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Encyclopedia about Chemistry

 

Chemical Reactions and Energy

Modern chemistry attempts to produce new materials which through their various characteristics and properties can be better used for all types of purposes. One prerequisite of choosing the necessary chemical reactions necessary to synthesise some new product is a detailed knowledge of the structure of the reactants and their characteristic properties, including some knowledge of the course of the chemical reactions and the mechanisms which make them go and influence them.

A chemical reaction is a change in molecules and elements which results in new molecules with new properties being formed. The course of a reaction is described by a chemical equation. The materials which react together are called reactants; the materials which are formed in a reaction are called products. A reaction equation, or a chemical equation, is used to abbreviate and symbolise a chemical reaction. The reactants, the materials which begin a chemical reaction, are written on the left side of a chemical equation, in front of an arrow, and the products are written on the right side of this arrow:

Fe(s) + S(s) ® FeS(s)

Iron (in the carbon group) and sulphur (same group) react to produce iron sulphide.

In many reactions, the state of matter of the materials changes. For this reason, whether the material, either reactant or product, is in the solid (s), liquid (l), or gaseous (g) state is indicated with the corresponding lower case letter, in parentheses as above. If a reaction results in the amount of products being less than reactants, we call this a combination, or synthesis reaction. If there are more products than reactants, this is a dissociation, or breakdown reaction.

Energy and chemical reactions

Elements try to attain a state which is the most natural or most energetically advantageous for them, that is, one where the outermost electron shells are filled. For this reason, electrons are very often transferred between atoms, either donated or accepted. Some elements donate their electrons more easily, while some elements accept electrons more readily. In extreme cases, the electrons of one atom are completely transferred to an atom of another or the same element. But most of the time, electrons are not completely transferred, but rather shared between two atoms, though those electrons may be attracted to one of the atoms more strongly than the other. This is a chemical bond.

The most ideal state for atoms and molecules is always that state with the lowest energy. In most chemical reactions, then, the energy that was included in higher-energy bonds is released to the surroundings. But in order for such an energy-releasing reaction to occur, the reactants must be infused with enough energy to break the original bonds and allow the formation of new ones. Most of the time, a certain amount of energy has to be added to the system (usually in the form of heat), to start the reaction, or to make it go. This energy is called the activation energy of a reaction.

In order for new compounds to be formed, the bonds of the reactants must first be broken. An activation energy must be introduced into the system. This helps in the formation of new bonds which are more energetically favourable for the atoms and molecules involved in the reaction. If a reaction evolves more energy than was necessary to begin it (activation energy), this reaction proceeds on its own, resulting in the release of some energy to the surroundings.

This is an exothermic reaction. If, however, the energy released in forming new compounds is less than its activation energy, energy must be constantly added as the reaction proceeds. This type of reaction does not proceed on its own. It is an endothermic reaction.

The energy released can be in the form of heat, but it can be light or electricity, too. The variety of energetic phenomena released by chemical reactions is called heat of reaction.

Every chemical reaction goes at its own pace (reaction rate). Influencing this rate is very important in chemistry. The concentration of individual reactants and products can be determined, as can changes in heat and temperature. In gaseous state of matter reactions, reaction rate can be influenced by pressure, with higher pressures resulting in more rapid reactions. Reaction rate increases as the concentration of reactants increases, too. Greater temperature also causes reaction rate to rise. A rise of 10 Kelvin (= 10° C) causes reaction rate to double.

Reaction rate is also markedly influenced by the size of the surface on which reactants are allowed to react. In other words, if reactants are divided into smaller particles, a reaction proceeds more quickly than if reactants are left in bulk.

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Formation of ions

In many compounds, atoms form what is called an ionic bond. In this type of bonding, electrons in one atom’s outer shell are transferred from that atom to another, which accepts them. This is a complete transfer. The atom which accepts the electron or electrons completely fills its outer shell, thus attaining a noble gas electron configuration. The donor atom, the one which gives up its electrons, also attains a noble gas electron configuration (at a lower energy level) by emptying its most outer shell.

The transfer of negatively charged electrons leads to an excess of positively charged protons in the donor atom, thus forming an ion which is overall charged positively (cation). The second atom, the one which accepts the electron or electrons, becomes a negatively charged ion (anion). An ionic bond is based on the electrostatic attraction of two ions of opposite charges.

Salts make up a great percentage of the compounds which form ionic bonds. They are composed of atoms or molecules with a positive charge (cations) and the second half of an acid, which is a negatively charged anion. The reaction mechanism begins when an atom (or atoms) of hydrogen escape the acid, forming a positive ion. This positively charged hydrogen atom is replaced with another cation (or cations).

For example: HCl (hydrochloric acid) + NaOH (sodium hydroxide) = NaCl (table salt)+ H2O (water)

The valence of a salt is given by the number of hydrogen ions which are able to be transferred in a given reaction.

In the above reaction, just one hydrogen ion is replaced by one sodium ion, forming sodium chloride (table salt, NaCl). For this reason, table salt has one valence. Salts are soluble (able to dissolve) in water, and they have high melting and boiling points. Salts, when they are found in the solid state of matter, are crystalline in form.

Ionic compounds are usually spatially repeating molecules. In other words, they form crystals. Crystals can grow out of, or crystallise from, a saturated solution (from a solution which has exceeded its maximum solubility, where there is more salt than can be dissolved). Or, crystals can be grown from the transformation of an amorphic material (from a material without a regular crystalline structure).

What is the difference between a crystal and an amorphous material? Amorphous materials are not repeating, fixed, regular structures. On the other hand, crystalline structures have completely determined inner arrangements – their crystal lattice.

Every crystal has specific angles which together form the sides of that crystal. These repeat in a formation, with proportions which are highly specific.

Other types of bonds can be integrated into a crystal lattice, as its constituent parts. Crystals can be of various shapes and sizes. These varying crystalline structures, with their different forms and sizes, are what differentiates atoms, molecules and ions. It all depends on the exact geometric arrangement of a crystal, with its defined borders and in some cases sharp angles. The ideal crystal lattice is a thing of beauty, in which all of the points of the lattice are perfectly arranged in their natural places. In reality, however, such perfect crystals are quite rare. Most of the time, crystals which occur in nature are imperfect. Some points on the crystal lattice contain components which do not belong. Sometimes, the lattice is quite flawed.

The growth of a crystal or crystals is dependent on external factors, such as temperature, the natural speed of crystal growth, solution concentration, the amount of crystallising material and the presence, if any, of foreign material in the solution.

Crystals can be described with the help of two terms:

proportion of crystal and type of crystal

Agglomerates which appear from various materials can combine to form a complex, varied, imperfect crystalline structure.

Crystals can also be differentiated according to their crystal lattice. According to this criterion, there are simple crystals, in which individual points of the crystal lattice are occupied by parts of the same kind. The growth of a crystal can be imagined as a kind of regular swelling, on all sides, at its walls and edges. Besides those, there are complex crystals which are composed of multiple simple crystals.

Crystals can be investigated by structural analysis procedures. There are 7 basic types of crystal lattices and 7 other derivatives of these. All together, around 1000 crystalline structures are presently known.

Polymorphic crystals can appear in various forms. Materials which are formed from crystals can actually change their crystal lattice depending on temperature. Graphite (a component of pencil leads) and diamond are both modifications of the crystalline structure of the carbon atom ( C ). The differing characteristics come from differing attractions and forces between the various atoms.

An allotrope (allos from the Greek – different, trope – change) is a compound which is able to take on various forms.

Monotropes are those crystals that can be arranged in various ways, but only one of these is stable. The other forms, when they are present, tend to transform into this most stable form. Since temperature differences are not relevant to this situation, these transformations may not be considered as temperature based. While allotropic materials can be found in a variety of forms, monotropes, on the other hand, will sooner or later transform to one, most stable form.

Enantiotropes are those crystals which have the ability to change their crystal lattices as a function of temperature. As temperature rises or falls, these crystals change their crystalline arrangements. One lattice exists above a certain temperature, with another in place below that critical temperature. Most of the time, these critical temperatures are very high. Of interest are a number of forms of iron which are assumed during production.

Isomorphs are those substances which share the same crystalline structure, although they are completely different compounds.

One of the simplest crystalline structures is the one which characterises table salt (NaCl). Its structure is that of a cube which has at its corners ions of chlorine. Sodium ions are at the centres of the sides and in the centre of the cube.

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Electron pairs, covalent bonds

Bonds between atoms or in some cases molecules can be different. Paired, covalent bonds are found in non-metallic molecules. The atoms in the molecules of basic gases such as oxygen, nitrogen and hydrogen are all joined together with covalent bonds. These types of bonds have atoms connected with the help of the electrons in the outermost shell. The result is the union of two electrons to form an electron pair. Negatively charged, bonded electrons are attracted to the positively charged nuclei of both atoms. Because both of the nuclei must now share the electrons, they stick together, joined by the union of their electrons, an electron pair.

Each of the two atoms, then, seemingly has one or more electron extra. The bond between the atoms is based on the attraction of the two nuclei of the atoms to the shared electron pair. The shared electrons belong to both atoms at the same time. All atoms, in whatever state they are found, have the tendency to want to fill their outer electron shells. In the hydrogen molecule (H2), each hydrogen atom has two electrons associated with it, in its one and only outermost shell. (An isolated hydrogen atom has only one electron.) When, however, two hydrogens are bonded together, they achieve the electron configuration of the second element, helium (He).

Covalent bonds are very stable, because the atoms involved in a covalently bonded compound fill their outermost shells completely, bringing the atoms to their most energetically desirable state. This type of electron arrangement is equivalent to that of a noble gas, because all of the noble gases have a stable electron configuration (filled outermost electron shell). Also, molecules of chlorine, oxygen and nitrogen can reach the stable electron configuration in their outermost shell – by bonding with another atom of their own kind. That is, two chlorines bonded together, two oxygens, two nitrogens.

In order to reach the noble gas electron configuration, it is often necessary to fill various spaces in the outermost electron shell. In this case, multiple electron pairs are needed to fill these "holes". In the oxygen molecule, two electron pairs are needed, with the nitrogen molecule three. This is necessary because all atoms taking part in these types of bonding reactions need either 2 electrons in their outermost shell (elements in the first energy level, or period, of the periodic table: H and He) or 8 (other groups of the periodic table which are at the right end). These atoms which have incomplete outermost electron shells must attract other electrons, from other atoms, to fill their shells completely. An atom like oxygen can join with two atoms, forming an electron pair with each of them, or it may join with one other atom to form two electron pairs with the one atom, called a double bond. There are also triple bonds. Carbon (C) is capable of forming single, double and triple bonds.

In a covalent bond, a shared electron pair in a molecule is attracted to both nuclei on both sides equally strongly, but only if the two atoms sharing that pair are the same. Attractive force depends on the charge of the atomic nucleus and on the amount of electrons in the atom’s electron cloud. The ability to attract electrons by an element was called electronegativity (EN) by L. Pauling (American chemist).

The quantity electronegativity is defined as the comparative ability of an atom to be attracted to an individual atomic nucleus. In other words, the flourine atom attracts bonded electrons most strongly of all atoms. It was therefore assigned the highest electronegativity of all elements – 4.0. Electronegativity values of all the elements can be found in the periodic table. In every period, every horizontal row of the periodic table, electronegativity rises from left to right across the period, with rising atomic number. On the other hand, in the main groups, as we move down the periodic table from top to bottom, or vertically, electronegativity decreases. So, the element with the largest value of electronegativity must logically be found in the top right of the period table. Besides the noble gases, which have their outermost electron shells filled, and do not need electrons, the element which attracts electrons most readily is flourine (F), with a value of 4.0. At the other end of the periodic table, bottom left, are elements with the lowest electronegativity (Fr 0.7).

In compounds composed of two different atoms, an electron pair is not shared equally among the two. Instead, it is attracted to the two sides with different attractive force, based on the atoms’ differing electronegativities. In the molecule hydrogen chloride (HCl), the hydrogen atom and the chlorine atom share one electron pair. But because of the greater size of the chlorine nucleus, this electron pair is more strongly attracted by the chlorine nucleus than by the hydrogen nucleus. In addition, the chlorine atom has another 6 electrons in its outermost shell. These are arranged into three electron pairs – all unbonded. For this reason, the chlorine atom has an overall negative charge to it, if only a partially negative charge. The hydrogen atom, on the other side, has the same value of partial positive charge. The molecule HCl, or hydrogen chloride, with its partial positive side (hydrogen) and its partial negative side (chlorine) is said to have a dipole, or dipole moment. This means that the one pair of shared electrons is not shared equally. In this case, the pair is closer to the chlorine atom. It is partially negatively charged because it now has more electrons than it has protons in its nucleus. Hydrogen, on the other side, has less electrons than it has protons, and is therefore positive. Bonded electrons are written as a dash, a short line between two element symbols, or between molecular chemical formulas. This type of designation is called a valence formula.

The electronegativity of an element is determined by the amount of protons it has in its nucleus, as well as the number of electrons it contains in its outermost shell. Thanks to the partial transfer of a bonded electron pair to the more electronegative atom in a molecule, that molecule can have a positive and negative side. These sides are called poles, and if they differ in a significant way, the molecule is said to have a dipole. The result is a molecule with one side positive, one side negative. This can, of course, affect neighbouring molecules, attracting or repelling them if they are partially charged. The water molecule has a partial negative charge, found on the oxygen atom. The two hydrogen atoms have a partial positive charge.

Both free electron pairs in the oxygen atom attract the centre of a partially positively charged neighbouring molecule with their electromagnetic attractive force. This type of bonding is called hydrogen bonding. Each molecule of water hydrogen bonds with other water molecules, aligning so as to produce a positive, negative repeating pattern. The positive side is hydrogen, the negative oxygen. This phenomenon, hydrogen bonding in water, explains water’s high surface tension. This means that the molecules on the surface are weakly bonded to the rest of the liquid, by these hydrogen bonds. For that reason, water, even at relatively high temperatures, is still a liquid, whereas other similar molecules have already changed to the gaseous state.

Bonds between atoms can be depicted in various ways:

H : H formula with points, or dots, indicating electrons

H – H or with hydrogen chloride H Cl valence formula

H2 HCl chemical formula of the molecule

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Acids, Bases, Salts

Intermolecular forces

Most inorganic compounds are categorised as either acids, bases or salts. S. Arrhenius (Swedish physical chemist) came up with one of the most often used definitions for an acid.

According to that definition, acids are materials which when dissolved in water release hydrogen cations (atoms of hydrogen with a positive charge). Bases, on the other hand, are materials which release hydroxide anions (negatively charged compounds of one atom oxygen, one atom hydrogen) into solution when dissolved.

Salts are made of atoms or molecules, with one side positively charged, the other negatively charged.

They are formed from an acid when that acid gives up its hydrogen atoms with their positive charges, only to replace the hydrogen with the ion from a metal.

A number of acids and bases were known long before their chemical makeups and reaction mechanisms were known. As pure substances they are not distinguishable from each other. So, acids have to be dissolved in water in order for chemists to determine their characteristic properties. Acids begin to react when placed in water. In an aqueous solution the ions of an acid separate from each other, into a hydrogen cation and the corresponding anion. Both of these ions, free in the water, interact with it. In essence, water molecules surround the ions, creating what is called hydrated ions. So, a hydrogen ion does not remain isolated, but undergoes a hydration reaction to produce a positively charged "water" molecule, in the reaction H2 O + H+ = H3O+. These ions cause a solution to be acidic in character, and cause the colour of an indicator to change, indicating an excess of H3O+

ions in solution. (An indicator is a substance which can differentiate whether an acid or base is present in a solution.) In addition, ions in solution cause a solution to conduct electricity, or be conductive.

When a base is dissolved in water, positive ions are released into solution, and so are negatively charged hydroxide ions. A solution which contains hydroxide ions is a basic solution, or an alkaline solution. Just like with acids, the ions released into solution are hydrated, or surrounded by water. These solutions also conduct an electric current. Basic solutions also affect the colour of an indicator, and can produce basic salts when reacted with acids. Bases are basically lattices of ions. Their solids can also conduct an electric current.

According to the Brönsted-Lowry theory of acids and bases, any compound which releases a proton, or a hydrogen atom, into solution is an acid. Any compound which accepts a proton is considered a base. Solutions which contain dissolved bases and acids, because they release protons or hydroxide ions, conduct electricity.

The chemical process in which an electrical current runs through a solution is called an electrolysis. Bonds are broken in the process due to the electrolysis, with new substances being formed on the ends of the conductors, or electrodes.

Electrolysis reactions require the kinds of solutions which contain dissociated ions, allowing the solution to carry an electrical current.

During the electrolysis of an ionic solution, negatively charged ions (anions) migrate to the positively charged electrode (anode), while positively charged cations migrate to the negatively charged electrode, the cathode. In the case of an acidic or basic solution, positive ions migrate to the cathode (the end of the electrode with a negative pole), whereas the negative hydroxide ions swim to the anode (electrode with a positive pole). In these types of solutions (called electrolytic), there is no movement of electrons as in a crystal lattice, but rather movement of free swimming ions to the corresponding electrode. The number of ions is the determining factor as to whether, and how well, a solution conducts electricity.

The volume of hydrogen ions in a solution is measured as the value of the pH of a solution. The value of pH is the negative base ten logarithm giving the concentration of protons (hydrogen (H), measured from 0 to 14. A pH of O means that the concentration of hydrogen = 1, while a value of 14 means a concentration of 0.00000000000001. Solutions with a pH from 0-7 are acidic.

The acidic character of a solution decreases with rising pH. At a pH of 7, a solution is neutral. As pH rises from 7, so does the alkalinity of a solution. At a pH of 7, there are the same amount of hydrogen ions as hydroxide ions.

Indicators are used in order to determine the acidic or basic character of a reaction. These substances have to have the property of changing their colour in the presence of an acidic or basic solution. For example, litmus paper changes its colour to blue in a basic solution. In a neutral solution, it is pink. In a basic solution, it is red. Colour changes differ from one indicator to another, but are characteristic for one specific indicator. With the right choice of an indicator, pH can be fairly accurately determined.

The degree with which an acid releases hydrogen ions into solution depends on the concentration of an acid. The stronger an acid, the more protons it releases into solution, and the more negative ions as well. Two well-known strong acids are sulfuric acid and hydrochloric acid (HCl). Weak acids, on the other hand, do not release as many ions into solution. In other words, they do not dissociate as completely. Examples of weak acids include citric acid and acetic acid.

If we mix an acidic solution with an equally strong basic solution in the same proportions, the resulting solution will be neutral. This is called a neutralisation reaction. In a neutralisation reaction, hydrogen ions are neutralised by hydroxide ions - forming water – and a salt. Heat is also released during neutralisation reactions.

Many chemical reactions that seem not to be working or go at an extremely slow pace can be accelerated by addition of a small amount of some material. The material, called a catalyst, is added to the reactants. A reaction which requires a catalyst is said to be catalysed.

Catalysts take part in a reaction, but they are not used up by the reaction and are unchanged by the reaction. In the type of reaction which requires a catalyst, the reactants would react either too slowly or not at all. In other words, a catalyst gives the system a boost, an increase in activisation energy. The presence of a catalyst in a chemical reaction makes the reaction easier, or in some cases, possible at all: A catalyst takes part in a reaction by reacting with one of the original reactants to form a an intermediate product, which goes on to produce the required end product. One possibility is that one of the reactants, with the help of interaction with a catalyst, acquires new spatial dimensions or other characteristics which make it more reactive with another of the reactants. We differentiate between homogeneous catalysts, which are the same state of matter as the other reactants, and heterogeneous catalysts, where the catalyst is in a different state of matter.

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