Reactions in Solution: Defining Solution Characteristics

Outline

• Solutes, Solvents, and Solutions
  • Units of Concentration
  • The Solution Process
• Practice with Concepts
• Discussion Questions
• Optional Website Reading

Solutes, Solvents, and Solutions

Units of Concentration

We tend to think of solutions as the result of dissolving a dry powder like salt in a liquid like water, but the term actually applies to a wide range of chemical situations, solid, liquid, and even gas! Metal alloys like bronze are solid solutions: bronze is a case where tin is dissolved into copper to form a homogenous solution as liquids, then allowed to cool and solidify. Oxygen dissolves in water, which is a good thing, or fish would not be able to survive.

A solution is a homogeneous mixture composed of only one phase. This means that when we dissolve a solid or gas solute into a liquid solvent, the solute takes on the liquid phase of the solvent. Since a solution is a mixture, its components may be separated by physical, rather than chemical means. We can recover common salt (solute) from a saltwater solution by evaporating the water (solvent). However, interesting chemical reactions occur in solutions: the dissolution of ionic substances into their ions is one such reaction. As we shall see, ions in solution are the basis of electrical systems that depend on chemical batteries.

Chemists have different ways of expressing the concentration of a solute in a solution, depending on what they want to do.

• Molarity — Determine the concentration of an existing solution: Often in titration experiments, an acid or base is added to a solution until a known pH is reached or the solution is neutralized. This method allows the chemist to easily determine the number of moles of solute reacting with the added materials. But since the original amount of solvent is unknown, or no longer available, the moles of solvent are expressed as a ratio to the total solution volume: moles solute / liters solution = Molarity, with the symbol M (italicized and capitalized).
• Molality — Prepare a solution of specific concentration: It is often useful to prepare a solution where we know the number of moles of the solute that can react; we measure the solvent by weight: moles solute/ mass of solvent = Molality, with the symbol m, (italicized, lower case).
• Mole Fraction — Moles available for reaction or contributing to pressure: Colligative properties depend on the total number of particles available, rather than the identity of the particle. In situations where we focus on these properties, knowing the ratio of solute moles can be important. The mole fraction is the ratio of moles solute to total moles present: X = (n moles solute) / (n moles solute A + x moles solute B + y moles solvent....)
• Percent weight — Consumer product measurements often use percent weight. This makes it easy to envision the amount of solute compared to the solvent. A 5% acetic acid solution means a solution where 5% of the weight is acetic acid and the rest is water ... in other words, commonly available commercial white vinegar, used for salad dressing.
• Parts per million — Environmental measurement used for pollution Because pollutants can be dangerous even in very small proportions, pollutants are often measured as "pollutant parts per million", or ppm. This is roughly equivalent to a single drop of water in a tankful of gas.

The Solution Process

As with any process, we need to examine the energy flow into and out of the solution system when a solute dissolves in a solvent.

A common example is the dissociation of an ionic compound (a salt) in water. This is a two-step process.

1. Dissociation occurs first as the bonds between the ions are broken. This usually involves energy input as work is done to separate the mutually attracted ions.
2. Hydration occurs as the ions form bonds with polar water molecules.

The enthalpy of solution is the net change in overall energy of the system between the dry salt + water state and the dissolved salt in solution state. While both breaking ionic bonds and forming H-bonds with water molecules may involve large amounts of energy, the difference between the two steps is often very small, and slightly exothermic — meaning that most reactions where a salt dissociates will be spontaneous and product-favored.

Not every substance dissolves easily in a solvent. A good rule of thumb is that "like dissolves like", where "like" refers to the polarity of the substances involved. Water is a good solvent because as a polar covalent substance, it can readily dissolve other polar covalent or ionic (the extremity of polar covalence) substances. It cannot readily dissolve non-polar substances. We say that non-polar substances are immiscible in water.

Another common situation involves fats and acids. Oil and vinegar don't mix well, leading to interesting taste sensations in salad dressings, and the classification of salad dressing as a suspension of oil droplets in vinegar.

Practice with the Concepts

Molality is used to indicate final concentration when we prepare a solution.

What is the molality of the solution when we dissolve 5.844g table salt (NaCl) in 1.0 L water?

Optional Readings

• Check out this presentation of the solution process at "Learners TV".

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