Hydrogenation refers to the treatment of substances with molecular hydrogen H 2 , adding pairs of hydrogen atoms to compounds generally unsaturated compounds. These usually require a catalyst for the reaction to occur under normal conditions of temperature and pressure. Most hydrogenation reactions use gaseous hydrogen as the hydrogen source, but alternative sources have been developed.
The reverse of hydrogenation, where hydrogen is removed from the compounds, is known as dehydrogenation. Hydrogenation differs from protonation or hydride addition because in hydrogenation the products have the same charge as the reactants. Hydrogenation : Hydrogen can be added across a double bond—such as the olefin in maleic acid shown—by utilizing a catalyst, such as palladium.
Hydrogenation reactions generally require three components: the substrate, the hydrogen source, and a catalyst. The reaction is carried out at varying temperatures and pressures depending on the catalyst and substrate used. The hydrogenation of an alkene produces an alkane. The addition of hydrogen to compounds happens in a syn addition fashion, adding to the same face of the compound and entering from the least hindered side.
Generally, alkenes will convert to alkanes, alkynes to alkenes, aldehydes and ketones to alcohols, esters to secondary alcohols, and amides to amines via hydrogenation reactions. Generally, hydrogenation reactions will not occur between hydrogen and organic compounds below degrees Celsius without metal catalysts. Catalysts are responsible for binding the H 2 molecule and facilitating the reaction between the hydrogen and the substrate.
Platinum, palladium, rhodium, and ruthenium are known to be active catalysts which can operate at lower temperatures and pressures. Research is ongoing to procure non-precious metal catalysts which can produce similar activity at lower temperatures and pressures. Nickel-based catalysts, such as Raney nickel, have been developed, but still require high temperatures and pressures. Heterogeneous Catalysis : The hydrogenation of ethylene C 2 H 4 on a solid support is an example of heterogeneous catalysis.
Catalysts can be divided into two categories: homogeneous or heterogeneous catalysts. Homogeneous catalysts are soluble in the solvent that contains the unsaturated substrate. Heterogeneous catalysts are found more commonly in industry, and are not soluble in the solvent containing the substrate.
Often, heterogeneous catalysts are metal-based and are attached to supports based on carbon or oxide. The choice of support for these materials is important, as the supports can affect the activity of the catalysts. Hydrogen gas is the most common source of hydrogen used and is commercially available. For heterogenous catalysts, the Horiuti-Polanyi mechanism explains how hydrogenation occurs. First, the unsaturated bond binds to the catalyst, followed by H 2 dissociation into atomic hydrogen onto the catalyst.
Then one atom of hydrogen attaches to the substrate in a reversible step, followed by the addition of a second atom, rendering the hydrogenation process irreversible. For homogeneous catalysis, the metal binds to hydrogen to give a dihydride complex via oxidative addition. The metal binds the substrate and then transfers one of the hydrogen atoms from the metal to the substrate via migratory insertion.
The second hydrogen atom from the metal is transferred to the substrate with simultaneous dissociation of the newly formed alkane via reductive elimination. Heterogeneous catalytic hydrogenation is very important in industrial processes. In petrochemical processes, hydrogenation is used to saturate alkenes and aromatics, making them less toxic and reactive. Hydrogenation is also important in processing vegetable oils because most vegetable oils are derived from polyunsaturated fatty acids.
Partial hydrogenation reduces most, but not all, of the carbon-carbon double bonds, making them better for sale and consumption. The degree of saturation of fats changes important physical properties such as the melting range of the oils; an example of this is how liquid vegetable oils become semi-solid at various temperatures.
Partial hydrogenation in margarine : Margarine is a semi-solid butter substitute created from vegetable oil, which is typically unsaturated and therefore liquid at room temperature. The process of partial hydrogenation adds hydrogen atoms and reduces the double bonds in the fatty acids, creating a semi-solid vegetable oil at room temperature. Incomplete hydrogenation of the double bonds has health implications; some double bonds can isomerize from the cis to the trans state. This isomerization occurs because the trans configuration has lower energy than the cis configuration.
The trans isomers have been implicated in contributing to pathological blood circulatory system conditions i. The hydrogen economy refers to a hypothetical future system of delivering energy through the use of hydrogen H 2. Advocates of this proposed system promote hydrogen as a potential fuel source. Free hydrogen does not occur naturally in quantities of use, like other energy sources, but it can be generated by various methods. As such, hydrogen is not a primary energy source, but an energy carrier.
The feasibility of a hydrogen economy depends on issues including the use of fossil fuel, the generation of sustainable energy, and energy sourcing.
As a potential energy source, the hydrogen economy stands to eliminate or reduce the negative effects of using hydrocarbon fuels, the currently dominant energy source that releases high amounts of carbon into the atmosphere. In the current hydrocarbon economy, transportation is fueled by petroleum, the use of which ultimately results in the release of carbon dioxide a greenhouse gas and many pollutants into the atmosphere.
In addition, the supply of raw materials that are essential for a hydrocarbon economy is limited, and the demand for such fuels is increasing each year. As a potential fuel, hydrogen is appealing because it has a high energy density by weight. In addition, it provides an environmentally clean source of energy that does not release pollutants.
However, there are several obstacles for the use of hydrogen as a fuel, including the purity requirement of hydrogen and difficulties that arise with its storage. Hydrogen production is a large and growing industry. Globally, 50 million metric tons of hydrogen equivalent to million tons of oil were produced in There are two primary uses for hydrogen today.
Half of the hydrogen produced is used to synthesize ammonia in the Haber process. The other half is used to convert heavy petroleum sources into lighter fractions which can be used as fuels.
The Hydrogen Economy : The hydrogen economy could possibly revolutionize the current energy infrastructure by transferring fuel demands from fossil fuels onto hydrogen.
Hydrogen production is mostly accomplished by steam reforming from hydrocarbons, but alternative methods are being developed.
The process involves methane and water and is highly exothermic:. Other ways of producing hydrogen from fossil fuels include partial oxidation and plasma reforming.
Hydrogen can also be produced from water splitting. Fuel cells are electrochemical devices capable of transforming chemical energy into electrical energy. Fuel cells require less energy input than other alternatives and perform water electrolysis at lower temperatures, both of which have the potential of reducing the overall cost of hydrogen production.
Water can also be split through thermolysis, but this requires high temperatures and catalysts. In addition, hydrogen can be produced via enzymes and bacteria fermentation, but this technology has not yet been prepared for main scale commercialization. Other methods include photoelectrocatalytic production, thermochemical production, and high temperature and pressure electrolysis.
One major obstacle in the hydrogen economy is its transport and storage. Although H 2 has high energy density based on mass, it has very low energy density based on volume. This is a problem because at ambient conditions molecular hydrogen exists as a gas. To be a suitable fuel, hydrogen gas must be either pressurized or liquified to provide enough energy.
Increasing the gas pressure will ultimately improve the energy density by volume, but this requires a greater amount of energy be expended to pressurize the gas. Alternatively, liquid hydrogen or slush hydrogen a combination of liquid and solid hydrogen can be used. Liquid hydrogen, however, is cryogenic and boils at 20 K, therefore a lot of energy must be expended to liquify the hydrogen.
Storing hydrogen in tanks is ineffective because hydrogen tends to diffuse through any liner material intended to contain it, which ultimately leads to the weakening of the container. Hydrogen can be stored as a chemical hydride or in some other hydrogen-containing compound.
These compounds can be transported relatively easily and then decomposed into hydrogen gas. Current barriers to practical storage stem from the fact that high temperatures and pressure are needed for the compound to form and for the hydrogen to be released. Hydrogen can be adsorbed onto the surface of a solid storage material and then be released upon necessity; this technology is still being investigated. Atomic bombs do not split hydrogen. Electrons can exist between shells.
It takes a finite time for an electron to make a transition. The electron s orbit the nucleus. The color of the light emitted would result from the amount of energy as it moves through shells. Absorption is shown by the energy levels increasing as the photon gains energy.
The wavelengths shown relate to the amount of energy in the photon. When an electron is hit by a photon of light, it absorbs the quanta of energy the photon was carrying and moves to a higher energy state. Electrons therefore have to jump around within the atom as they either gain or lose energy. The simplest answer is that when a photon is absorbed by an electron, it is completely destroyed. All its energy is imparted to the electron, which instantly jumps to a new energy level.
The photon itself ceases to be. The opposite happens when an electron emits a photon. Exceptions to the Aufbau principle are based on the fact that a few atoms are more stable when their electrons fill or half-fill an electron shell or subshell. According to the Aufbau principle, these electrons should always fill shells and subshells according to increasing energy levels.
Answer: Electronic Configuration of Lanthanides: as the 4f and 5d electrons are so close in energy it is not possible to decide whether the electron has entered the 5d or 4f orbital. According to the Aufbau principle, the orbital with the lower energy level must be filled first completely, before moving on to the next orbital.
Orbitals fill in order of energy. So 5D fills before 4F in some cases simply because the 5D energy levels are lower than the 4F levels for some. The nuclear charge is insufficient to contract the 4F orbitals and lower their energy well below the 5D. Both protons and neutrons have a mass of 1, while electrons have almost no mass. The element hydrogen has the simplest atoms, each with just one proton and one electron.
The proton forms the nucleus, while the electron orbits around it. All other elements have neutrons as well as protons in their nucleus, such as helium, which is depicted in Figure 2. The positively charged protons tend to repel each other, and the neutrons help to hold the nucleus together. The number of protons is the atomic number , and the number of protons plus neutrons is the atomic mass. For hydrogen, the atomic mass is 1 because there is one proton and no neutrons.
For helium, it is 4: two protons and two neutrons. For most of the 16 lightest elements up to oxygen the number of neutrons is equal to the number of protons. For most of the remaining elements, there are more neutrons than protons, because extra neutrons are needed to keep the nucleus together by overcoming the mutual repulsion of the increasing numbers of protons concentrated in a very small space.
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