Chemical Equilibria Lab Report

Danika Ng
CHEM 1252 L
Exp 12: Thermodynamic Analysis of Chemical Equilibria
The reaction being done with borax and HCl is an acid base reaction. The K constant expression of the reaction is Kc = [Na^+][B4O7^-2]. A titration is needed for this experiment because when a strong acid with a known concentration is added to a weak base, the unknown concentration of the weak base can be found, which can then find the moles of the weak base, which can then find the Kc. The first law of thermodynamics is that energy can’t be created or destroyed. Based, off of Google, the second law of thermodynamics is that “entropy in the universe can never be negative”. They relate to the expression for deltaH, deltaS, and deltaG because the equation
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Solubility increases with high temperature and more agitation (stirring). The less the solution is stirred or if there’s a decrease in temperature, the solution becomes less soluble and forms a solid at the bottom of the solution. The forward reaction is endothermic because the deltaHrxn was positive, meaning that the solution was absorbing heat rather than releasing heat. An increase in Kc shows the increase of solubility in borax solution. If there’s a decrease in Kc values, that means there is a decrease in solubility. Increase in temperature means an increase in Kc values because solubility increases with temperature, and vice versa. For example, at 323.2 K, the Kc was 4.54*10^-7, which is more than at 303.2 K, which is Kc = 8.19*10^-9. To calculate the sign and value of deltaSrxn: 45.6(8.314 J/mol*K) = +379.12 J/mol. The positive sign of the entropy change makes since because Borax is placed in distilled water to create a Borax solution, the Borax breaks down into particles; the particles of Borax are random in the solution and are very disordered, meaning that the entropy increases; the positive sign of deltaSrxn shows the increase in disorder.
To calculate deltaHrxn: (19573 K)(8.314 J/mol*K) =162729.922 J/mol (1/1000) = +162.73 kJ/mol. The positive sign of enthalpy change makes sense because when borax is dissolved in distilled water, the bonds are broken. When bonds break, it is an endothermic reaction justifying positive deltaHrxn. Positive deltaG shows that reaction is nonspontaneous, negative deltaG is spontaneous reaction. Decreasing temperature increases deltaG. Increasing temperature decreases deltaG.

Figure 1:
Molecular-level view of Supernatant borax

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