Definition of Precipitation Reaction

Definition of Precipitation Reaction A precipitation reaction is a  type of chemical reaction in which two soluble salts in aqueous solution combine and one of the products is an insoluble salt called a  precipitate. The precipitate may stay in the solution as a suspension, fall out of solution on its own, or can be separated from the liquid using centrifugation, decantation, or filtration. The liquid that remains when a precipitate forms is called the supernate. Whether or not a precipitation reaction will occur when two solutions are mixed may be predicted by consulting a solubility table  or the solubility rules. Alkali metal salts and those containing ammonium cations are soluble. Acetates, perchlorates, and nitrates are soluble. Chlorides, bromides, and iodides are soluble. Most other salts are insoluble, with exceptions (e.g., calcium, strontium, barium sulfides, sulfates, and hydroxides are soluble). Note that not all ionic compounds react to form precipitates. Also, a precipitate may form under certain conditions, but not others. For example, changes in temperature and pH can affect whether or not a precipitation reaction will occur. Generally, increasing temperature of a solution increases the solubility of the ionic compounds, improving the likelihood of precipitate formation. The concentration of the reactants is also an important factor. Precipitation reactions are usually single replacement reactions or double replacement reactions. In a double replacement reaction, both ionic reactants dissociate in water and their ions bonds with the respective cation or anion from the other reactant (switch partners). In order for a double replacement reaction to be a precipitation reaction, one of the resulting products must be insoluble in aqueous solution. In a single replacement reaction, an ionic compound dissociates and either its cation or anion bonds with another ion in solution to form an insoluble product. Uses of Precipitation Reactions Whether or not mixing two solutions produces a precipitate is a useful indicator of the identity of the ions in an unknown solution. Precipitation reactions are also useful when preparing and isolating a compound. Precipitation Reaction Examples The reaction between silver nitrate and potassium chloride is a precipitation reaction because solid silver chloride is formed as a product.AgNO3(aq) KCl(aq) → AgCl(s) KNO3(aq) The reaction may be recognized as a precipitation because two ionic aqueous solutions (aq) react to yield a solid product (s). Its common to write precipitation reactions in terms of the ions in the solution. This is called a complete ionic equation: Ag  (aq)   NO3−(aq)   K  (aq)   Cl−(aq)  Ã¢â€ â€™ AgCl  (s)   K  (aq)   NO3−(aq) Another way to write a precipitation reaction is as a net ionic equation. In the net ionic equation, the ions that dont participate in the precipitation are omitted. These ions are called spectator ions because they seem to sit back and watch the reaction without taking part  in it. In this example, the net ionic equation is: Ag(aq)   Cl−(aq)  Ã¢â€ â€™ AgCl  (s) Properties of Precipitates Precipitates are crystalline ionic solids. Depending on the species involved in the reaction, they may be colorless or colorful. Colored precipitates most often appear if they involve transition metals, including the rare earth elements.

Cylinder Deactivation Variable Engine Displacement

Cylinder Deactivation Variable Engine Displacement What is cylinder deactivation? It is a method used to create a variable displacement engine that is able to supply the full power of a large engine under high load conditions as well as the fuel economy of a small engine for cruising. The Case for Cylinder Deactivation In typical light load driving with large displacement engines (e.g. highway cruising), only about 30 percent of an engine’s potential power is utilized. Under these circumstances, the throttle valve is only slightly open and the engine has to work hard to draw air through it. The result is an inefficient condition known as pumping loss. In this situation, a partial vacuum occurs between the throttle valve and the combustion chamber- and some of the power that the engine makes is used not to propel the vehicle forward, but to overcome the drag on the pistons and crank from fighting to draw air through the small opening and the accompanying vacuum resistance at the throttle valve. By the time one piston cycle is complete, up to half of the potential volume of the cylinder has not received a full charge of air. Cylinder Deactivation to the Rescue Deactivating cylinders at light load forces the throttle valve be opened more fully to create constant power, and allows the engine to breathe easier. Better airflow reduces drag on the pistons and the associated pumping losses. The result is improved combustion chamber pressure as the piston approaches top dead center (TDC) and the spark plug is about to fire. Better combustion chamber pressure means a more potent and efficient charge of power is unleashed on the pistons as they thrust downward and rotate the crankshaft. The net result? Improved highway and cruising fuel mileage. How Does it All Work? In a nutshell, cylinder deactivation is simply keeping the intake and exhaust valves closed through all cycles for a particular set of cylinders in the engine. Depending on the design of the engine, valve actuation is controlled by one of two common methods: For pushrod designs- when cylinder deactivation is called for- the hydraulic valve lifters are collapsed by using solenoids to alter the oil pressure delivered to the lifters. In their collapsed state, the lifters are unable to elevate their companion pushrods under the valve rocker arms, resulting in valves that cannot be actuated and remain closed.For overhead cam designs, generally a pair of locked-together rocker arms is employed for each valve. One rocker follows the cam profile while the other actuates the valve. When a cylinder is deactivated, solenoid controlled oil pressure releases a locking pin between the two rocker arms. While one arm still follows the camshaft, the unlocked arm remains motionless and unable to activate the valve. By forcing the engine valves to remain closed, an effective â€Å"spring† of air is created inside the deactivated cylinders. Trapped exhaust gasses (from previous cycles before the cylinders were deactivated) are compressed as the pistons travel on their upstroke and then decompressed and push back on the pistons as they return on their down stroke. Because the deactivated cylinders are out of phase, (some pistons traveling up while others are traveling down), the overall effect is equalized. The pistons are actually just going along for the ride. To complete the process, fuel delivery for each deactivated cylinder is cut-off by electronically disabling the appropriate fuel injection nozzles. The transition between normal operation and deactivation is smoothed by subtle changes in ignition and camshaft timing as well as throttle position all managed by sophisticated electronic control systems. In a well-designed and executed system, the switching back-and-forth between both modes is seamless- you really don’t feel any difference and have to consult the dash gauges to know that its happened. Read more about cylinder deactivation at work in our review of the GMC Sierra SLT flex-fuel, and see the instant fuel economy it generates in the GMC Sierra test drive photo gallery.