Analytical Chem Ch. 8: Activity and the Systematic Treatment of Equilibrium

At low ionic strength, what happens?

(u -> 0M), y approaches unity; K approaches the concentration quotient

Extrapolation

estimates a number that lies beyond the values on a table

ionic strength (u)

measure of the total concentration of ions in solution

Activity coefficient (y)

measures the deviation of behavior from ideality; numeric values is used to adjust concentration for ionic strength; if y=1, behavior would be ideal

What is true of activity coefficients of nonionic compounds?

neutral molecules have an activity coefficent of y = 1 (when u < 0.1 M); activity = concentration for neutral species

Linear interpolation

the use of a linear equation to predict values that are inside of the data range; fast alternative to the Debye-Huckel equation; Unknown y/change in y = known x/ change in x

Activity coefficient (y) approaches unity as ionic strength (u) approaches _______

zero

The systematic approach to solving complex equilibria

1. Write the pertinent reactions (balanced chemical reactions) 2. Write the charge balance equation (if possible) 3. Write the mass balance equations (there may be more than one) 4. Write the equilibrium constant expressions for each chemical reaction from step 1. (use activities here) 5. Count the equations and unknowns (if # equations = # unknown chemical species, there is enough info to solve 6. Solve for all the unknown chemical species

With an increase in ionic strength, what happens?

A second salt was probably added, reactants would increase and activity coefficients would decrease

Mass balance equation

Formal concentration = the sum of all relevant species after reaching equilibrium

What is true of gases and their activity coefficients?

Gases have an activity coefficent y = 1 (when pressure < 1 bar); activity = pressure of the gas; fugacity = the activity of a gas

Activity corrections are more important for ions with higher or lower charges?

Higher; +/-3 > +/- 1

The greater the ionic strength of solution, the solubility of salt is ___________

Increased

Ksp with activities considered

Ksp = [C]yc [D]yd

Activity (A)

Value that replaces concentration in a thermodynamically correct equilibrium expression; Ac = [C]yc

When a pH meter is used to measure pH, we are attempting to measure what?

The negative logarithm of hydrogen ion activity, not concentration

ionic atmosphere

The region of solution around an ion or a charged particle. It contains an excess of oppositely charged ions.

Systematic Treatment of Equilibrium

a way to deal with all types of chemical equilibria, regardless of their complexity; key is to write as many independent algebraic equations as there are unknowns in solution including: charge balance, mass balance, and chemical equilibria; if unknown species > algebraic equations, cannot solve

pH with activities

activities can be used to calculate a more accurate value for pH

As ionic strength increases, ____________ decreases

activity coefficient

At high ionic strengths, where u > 1M ________

activity coefficients deviate from trends observed on the table; y increases with increasing u

The smaller the ion size (a), the more important ___________________ become

activity effects

charge balance

algebraic statements of electroneutrality; sum of positive charges in solution = sum of negative charges in solution; solutions must have zero total charge; always need to include [H+] and [OH]

mass balance

an equation describing a system that is based on the law of conservation of mass, but focuses on the conservation of atoms; quantity of all species containing a particular atom (or group of atoms) must be equal to the amount of that atom (or group) initially delivered to solution

extended Debye-Huckel equation

an equation used to calculate the activity coefficient for an ion at moderate ionic strengths (0M < u < 0.1 M)

As the ionic strength of the solution increases, the activity coefficients of the ions _______________

decrease

As magnitude of the charge (z) of the ion increases, activity coefficient (y) ___________________

deviates from unity