Главная страница Случайная страница
КАТЕГОРИИ:
АвтомобилиАстрономияБиологияГеографияДом и садДругие языкиДругоеИнформатикаИсторияКультураЛитератураЛогикаМатематикаМедицинаМеталлургияМеханикаОбразованиеОхрана трудаПедагогикаПолитикаПравоПсихологияРелигияРиторикаСоциологияСпортСтроительствоТехнологияТуризмФизикаФилософияФинансыХимияЧерчениеЭкологияЭкономикаЭлектроника
Writing Net Ionic Equations
|
|
We will fi nd net ionic equations extremely useful for summarizing a great deal of information with relatively little effort.
Three Types of Equations Are Used to Describe Reactions in Solution:
1. The formula (total) equation gives the overall reaction stoichiometry but not necessarily the actual forms of the reactants and products in solution.
2. The complete ionic equation represents as ions all reactants and products that are strong electrolytes.
3. The net ionic equation includes only those solution components undergoing a change. Spectator ions are not included.
When sodium chloride solution is added to silver nitrate solution, a precipitate of silver chloride is produced, and the solution contains sodium nitrate:
This type of equation can be called the total equation. (A total equation is sometimes referred to as a molecular equation because the compounds in it are written “as if they were molecules.”)
Even more informative than a total equation is an ionic equation. An ionic compound in aqueous solution may be represented as separate ions, but an ionic solid that is not dissolved in water is written as a complete compound. We can write an ionic equation for the reaction of sodium chloride with silver nitrate in aqueous solution as follows:
Because the Na+ and NO3- ions appear on both sides of this equation (unchanged by the reaction), they are called spectator ions. They may be eliminated from the equation:
This equation is an example of a net ionic equation. All the spectator ions are omitted from a net ionic equation.
While we are learning to write net ionic equations, we will use the following procedure. After we get used to working with them, we will be able to write them directly and save even more effort with their use.
1. Start with a total equation for the reaction, making sure that it is balanced.
2. Write all compounds that are both soluble and ionic in the form of their separate ions, making sure to have the correct number of ions of each type. Write all other compounds (for example, gases, other covalent compounds, and all solids) as complete compounds.
3. Eliminate the ions that are unchanged on both sides of the equation to obtain the net ionic equation.
Be careful not to misinterpret the name net ionic equation. It is not necessarily true that all the substances appearing in such an equation are ionic. Covalent compounds often occur in net ionic equations. Also, just because the formula for a complete compound is written in such an equation does not mean that the compound is not ionic; it might simply be insoluble.
Don’t be confused about what should be included in net ionic equations. It is easier to remember what should be left out: Only ions in solution that remain unchanged in solution should be left out to produce net ionic equations; all other species must be included. Thus, insoluble compounds (ionic or not), covalent compounds, elements, and ions that change in any way between reactants and products are all included.
48.: Colloidal Chemistry. Types of disperse systems. Materials may be mixed together to form a true solution, a coloidal dispersion, or a coarse dispersion.
A true solution - mixture of two or more components that form a homogenous molecular dispersion, i.e. a one-phase system, the composition of which can vary over a wide range.
A colloidal dispersion - represents a system havin a particle size intermediate between that of a true solution and a coarse dispersion, roughly 10Å to 5000Å (0.1mm = 1000Å)
A coarse dispersion the diameter of the particles in emulsions and suspensions for the most part being larger than 0.1mm (1000Å).
A colloid is any substance which is dispersed throughout another substance very evenly, to the point of even distribution on the microscopic level. In order to be distributed in this way, the colloidal mixture has to be broken down into very small particles, called colloidal particles, which are too small to be directly seen by a conventional microscope. There are both biological and man-made examples of colloids which are evenly distributed throughout another substance in the world, such as within milk, fog, smoke and pears, or aerosol sprays, marshmallows, styrofoam, and shaving lather.
A colloid is prepared by reducing large particles to colloidal size (generally between 1 nanometer and 1 micrometer), or increasing small particles (usually single molecules) to the size of colloidal particles. There are a wide variety of techniques in practice, and the type of method used depends on what form state of colloid is desired. Aerosols are produced by using a jet of pressured gas to tear away liquid from the mixture, sometimes facilitated by the ionization of the liquid, then using the repulsion between the similarly ionized liquid to separate into easily removed droplets. Emulsions are prepared by vigorously shaking the two constituent liquids together, sometimes with the use of surfactants like soap in order to help emulsify and stabilize the product formed here.
Semi-solid colloids are what are known as gels, sometimes formed by taking lycophilic (attracting solvents) sols (a stable dispersed mixture of a solid and a liquid) and cooling them, such that their large linear molecules and their natural viscosity cause the solution to disperse. Colloids can be purified through dialysis, which is the process of removing any ionic material that may have accompanied the colloid during its formation. In order to carry out this dialysis, a membrane that allows ions and solvents to pass through, but which stops colloids is used, exhibiting the concepts of diffusion, osmosis, and ultrafiltration.
Colloids have a number of unique properties. Colloids have strong absorption qualities, caused by the large exposed surface area when colloidal particles are finely divided. Colloidal particles can carry an electrical charge, resulting in attraction or repulsion among itself, known as electrostatic interaction.
Stable colloidal systems are simply colloids with the capability to remain in the colloidal state. Unstable colloidal dispersion results in aggregation, where the dispersed substance builds up, and becomes unevenly distributed. Aggregation can be avoided through the use of electrostatic stabilization, by which the colloidal substance is induced with a like electrical charge throughout, resulting in repulsion between all dispersed particles.
A solution is a homogeneous mixture of one or more solutes dissolved in a solvent.
solvent: the substance in which a solute dissolves to produce a homogeneous mixture
solute: the substance that dissolves in a solvent to produce a homogeneous mixture
Note that the solvent is the substance that is present in the greatest amount.
Many different kinds of solutions exist. For example, a solute can be a gas, a liquid or a solid. Solvents can also be gases, liquids or solids.
Colloids are on the dividing line between solutions and heterogeneous mixtures. Like solutions, colloids can be gases, liquids, or solids.
| Disperse Phase | Dispersion Medium | Solution | Collodial Dispersion | Coarse Dispersion |
| Gas | Gas | Air | n/a | n/a |
| Liquid | Gas | Water Vapour | Fog | Spray |
| Solid | Gas | Sublimed Idoine | Smoke | Dust |
| Gas | Liquid | Carbonated Water | Foam | Foam |
| Liquid | Liquid | Alcohol in Water | Liquid surfactant micelles | Emulsions |
| Solid | Liquid | NaCl in Water | Polymer or protein solutions | Suspensions |
| Gas | Solid | Hydrogen in palladium | Solid Foam | Solid Foam |
| Liquid | Solid | Mineral oil in paraffin | Solid Emulsion | |
| Solid | Solid | " Solid dispersion" | Colloidal Gold in glass | " Solid suspensions" |
A suspension is a heterogeneous mixture in which do not dissolve fully, but remain suspended throughout the solution. The particles are larger than 10, 000 Angstroms which allows them to be filtered. If a suspension is allowed to stand the particles will separate out.
A colloid is somewhere between a solution and a suspension. A suspension separates out but a colloid does not. When light passes through a colloidal dispersion, such as smoky air, it gets reflected by the larger particles and the light beam becomes visible.