Effect of temperature and concentration on equilibrium system

Objectives:

1. To study the effect of concentration on equilibrium

2. To study the effect of temperature change on equilibrium

3. To predict the direction of the net reaction in an equilibrium system by Le Chatelier’s principle

Introduction:

Chemical equilibrium applies to reactions that can occur in both directions. In a reaction such as:

aA(g) + bB(g) cC(g) + dD(g)

The reaction can happen both ways which is a reversible reaction. Some of the products are created the products begin to react to form the reactants. In a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have no net change with time. Chemical equilibrium is a dynamic process that consists of a forward reaction, in which the reactants are converted into the products; and a reverse reaction, in which the products are converted into the product. Usually, this state results when the forward reaction proceeds at the same rate as the reverse reaction. The reaction rates of the forward and reverse reactions are generally not zero, but with the net change in concentration of the reactant and product are the same. The ideal equilibrium constant may expressed as where [ ] is the concentration of the chemicals

For the gaseous reactant, it is express as the equilibrium condition in the terms of partial pressures of reactants and products where P is the partial pressure

Factors that affect the equilibrium constant are the changes in the experimental conditions such as concentration, pressure, volume, and temperature disturb the balance and shift the equilibrium position so that more or less of the desired product is formed. The changes of outcome of these changes can be predicted by Le Chaptelier’s principle. According to the Le Chatelier’s Principle, changes the conditions of a chemical system at equilibrium such as temperature or pressure. The equilibrium shifts to the right or left to relieve the disturbance places on the system. To See how Le Chatelier’s Principle works, consider the reaction used to prepare hydrogen gas:

C(s) + H₂O(g) CO(g) + H₂(g) H = + 131 kJ

Le Chatelier’s Principle states that when a chemical system at equilibrium is disturbed, it retains equilibrium by undergoing a net reaction that reduces the effect of the disturbance. One of the ways is by adding or removing reactants or products, which is means that changing the concentration of reactants or products. In the hydrogen gas reaction if more steam (H₂O) was added, the reaction would shift to the right producing more hydrogen and carbon monoxide. The reaction shifts to cause more hydrogen to react. This shifts counteracts the stress of adding more hydrogen to the system. Likewise, by removing hydrogen and carbon monoxide, the equilibrium, shifts to the right, or if more of these products are added, the reaction shifts to the left.

Another factor of a chemical system is by changing the pressure. The key to this lies in the stoichiometry of the reaction. The effect of pressure on equilibrium depends on the number of moles of gas particles on the right and left sides of the balanced equation. In reactions where the numbers of moles of gas are equal, a change in pressure has no effect on the equilibrium. If Δn_gas = 0, there is no effect on the equilibrium position. Changes in the pressure are directly related to changed in volume. This is because a decrease in volume gives the same result as increasing the pressure, while an increase in volume is the same as a decrease in pressure. For example, this situation exists when equilibrium occurs in a cylinder with a movable piston and the volume is changed by moving the piston. Pure solids and liquids do not need to be considered when using Le Chatelier’s Principle because the concentrations of pure solids and liquids are constant during the course of the reaction.

The final factor to be considered is a change in temperature. Other than the three types of disturbances, only temperature changes alter K_c. To apply Le Chatelier’s Principle with temperature changes, the sign of H for the reaction needs to be known. A temperature rise will increase K_c for a system with a positive ΔH whereas a temperature rise will decrease K_c for a system with a negative ΔH. The increase of K_c will shift the equilibrium position to product side. Likewise, the decrease in K_c will shift the position of equilibrium to reactant side. If the heat is supplied to the endothermic reaction ( H is positive), the equilibrium will favor to the product direction. If the heat is removed from the exothermic reaction ( H is negative), the equilibrium will favor to the product direction. For the temperature is not favor to the reaction, the equilibrium position will shift to the reactants side.

Thymolphthalein and phenolphthalein are the acid base indicators. Both indicators are often written as HIn in shorthand notation. C₂₈H₃₀O₄ and C₂₀H₁₄O₄ are the formula of the thymolphthalein and phenolphthalein respectively. The thymolphthalein has transition range is at approximately pH 9.3-10.5. Below this pH 9.3, it is colorless whereas the colour is blue when above pH 10.5. For phenolphthalein, the colour of the indicator is pink within the range of pH 8.2 to pH12. The following diagram A and B shows the structure of the thymolphthalein and phenolphthalein

 

Apparatus: test tubes, thermometer ( 0-200 )

Materials: solid copper (II) nitrate, 0.4M cobalt (II) chloride, 0.1 M potassium chromate, 0.1 M potassium dichromate, 0.2 M HCl, 0.2M NaOH, thymolphthalein indicator, 0.1M ammonia solution, solid ammonium chloride, phenolphthalein indicator

Experimental procedure:

i) Effect of temperature changes on equilibrium

a) Investigate the equilibrium between a solution and a solid

Cu(NO₃)₂ (aq) Cu(NO₃)₂ (s)

1. Some solid Cu(NO₃)₂ is poured into a test tube and half of test tube is filled with distilled water. The test tube is shaked until form a saturated solution. If all the Cu(NO₃)₂ is dissolves, additional Cu(NO₃)₂ is added into test tube and shake it until a saturated solution with some excess solid is obtained.

2. The colour of solution at room temperature is noted. A waterproof pen is used to mark the level of the top of the solid.

3. Test tube is placed in a beaker with hot water at 60 for over 30 minutes. Observation is recorded.

4. Test tube is placed in an ice bath for another 30 minutes and observation is recorded.

b) Investigate the aqueous equilibrium of a cobalt complex

[Co[H₂O]₆]²⁺ (aq) + 4Cl^-(aq) [CoCl₄]^2- (aq) + 6H₂O (l)

1. Test tube is placed in 60 water bath for 15 min, and then observation is recorded.

2. Same test tube is placed in an ice bath for 15 min and observation is recorded.

ii) Effect of concentration changes on equilibrium

a) Dichromate/chromate solution

2CrO₄^2- (aq) + 2H⁺ (aq) Cr₂O₇^2- (aq) + H₂O (l)

1. 5 drops of 0.1M potassium chromate solution is placed into two small teest tubes and the colour is noted. Test tube is labeled as 1 and 2.

2. 5 drops of 0.1 M potassium dichromate solution is placed into another small test tube and the colour is noted. Test tube is labeled as 3.

3. 0.2 M HCl is added drops by drops into tube 1 and is shaked when adding, until a colour change. Observation is noted. The colour change is compared with tube 2 and 3 and observation is recorded.

4. 0.2m of NaOH is added drop by drop into tube 1 and is shaked when adding, until a colour change. Observation is noted.

5. Some acid is added into tube 1 and observation is noted.

b) Thymolphthalein equilibrium

Hln (aq) H⁺ (aq) + ln^- (aq)

1. Small amount of thymolphthalein solution is placed on a small piece of cloth with paint brush and leave it exposed to air.

2. The colour change of the blue stain is noted. Time is recorded it took to change colour.

c) Ammonium ion / ammonium solution

NH₃ (aq) + H₂O (l) NH⁴⁺ (aq) OH^- (aq)

1. 5cm³ of 0.1M ammonia solution is placed in two small test tube, 2 drops of phenolphthalein is added into each test tube.

2. One of the test tube is added with solid ammonium chloride, a little at time.

3. The colour of both test tube are compared and observation is recorded.

Result and calculation:

Part (i)

Table 1.1 Observation of copper(II) nitrate and its colour change at 60 and 0

Temperature,	Observation
Room temperature	Blue colour solution is formed after solid copper(II) nitrate dissolved.
60	Blue colour solution remains unchanged but the level of solid decreases.
0	Blue colour solution remains unchanged but the level of solid increases.

Table 1.2 Observation of colour change of cobalt(II)chloride at 60 and 0

Temperature,	Observation
Room temperature	Pink colour solution is formed
60	Colour of solution turns from pink to dark pink
0	Dark pink solution turns to light pink solution.

Part (ii)

Table 2.1 Comparison between colour of potassium dichromate and colour changes of potassium chromate solution with addition of HCl and NaOH

Table 2.2 Observation of colour change of thymolphthalein on cloth and time taken to decolourise

Solution	Observation
Thymolphthalein	The blue colour solution is turns to colourless after exposed to air for two to three second.

Table 2.3 Colour change of ammonium solution after with addition of ammonium chloride

Solution	Observation
Ammonium solution + phenolphthalein	The light purple colour is formed.
Ammonium solution + phenolphthalein + ammonium chloride	The light purple solution is decolourized.

Discussion:

In the experiment part (i) (a), the solid copper(II) nitrate is used to dissolve in a test tube filled with half distilled water. The solid copper(II) nitrate ionize in water to form copper(II) ions and nitrate ion. Hence, a blue solution is formed due to the blue copper(II) ion present in the solution. When excess solid is added, the solution will become saturated and do not allow any solid to dissolve, so excess solid will remain in the solution. This is shows that the equilibrium between solid and aqueous copper(II) nitrate is achieved. When the test tube is being placed in the water bath of 60 , the level of solid copper(II) nitrate tends to decrease to ionize in the solution but the blue intensity of the solution remains the same. The dissolution of copper(II) nitrate is an endothermic reaction. So, the equilibrium constant of the reaction increases as the temperature of the solution increases which allow more products are formed. The equilibrium position is shift to the left in the equilibrium equation below:

Cu(NO₃)₂ (aq) Cu²⁺ (aq) + NO₃^- (aq)

Eventually, more copper(II) ions and nitrate ion are formed at the high temperature due to the equilibrium effect. Oppositely, the level of solid copper(II) nitrate is increases at the 0 . When the test tube is placed in an ice bath, the low temperature causes the equilibrium constant of the solution decreases and hence the formation of solid copper(II) nitrate is increases. The equilibrium position shifts to the left at the condition of 0 .

Next, the cobalt(II) chloride is added with hydrochloric acid in this experiment in order to investigate its equilibrium at different temperature. The cobalt (II) chloride is added with hydrochloric acid, which a pink solution is formed. When the test tube is placed in a water bath with 60 , the solution turns from light pink to dark pink. This is because the cobalt(II) chloride dissolve in solution is an endothermic reaction which it tend s to form blue cobalt (II) chloride in the solution. The dark pink solution is formed instead of light pink because the dark pink colour of solution is the mixture of pink and blue. The equilibrium position shift to right in the equilibrium equation below:

[Co[H₂O]₆]²⁺ (aq) + 4Cl^-(aq) [CoCl₄]^2- (aq) + 6H₂O (l)

Since the equilibrium equation above is an endothermic reaction, the position of equilibrium tends to shift to right at high temperature, 60 in this experiment. The increase in equilibrium constant leads to more products is formed. So, the colour changes to dark pink due to the amount of cobalt(II) chloride formed is increases. On the other hand, the equilibrium position shifts to the left at the temperature of 0 . When the equilibrium shifts to left, the concentration of hydrated cobalt ion form and hence the solution forms at a light pink colour.

In the experiment part (ii) (a), the potassium dichromate solution is orange colour while the potassium chromate solution is appears in yellow colour. When there is addition of two drops of 0.2M hydrochloric acid,HCl, the colour of potassium chromate solution turns from yellow to orange due to the equilibrium effect. The equilibrium equation between chromate ion and dichromate ion is

2CrO₄^2- (aq) + 2H⁺ (aq) Cr₂O₇^2- (aq) + H₂O (l)

When HCl is being added, the acid will ionize to become H⁺ ion and Cl^- ion. The chromate ion, CrO₄^2- ion accept the excess H⁺ and shift to the right to become dichromate ion, Cr₂O₇^2- which is orange colour ion. But, when two drops of sodium hydroxide ion, NaOH is added, the OH^- ion is ionize and tends to combine with H⁺ to become water and hence the concentration of H⁺ ion in the solution decreases. The dichromate ion, Cr₂O₇^2- will release H⁺ ion to form chromate ion, CrO₄^2- in the solution, thus the colour change to yellow colour. When second time of HCl is added, the chromate ion, CrO₄^2- will accept the H⁺ ion to dichromate ion, Cr₂O₇^2- form if the H⁺ ion concentration is excess.

In the part (ii) (b) experiment, the thymolphthalein is undergoes reduction which its blue colour solution is decolorized. When the blue thymolphthalein is exposed to the air, the carbon dioxide in the air will react with the thymolphthalein since it is a basic solution. Initially, dilute sodium hydroxide solution is added into the thymolphthalein when preparing the thymolphthalein indicator in order to maintain it pH in basic condition. If hydroxide ion, OH^- is excess in the solution, the blue colour can be perceived. However, when thymolphthalein is exposed to the air, the sodium hydroxide will tends to react with carbon dioxide to form sodium carbonate and water as shown in the chemical equation below:

At the same time, the carbon dioxide will react with water in the solution to form carbonic acid, H₂CO₃. Due to the partial ionization of carbonic acid, the hydronium ion, H⁺ is produced which caused the condition become more and more acidic (pH value dropping).

The ionized H⁺ ion will react with the thymolphthalein which driving the equilibrium shift to the right to form colourless Hln in the solution.

Hln (aq) H⁺ (aq) + ln^- (aq)

The equilibrium between ammonia molecule and ammonium ion is studied in this experiment. When phenolphthalein is added into the ammonia solution, the colour present in the solution is light purple colour. This is because the ammonia solution is a weak acid with pH value of 11.6. The phenolphthalein turns from colourless to light purple when it is added into the solution with pH 11.6. When solid ammonium chloride is added, the ammonium chloride dissolves quickly in the solution to form ammonium ions and chloride ions. The ammonium chloride dissociates completely in the solution. The formation of ammonium ions in the solution combine with hydroxide ions and hence shifts the equilibrium position to left to form more ammonia and water as shown below:

NH₃ (aq) + H₂O (l) NH⁴⁺ (aq) OH^- (aq)

At the same time, the pH value of the solution decreases due to the concentration of the hydroxide ions is reduced by the reaction. As a result, the phenolthalein in the solution is decolourized due to the drops in pH of the solution.