Values of K* as a function of temperature are as follows: Is the auto ionization

Question

Values of K* as a function of temperature are as follows: Is the auto ionization of water exothermic or endothermic? Explain. What is the pH of water at 50degreeC? Determine K_w at 37degreeC, normal physiological temperature. Determine the pH of a neutral solution at 37degreeC (using an Arrhenius plot of the data). What is the bond dissociation enthalpy for breaking one O-H bond in water to form H^* and OH^-? Compare this calculated value to that found in bond dissociation enthalpy table (is this the identical reaction? Write the (comparable) reaction Explain)

Explanation / Answer

1)

a)
The equilibrium constant, Kw for the autoionization of water increases with increasing temperature. This implies that heat drives the reaction to the right. Endothermic reactions are driven by the addition of heat, and so the autoionization of water is endothermic.

Ex: At 25 degrees Celsius, Kw=1.0 x 10^-14
At 35 degrees Celsius, Kw=2.09 x 10^-14

b)

square root the kw = 5.47 *10-14, take the negative log of the answer
pH=6.63

c)

At 37 degrees C, (H+) (OH-_ = 2.51 x l0-14th.

so, (H+) = square root of 2.51 x l0-l4th which = 1.58 times l0-7th

pH = -log (H+) or -log (l.58 times l0-7th equals 6.80

d)

The Bond dissociation energy for breaking one O-H bond in water is = -481. kJ mol-1

Thank you

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