Many of us must have suffered from much confusion after reading about the terms like alkali, alkaline, alkali metals, alkaline earth metals, alkane, alkene and alkyne. Since the terms have very similar names so it is difficult to distinguish b/w them. In this article, definitions, properties, structures, formulas and uses of all these chemical terms will be discussed briefly i.e. what is alkaline, what are its properties and what are its uses?
Alkali metals are those metals which form alkalis (the bases that can neutralize the acids) when they get dissolved in water. These are the 6 alkali metals from periodic table forming Group 1: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr). Alkali metals are very reactive in nature and are usually found in form of compounds in nature like NaCl, KCl etc. since these alkali metal compounds are easily dissolvable in water so they can be easily extracted. Alkali metals have high ductility, silver like luster (shiny) and are soft, highly reactive and excellent conductors of electricity and heat. Their melting points are low (28 degree Celsius to 179 degrees).
Alkali metals react with water to release hydrogen and to make caustic solutions as each alkali metal has a single electron in the outer shell in s-orbital. So when these metals react with non-metals, they tend to make cations (positive charged ions) by giving this single valence electron, gaining the charge of +1. Thus the products are hard crystals that are bound by ionic bonds, thus they have higher melting points as compared to alkali metals.
Alkali is defined as a subset of some base: a base that can get dissolved in water. A solution which has a soluble base has the pH greater than 7. Alkali is actually an ionic and basic salt of some alkali metal. All alkalis (Arrhenius bases) are capable of making hydroxide ions (OH–) after getting dissolved in water. Usually alkaline solutions are very slippery to the touch.
These are the salts that are soluble hydroxides of the alkali metals, like
NaOH — sodium hydroxide, also known as caustic soda
KOH — potassium hydroxide, 2nd name is caustic potash
Ca(OH)2 — calcium hydroxide, also known as limewater when it is in form of saturated solution.
What is alkaline? On pH scale, substances below 7 are known as acidic and substances above 7 are called basic or alkaline. A simple liquid can be made alkaline by adding a base into it. For example, when natural water runs over the rocks (containing basic minerals), water becomes alkaline slightly. Basically, when a base is adde to water then hydrogen and hydroxide ions are formed by dissociation of water. The overall balance of ion concentration moves in favor of hydroxyl ion concentration, thus resulting an alkaline solution. Sodium bicarbonate (baking soda), ammonia, magnesium hydroxide (milk of magnesia) and sodium borate (borax) are some common examples of bases.
What is alkaline soil? The soil which have pH greater than 7.3 is known as alkaline soil. Because of presence of alkali salts, such soils can occur naturally. Alkaline soils can cause problem for many plants as most of plants need slightly acidic soil having pH in the range 6 to 6.8.
Water having pH upto 8 or 9 is known as alkaline water. Natural water like spring water contains those minerals which make it more alkaline. According to the present research date, alkaline water is helpful to boost energy, fight cancer and strengthen the bones.
Alkaline earth metals
Alkaline earth metals belong to the 2nd group of the periodic table including beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and radium (Ra). Since all of them lie in same group, so they have similar properties like they are very silvery white, shiny and reactive. All of them have 2 electrons in outer most shell in S-orbital, including helium. They gain an oxidation state of +2 when they lose these electrons to be cations with +2 charge.
All the alkaline earth metals occur in nature but only radium is produced by the decay of uranium and thorium. All these metals are soft and have low melting and boiling points and densities too. Except beryllium, all these metals react with water to form alkaline hydroxides. Since beryllium doesn’t react with water or steam so its halides are covalent. In fact all compounds of beryllium have covalent bonds.
- These metals interact with halogens to form ionic halides like calcium chloride CaCl.
- They react with oxygen to form oxides like strontium oxide SrO.
- Barium reacts with water to form hydroxides and to produce hydrogen gas.
Alkane is an acyclic saturated hydrocarbon consisting of hydrogen and carbon atoms (saturated because they have single bond), which are arranged in a tree structure in which all the carbon–carbon bonds are single. The general chemical formula of alkanes is CnH2n+2. The simplest case of alkanes is methane (CH4), where n = 1 (also termed as parent molecule) ad the complex molecules of alkanes include penta-contane (C50H102) and many more.
In alkanes, each hydrogen atom is joined to carbon atom in a C–H bond. The longest series of linked carbon atoms in a molecule is known as its carbon skeleton. The number of carbon atoms are considered as the size of the alkane. There are three types of saturated alkanes: linear alkanes, branched alkanes and cyclic alkanes. Depending upon their names, all of them have different structures. Boiling points of linear alkanes are higher than that of branched chain alkanes. This is due to the greater surface area in contact, thus causing greater van der Waals forces b/w adjacent molecules. On the other side, cyclo-alkanes have higher boiling points than that of linear, because of locked confirmations of molecules, giving a plane of intermolecular contact.
- Alkanes having lowest molecular weight are in form of gases.
- Alkanes of intermediate molecular weight are in liquid form.
- The heaviest alkanes are in form of waxy solids.
All alkanes are colorless. Alkanes have higher boiling points due to stronger intermolecular Vander Waals forces. Boiling point is linked to the size of the molecule.
There are two main commercial sources of alkanes: petroleum and natural gas. From CH4 to C4H10 alkanes are in gaseous state, from C5H12 to C17H36 they are in form of liquid and after C18H38 they are all solids. In the table below, alkanes list with their formulas and their states is shown:
- Methane CH4 gas
- Ethane C2H6 gas
- Propane C3H8 gas
- Butane C4H10 gas
- Pentane C5H12 liquid
- Hexane C6H14 liquid
- Heptane C7H16 liquid
- Octane C8H18 liquid
- Nonane C9H20 liquid
- Decane C10H22 liquid
- Undecane C11H24 liquid
- Dodecne C12H26 liquid
- Tridecane C13H28 liquid
- Tetradecane C14H30 liquid
- Pentadecane C15H32 liquid
- Hexadecane C16H34 liquid
- Heptadecane C17H36 solid
- Octadecane C18H38 solid
- Nonadecane C19H40 solid
- Icosane C20H42 solid
- Triacontane C30H62 solid
- Tetracontane C40H82 solid
- Pentacontane C50H102 solid
- Hexacontane C60H122 solid
- Heptacontane C70H142 solid
Alkanes do not conduct electricity, nor are they polarized by an external electric field. This is the reason due to which they do not form hydrogen bonds and are unable to get dissolved in polar solvents like water. Alkanes are said to be hydrophobic as they repel water. However, their solubility in non-polar solvents (lipophilicity) is high. The density of alkanes is less than that of water but at the same time, it is dependent on the number of carbon atoms. That’s why alkanes form the upper layer in an alkane water mixture. Usually alkanes are very weakly reactive with most of compounds.
Applications of alkanes are dependent on the number of carbon atoms. Methane and ethane are usually stored as liquids after cooling and compressing. 1st 4 alkanes are used for cooking and heating purposes. At atmospheric pressure, propane and butane are the gases that can be liquefied at relatively low pressures and are commonly known as liquefied petroleum gas (LPG). Propane is used in propane gas burners and butane in space heaters & disposable cigarette lighters, and as a fuel for road vehicles. Both are used in aerosol sprays as propellants.
Alkanes are extremely volatile liquids, ranging from pentane to octane. They are used in internal combustion engines as fuels, as they vaporize quickly without forming droplets on entry into the combustion chamber, which would impede the uniformity of the combustion.
Branched-chain alkanes are favored as they are much less vulnerable than their straight-chain homologues to premature ignition, which triggers knocking. The middle alkanes are strong solvents for non-polar compounds, apart from their use as fuels.
Nonane-to-hexadecane alkanes (16 carbon atoms) are liquids with higher viscosity that are less and less suitable for gasoline use. They represent the bulk of diesel and aviation fuel.
The most important components of fuel oil and lubricating oil include alkanes from hexadecane upwards. They act at the same time as anti-corrosive agents in the latter role, as their hydrophobic nature ensures that the metal surface cannot be penetrated by water. Several solid alkanes, for example, find use in candles as paraffin wax.
In bitumen, used for road surfacing, alkanes with a chain length of approximately 35 or more carbon atoms are found. The higher alkanes, however, have little meaning and are usually separated by cracking into lower alkanes.
Alkenes are those hydrocarbons which have double bond b/w carbon atoms. They are the unsaturated compounds of carbon with the general formula of CnH2n. Unsaturated hydrocarbons means those hydrocarbons which have double or triple bond. Sometimes, alkenes are also termed as olefins. They are weakly polar, colorless, odorless but are more reactive than alkanes. However, ethane has a sweet odor. Ethene and propene are 1st 2 members of this family. In alkenes, the simplest compound is ethylene C2H4. Initial 3 members of alkenes series are in gaseous state, next 14 in liquid form and remaining are in waxy solid form. Alkenes which have 4 or more carbon atoms form structural isomers. Many alkenes are actually the isomers of cycloalkanes.
Alkenes have no ability to get dissolved in water due to non-polar characteristics, however they can get dissolved in non-polar solvents like benzene. Boiling points of alkenes are dependent on the number of carbon atoms in relevant compound i.e. as number of atoms increase, boiling point also increases. Just like alkanes, boiling point of linear alkenes is greater than that of branched alkenes. However melting points are dependent on the positioning of carbon atoms. The density of alkenes is less than that of water.
Structural isomers of acyclic alkenes having only one double bond are as follows:
- C2: ethylene only
- C3: propylene only
- C4: 3 isomers: 1-butene, 2-butene and isobutylene
- C5: 5 isomers: 1-pentene, 2-pentene, 2-methyl-1-butene, 3-methyl-1-butene, 2-methyl-2-butene
- C6: 13 isomers: 1-hexene, 2-hexene, 3-hexene, methyl-pentene (7 isomers), dimethyl-butene (3 isomers)
- C7: 27 isomers
- C12: 2,281 isomers
- C31: 193,706,542,776 isomers
Alkenes are combustible and their physical state depends on the molecular mass: as simplest few alkenes are gases at room temperature. The melting point of the solid alkenes increases with increase in molecular mass. As compared to corresponding alkanes, alkenes smell stronger as ethylene has musty sweet odor. Usually alkenes are stable but they are more reactive than alkanes due to reactivity of C-C pi-bond.
The applications of alkenes are given as: The plastics that we use as marketing buckets, bags etc are made up of polythene. 2ndly alkenes are used to manufacture polystyrene that is used in making car battery cases and parts of refrigerator. They are used in production of ethanol. They are also used in manufacture of propanol that is used in formation of acetone. Packaging material and ropes are made by poly-propene, which is another application of alkenes.
These are the unsaturated and non-polar hydrocarbons which contain at least one triple bond b/w carbon atoms. The general formula of alkynes is CnH2n-2. Alkynes are also termed as acetylenes. The simplest alkyne is ethyne with C2H2 formula. The triple bond that alkynes contain is very strong i.e. about 839 KJ/mole energy is required to break it. Usually alkynes are soluble in organic solvents like benzene and acetone and are in gaseous form. Like alkenes, they are also insoluble in water.
Alkynes are colorless and odorless. First 3 alkynes are gases, next 8 are in liquid form and remaining are solids. Boiling points of alkynes are greater than that of alkanes or alkenes and they are more acidic.
Some of applications of alkynes are given below:
- Polyethylene is used in formation of plastics.
- Propyne is used as fuel for welding torches and also as fuel for rockets.
- Ethyne is used to make organic compounds like ethanoic acid, ethanol and acrylic acid etc.
- Alkynes are used to artificially ripe fruits.