Chemical Equilibrium Purpose The purpose of this experiment is to observe Le Cha
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Chemical Equilibrium Purpose The purpose of this experiment is to observe Le Chatelier's Principle and calculate the equilibrium constant for a chemical reaction. Introduction When chemical substances react, the reaction may not always proceed to completion. In many cases equilibrium is established in which both products and reactants exist. To represent a reaction at equilibrium, a double-headed arrow is used (versus a single-headed arrow that denotes a reaction going to completion). For example, the reaction aA + bB cC represents a reaction that has gone to completion- only products are present. However, using a double-headed arrow shows that the reaction is at equilibriurm In an equilibrium reaction, the ratio of products to reactants is constant at a given temperature. This ratio, known as the equilibrium constant Kc, can be expressed for the reaction above as a ratio between products and reactants [A]a [B]b The brackets represent the molarity of the compound at equilibrium, and the superscript is the stoichiometric coefficient of each compound as shown in the chemical reaction equation Le Chatelier's Principle is an important concept when working with reactions at equilibrium. Le Chatelier's Principle states that if a system at equilibrium experiences a stress, the system will shift in such a fashion so as to minimize the stress. The shift may result in an increase in product concentration while the reactant concentration decreases (a shift to the right of the reaction), or vice versa (an increase in reactant concentrations, or a shift to the left of the reaction). There are several types of 'stresses" that can be applied to a system, the most common of which is a change in the concentration of either the products or reactants. Other stressors may include a change in temperature or pressure (when working with gasses at equilibrium). In the case of a temperature change, the value of Kc itself is changing (remember that Kc is constant at a given temperature, so changing the temperature changes the value of Kc). This change in Kc is what causes the system to shift In order to determine the value of Kc, the concentrations of both the products and reactants at equilibrium are required. In order to do so, and "ICE" Table is used. An example of an ICE table is shown below: 61Explanation / Answer
In spectroscopy the wo important concepts studied are: Absorbance and transmittance
Fe+3 + SCN- --> FeSCN+2(aq)
For Table 2:
Test Tube
Stress applied
Observation
Equilibrium shift (left / right)
Explanation
2
Add KSCN
Dark orange
right
Increase in concentration of SCN- will increase concentration of FeSCN+2
Hot water
Light orange
No shift
Will dilute the solution and will lighten the organge colour
Cold water
No change
No change
3
Add Fe(NO3)3
Dark orange
Right
More Fe+3 ion concentration will increase formation of more of FeSCN+2 and hence will darken the colour
Add NaOH
Orange ppt
left
The hydroxide draws off the iron forming iron hydroxide:
4
Add AgNO3
White cloudy
Left
Ag+ will consume SCN- from the system
5
Add NaOH
Much lighter yellow
Left
The hydroxide draws off the iron forming iron hydroxide:
Test Tube
Stress applied
Observation
Equilibrium shift (left / right)
Explanation
2
Add KSCN
Dark orange
right
Increase in concentration of SCN- will increase concentration of FeSCN+2
Hot water
Light orange
No shift
Will dilute the solution and will lighten the organge colour
Cold water
No change
No change
3
Add Fe(NO3)3
Dark orange
Right
More Fe+3 ion concentration will increase formation of more of FeSCN+2 and hence will darken the colour
Add NaOH
Orange ppt
left
The hydroxide draws off the iron forming iron hydroxide:
4
Add AgNO3
White cloudy
Left
Ag+ will consume SCN- from the system
5
Add NaOH
Much lighter yellow
Left
The hydroxide draws off the iron forming iron hydroxide:
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