Ionic Bond

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When ionic solids dissolve, they divide to give their positive and negative ions that make up the solids. These ions become hydrates and have the same relative proportions when in solution and when solid. The more the solid dissolves, the more the ion’s concentration increases. This increase and build-up allows for the reverse reaction to occur. In this phase of the reaction the ions crystallise out in order for the reaction to have a greater chance of occurring. Eventually the rate of dissolving will equal the rate of crystallisation. This is the state of saturation. This can be recognised by a constant colour or constant mass.

The solubility product constant, Ksp is given in the following example:
Ksp for AgCl is
Ksp = [Ag][Cl]
Ksp for PbI2 is
Ksp = [Pb][I]2

This gives the relationship between the ions in the saturated solution and is the maximum concentration possible without creating precipitation. In this lab, solutions of lead nitrate and potassium iodide will be mixed at a number of dilutions. The reactions will then be observed to see at which point a precipitate no longer occurs. Ksp will then be stated as a range of values at room temperature, and the precipitate test tubes will be heated until the precipitate is dissolved so that Ksp may be observed and determined at different levels.

In this experiment various solutions of lead nitrate and potassium iodide were mixed at a number of different dilutions. Through the observation of the amount – or lack of precipitate formed in each dilution, the mathematical relationship between the ions in a saturated dilution may be determined. This relationship is known as the solubility product constant, or Ksp, and is defined as follows, “The Ksp for an ionic solid is given by the product of the concentrations of the ions, each raised to the power of the coefficients in the dissolving reaction.'; (Heath Chemistry). The Ksp expression gives the maximum possible concentration of ions in a saturated solution without causing precipitation. Based on the equation for this experiment we can conclude that the ksp expression for this experiment is:
Ksp = [Pb][I]2

The Ksp for lead iodide at 25oC is 8.5x10^-9. Through the substitution of the values for each test tube, the trial product, or KSPtrial was obtained. A precipitate then formed in those test tubes who’s KSPtrial was greater than the Ksp, whereas if the KSPtrial was calculated as being less than the Ksp, no precipitate formed.
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