Shifts in Equilibrium

Observing and Inferring Le ChMelier s principle states that if a stress is placed on a reaction at equilibrium, the system will shift in a way that will relieve the stress. In this experiment, you will witness an equilibrium shift in a colorful way. 

Materials test tubes (2) 10-mL graduated cylinder 250-mL beaker concentrated hydrochloric acid 0.1M C0CI2 solution ice bath table salt hot plate 

Place about 2 mL of 0.1M C0CI2 solution in a test tube. Record the color of the solution. 

Add enough water to the test tube to make a color change occur. Record the color. 

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To initiate a chemical relaxation it is necessary to perturb the system from its initial equilibrium position. This is done by applying a forcing function, which is an appropriate experimental stress to which the system responds with a shift in equilibrium configuration. Forcing functions can be transient (a sudden, essentially discontinuous Jolt ) or periodic (a cyclic stress of constant frequency). 

In addition to COg, Cl and BPG also bind better to deoxyhemoglobin than to oxyhemoglobin, causing a shift in equilibrium in favor of Og release. These various effects are demonstrated by the shift in the oxygen saturation curves for Hb in the presence of one or more of these substances (Figure 15.35). Note that the Og-binding curve for Hb + BPG + COg fits that of whole blood very well. 

After dark-condition equilibrium was established, as indicated by the visible spectra, the photo-shift in equilibrium was observed to be completely reversed when the illumination ceased. This photogalvanic effect maintained a mass balance in the system, with no reagent consumed or generated during the dark-light-dark cycle. This observation suggested that the plutonium system in the proper network of a concentration cell... 

Example provides practice in analyzing changes that may lead to shifts In equilibrium. 

Two types of notion exist with respect to the term low concentrations [i.e., a low absolute concentration (highly dilute solutions) and a low equilibrium concentration (as in the formation of complexes or compounds of low solubility)]. In the latter case, when potential-determining substances start to be withdrawn from the solution, they re-form because of the shift in equilibrium (i.e., their potential supply is large). 

Figure 5.176. A sudden increase in concentration of the second solute concentration in phase 2 (Zj from 0.2 to 0.5) caused a shift in equilibrium.

Processes accompanied by a decrease in volume, such as C—C bond formation, in which the distance between two carbon atoms decreases from the van der Waals distance of ca 3.6 A to the bonding distance of ca 1.5 A, are accelerated by raising the pressure and equilibria are shifted toward the side of products (AV < 0, AV < 0). The reverse reaction, a homolytic bond cleavage, leads to an increase in volume (AV / > 0, AV > 0). Pressure induces a deceleration of such a process and a shift in equilibrium toward the side of reactants. However, in an ionization, such as an ionic dissociation, the attractive interaction between the ions generated and the solvent molecules leads to a contraction... 

For cryptands in which the molecular cavity is larger than in the case of the [l.l.l]-species [78], proton transfer in and out of the cavity can be observed more conveniently. Proton transfer from the inside-monoprotonated cryptands [2.1.1] [79], [2.2.1] [80], and [2.2.2] [81 ] to hydroxide ion in aqueous solution has been studied by the pressure-jump technique, using the conductance change accompanying the shift in equilibrium position after a pressure jump to follow the reaction (Cox et al., 1978). The temperature-jump technique has also been used to study the reactions. If an equilibrium, such as that given in equation (80), can be coupled with the faster acid-base equilibrium of an indicator, then proton transfer from the proton cryptate to hydroxide ion... 

Test Ttibe 5 0.1 M A1(N03)3 When I mixed samples of test tube 5 with a sample from test tube 3, a white precipitate formed. No heat was evident. The white precipitate could have been either Al(OH)3 or Sn(OH)4, however, since no heat was produced and the A1(N03)3 did not contain an acid, it can be concluded that test tube 5 contained A1(N03)3. When I mixed samples of test tube 5 which I believed contained Al3+ with samples of test tube 2 which I believed contained OH, a white precipitate formed which I believed to be Al(OH)3. When I added more solution from test tube 2, increasing the OH" concentration, the precipitate dissolved, consistent with a shift in equilibrium. 

Test Hibe 7 0.1 M SnCl4 in 3 M HC1 When I mixed samples from tube 3 (6 M NH3) with samples from test tube 7, a white precipitate formed and heat was produced. Tube 7 could contain Al3+ or Sn4. Since heat was produced, an acid must have been present. Between the two choices, only SnCl4 was originally mixed with acid, therefore I concluded that test tube 7 contained SnCfi. When I mixed samples of test tube 7 with samples from test tube 2 that I believed contained OH, a white precipitate formed, consistent with Sn(OH)4. Upon adding more NaOH, the precipitate dissolved resulting from a shift in equilibrium. 

A pressure perturbation results in the shifting of the equilibrium the return of the system to the original equilibrium state (i.e., the relaxation) is related to the rates of all elementary reaction steps. The relaxation time constant associated with the relaxation can be used to evaluate the mechanism of the reaction. During the shift in equilibrium (due to pressure-jump and relaxation) the composition of the solution changes and this change can be monitored, for example by conductivity. A description of the pressure-jump apparatus with conductivity detection and the method of data evaluation is given by Hayes and Leckie (1986). 

These three tubes contain a mixture of N02 g and N204(g). The tube on the left is in an ice-water mixture. The centre tube is at room temperature. The tube on the right is in boiling water. Given that N02 g is brown, can you explain the shift in equilibrium Think about Le Chatelier s principle and the enthalpy of the reaction between the two gases. 

In which tests did you increase the concentration of a reactant or a product Did your observations indicate a shift in equilibrium to form more or less of the reactant or product ... 

Indicators have different colors in the combined and dissociated forms. The equilibrium of the indicator in solution shifts according to Le Chatelier s principle as an acid or base is added to the solution. The shift in equilibrium to the right or left causes the color to change accordingly. 

Enzymes that are regulated by effector molecules in an allosteric maimer possess, apart from the binding site for the substrate, a specific binding site for the effector molecule. The binding of effector molecules to the effector site leads to a shift in equilibrium between the various conformations and thus to a change in activity (see 2.3). 

The ratio is known as the partition coefficient and is a constant. For the most part, this ratio holds regardless of the concentration. The reason for this goes to the thermodynamic driving force to eliminate potential energy. It is an equilibrium constant and therefore obeys all applicable thermodynamic laws. One of these is a shift in equilibrium in response to temperature changes. 

Indolo or benzo fusion at the 5,6-position in perhydropyrido[l,2-c]pyrim-idine (335-340) (Fig. 13), as in 360 and 361 shifts the equilibrium in favor of the cis-fused conformer 364 and the axial JV-methyl trans-fused con-former 362, with 54% of the equatorial iV-methyl trans-fused conformer 363 present at room temperature (CDC13 solution).284 285 The shift in equilibrium (compare corresponding shift for 192-194 compared to quinolizidine 179-181, Section III,B,3) is presumably a result of a destabilization of 362 and 363 by the peri-type interaction involving the C-l-H and either the... 

The Negative-Temperature-Coefficient Region The equilibrium constant for the reaction R + O2 ROO (R64) is strongly temperature dependent, and as the temperature increases, the equilibrium shifts in favor of R + O2. The shift in equilibrium is the primary reason for the existence of the region where the conversion decreases with an increase in temperature (i.e., where there is a negative temperature coefficient). Above about 650 K, the alkyl peroxy radical becomes less thermally stable, and alternative reaction paths for ROO begin to compete with the isomerization reaction (R65). A new product channel opens up for the R + O2 reaction... 

Wierzchowski et al.325 recorded in aqueous medium the IR specta of the monoanions of thymine and of 1-methyl- and 3-methylthymine. The data demonstrated that the monoanions of thymine consist of an equilibrium mixture of two tautomeric forms 34 and 35 corresponding to the dissociation of the N-l or the N-3 protons, respectively. The fractional content of the thymine monoanion 35 in the mixture of the two monoanions was evaluated from the extinction of the characteristic bands of the anion of thymine and 1-methylthymine to be 0.4. A marked shift in equilibrium of the tautomers was observed on going from a NaCO in D20 solution of thymine to NaOD in 75% dioxan-D20. Calculation of the tautomeric equilibrium showed that in 75% dioxan the fraction of 35 was 0.25, compared to 0.4 in aqueous medium. 

As amine concentration was increased, hydrogenation rates increased, passing through a maximum at 1-2M. This concentration dependence is readily understood in terms of a shift in Equilibrium 3 to favor the inactive ammonium carbonyl rhodate (I). In this equilibrium the concentration of active hydrogenation catalyst (II) passes through a maximum as the [amine] is increased. With stronger bases this maximum occurs at progressively lower concentration. 

There are Other forms of tautomerization but keto-enof tautomerization is the most likely form to be tested on the MCrtr. fn Older to recognize other forms, simply watch for the proton shift in equilibrium. 

Scheme 24 Chiral photoswitchable polyisocyanates A) schematic representation of the shift in equilibrium between P and M helices upon irradiation. B) illustration ofPto M helix transition in polyisocyanates upon photoisomerization ofthe azobenzene unit (adapted from references 75-78).

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