube
Amount of Acid Present
Amount of Base Present
Chromate Solution
Observations
A
5cm3
0
A reddish solution was observed which specifies it was a strong acid
B
0
5cm3
The solution turned blue-purple which specifies it was a base/alkali
C
0
0
A green solution which specifies it was neutral.
A+B
5cm3
5cm3
When sulfuric acid and sodium hydroxide solution is realized after mixing it appears as a brown colour.
C+B
0
5cm3
When water (green) and sodium hydroxide (purple) was seen it has a dark-greenish brown.
C+A
5cm3
0
When water (green) and sulfuric acid (red) was seen it has a yellow colour.
What factor is responsible for the colour change in this experiment?
The factor responsible for the colour change in the experiment is the H+ concentration and the OH- ion concentration. The acid (Hln) and its conjugate base (ln-) have different colors. At low pH values the concentration of H3O+ is high and so the equilibrium position lies to the left. The equilibrium solution has the color A. At high pH values, the concentration of H3O+ is low - the equilibrium position thus lies to the right and the equilibrium solution has color B. the H2SO4 acts as the source of hydrogen and NaOH acts as the source of the hydroxide ion so it shifts the equilibrium. Acids and bases are supplementary to a system so as to shift the position of a chemical equilibrium. The ions have diverse colors, so that changes are perceived visually. Yellow chromate turns orange by addition of acid, while the orange dichromate in reaction with bases turns yellow. The equilibrium depends on the acidity of the solution, so the colour in this case is pH dependent.
2. Chromate (VI) – Dichromate (VI) Experiment
The reaction investigated in this experiment is: 2 CrO4- (aq) + 2H+ (aq) ↔
Cr2O72- (aq) + H2O (l)
The shift in equilibrium is conveniently illustrated by a colour change.
Apparatus and Materials: Test tubes
Teat pipette Dil. Sulphuric acid Dil. Sodium hydroxide Potassium chromate.
Procedure: •
A small amount of potassium chromate (VI) was dissolved in about 5 cm3 of water in a test tube.
•
Some dilute sulfuric acid was added drop wised. The colour change was observed.
•
Some dilute sodium hydroxide was added drop wised to the same test tube. The colour change was observed.
•
Alternate additions of acid and base were repeated several times to the mixture to affirm that the colour change is due to the addition of acid and base.
Table Showing The # of additions to chromate (VI) solution of H2SO4 (dil.) and NaOH (dil.) with Observation # of additions to chromate (VI) solution of
Observations
H2SO4 (dil.)
When sulfuric acid (yellow) was added to Potassium Chromate (VI) it turned bright orange.
NaOH (dil.)
When Sodium Hydroxide was added to which was bright orange it turns back to yellow.
What factor is responsible for the colour change in the equilibrium mixture in this
experiment? •
Upon adding the H2SO4 to the chromate (VI) solution it turned orange, which indicated that the reaction was shifted to the reverse direction. The acid provided the source of H+ ion so in the act of trying to counterattack the stress on the equilibrium it shifted to the reverse direction.
•
Upon the addition of the NaOH to the chromate (VI) solution it turned yellow. This indicated that the equilibrium shifted from the dichromate ion to the
chromate ion. The base solution provided a source of OH- ion which caused the reaction to proceed in the forward direction. Acids and bases are supplementary to a system so as to shift the position of a chemical equilibrium. The ions have diverse colors, so that changes are perceived visually. Yellow chromate turns orange by addition of acid, while the orange dichromate in reaction with bases turns yellow. The equilibrium depends on the acidity of the solution, so the colour in this case is pH dependent. 3. Reversible Reaction Between Bismuth (III) Chloride and Water
The reaction under investigation here is the hydrolysis of bismuth (III) chloride: BiCl3 (aq) + H2O (l) ↔ BiOCl (s) + 2HCl (aq) Apparatus and Materials: Test tubes, teat pipettes, conc. HCl, bismuth (III) chloride Procedure: •
A little bismuth (III) chloride was dissolved in about 5cm3 of water in a test tube. The result of the reaction was examined.
•
Some concentration of HCl was added drop wise to the mixture. Change was observed.
•
A little amount of water was added drop wise to the mixture. Change was observed.
•
The alternate addition of the concentration of HCl and water was repeated several times to the same mixture to affirm that the change is due to the addition of acid and water.
Table Showing The Observation Of # of Additions to Bismuth (III) chloride solution Of Water and HCL (conc) # of additions to Bismuth (III) chloride solution of Water HCl (conc.)
Observations A white precipitate was formed. When white precipitate dissolves in the solution it leaves a colorless appearance of the solution.
What factor is responsible for the observed change which occurs in the reversible
reaction studied in this experiment? o
Upon bestowing additional HCl(aq) to the reaction mixture, it operated as a stress to the equilibrium and in so doing the reaction go in the reverse direction to neutralize the stress added, hence this is the reason why the white bismuth (III) Oxychloride precipitate dissolve in excess HCl since with will favor the production water with each extra addition of HCl.
Upon administration of additional water to the reaction mixture, the added stress initiates the movement in the forward reaction to stabilize the stress on the equilibrium, hence bismuth (III) Oxychloride was produced accounting for the white precipitate observed. Forward reaction Bismuth chloride + water bismuth oxychloride + hydrochloric acid BiCl3(aq) + H2O(l) ==> BiOCl(s) + 2HCl(aq) o
A reaction in which a molecule reacts with water to give at least two products is called a hydrolysis reaction. If hydrochloric acid is added to the mixture, the bismuth oxychloride dissolves to reform the bismuth chloride solution. Backward reaction Bismuth oxychloride + hydrochloric acid ==> bismuth chloride + water BiOCl(s) + 2HCl(aq) ==> BiCl3(aq) + H2O(l) •
Therefore the reaction is reversible and what is formed depends on the relative amounts of hydrochloric acid and water.
Le Chatelier’s principle is responsible for the reversible reaction. Concentration is also responsible for the observed changed. Limitations: 1. They maybe some built in errors in the method as a source of a limitation. 2. Inaccurate measurement of the volume of the various reactants. 3. Incorrect interpretation of the color change, so causing incorrect inferences 4. Insufficient amount of reactants was used in the experiment Precautions: 1. The experimenter must consider speed and accuracy when doing the procedures, so that environmental factors wouldn’t affect the experiment. 2. Safety gears such as lab coat and gloves should be worn, when handling chemicals. Conclusion:
It can be established that variations were observed by using a universal indicator. 1. For experiment 1, the test on the universal indicator solution. The factor affecting the color change is the actual the H+ ion concentration. 2. For experiment 2, the same corollary was made since it’s the H+ ion and OH- ion caused the equilibrium to shit in errands of the dichromate and chromate ion respectively. 3. In experiment 3, the equilibrium position was affected directly by HCl and Water.