Mixtures and Compounds in Chemistry Essay

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Thesis Statement

Understanding the process of chemical bonding is an important undertaking as it provides the basis for designing and identifying appropriate methods for separation of different compounds and mixtures.

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Introduction

For elements to combine and form useful compounds, they need to couple through bonds. These are possible because of differences in electron affinity of different elements across the periodic table. The type of bonds and the combination depends on the position of each element in the periodic table. As a result, two broad categories of compounds are obtained. These are ionic and covalent compounds.

Difference between a Mixture and a Compound

When two or more substances are mechanically combined, it results to a mixture. The constituents of the consequent mixture lack a chemical bond and for this reason, the mixture can separated using suitable physical means. Each component in a mixture retains its chemical attributes. Mixtures can either be homogeneous where the components exhibit uniformity or heterogeneous where the components are easily identifiable. If water is mixed with sugar, a homogeneous mixture is obtained whereby the components can be recovered by evaporation. If iron filings and sulphur powder are mechanically blended, a mixture is obtained. The individual components can be obtained in their pure form using a magnet (Tillery, Enger & Ross, p. 23).

A compound is a chemical entity consisting of two or more chemical elements, which are combined by a chemical process. In a compound, the elements are held together by chemical bonds, which can be ionic, covalent or dative. For example, two hydrogen atoms are covalently linked with an oxygen atom to form a water molecule. A compound exhibits unique properties, physical and chemical, which significantly differ from those of its elements. The elements in a compound are always in a fixed ratio and the arrangement is defined. Therefore, a compound can be represented by a chemical formula, which depicts its exact elemental composition. The elements of a compound can only be separated by chemical means for example, by hydrolysis to break water into hydrogen and oxygen (Ramsden 2001, p. 47).

By mere observation, it is hard to differentiate a pure element from a compound. Therefore, there is obvious necessity to conduct various tests to ascertain the true nature of a substance. Since compounds are composed of elements that are chemically combined, chemical means must be used to differentiate a compound from a pure element. The sample provided should be subjected to a chemical separation process for instance, hydrolysis where the analyst will have means of collecting the various products liberated. These products are then subjected to similar chemical and physical tests. If the outcome is similar for the samples collected, then the initial sample was an element. If the samples collected react differently with the same reactant, then the sample involved was a compound.

Ionic and a Covalent Bond

When two or more atoms of the same element or of different elements whose electronegativities are almost similar combine, they do so via covalent bonds. Electronegativity refers to the ease with which an atom can release the electron occupying the outermost energy level. If atoms are highly electronegative, then they are more likely to share their valence electron as opposed to losing it. This coupling is necessary because it ensures stability when the atoms combine to form molecules. When a covalent bond is formed, the atoms are brought together such that there is repulsion between the positively charged nuclei of the atoms making the molecule. The molecules formed have no charge but can have a partial charge due to differences in electronegativities of constituent atoms. The electron cloud usually shifts to the more electronegative atom. This results in a partial negative charge on the atom. This creates dipoles in molecules made up of atoms with different electronegativities. If the atoms are of the same element, then the net dipole charge will be zero and therefore the molecule will be neutral.

The opposing forces of attraction and repulsion result in the formation of a molecule with a defined shape and characteristics. This can be used in its identification. According to Zumdahl & Zumdahl (2007), “covalent, compounds lack localized electrons and are therefore very poor conductors of electricity” p.346. They also melt or boil at low temperatures as compared to ionic compounds and have low solubility in inorganic solvents.

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Some atoms easily lose their valence electrons in order to gain an electron configuration, which is stable. This is characteristic of atoms with low electron affinity. Ionization energy of these atoms is low and consequently they require less energy to ionize. When this happens, these atoms become positively charged ions or cations. The electrons lost are gained by atoms with high electronegativities, which become negatively charged ions or anions. Due to the difference in charge, the two sets of ions attract each other electrostatically. This is called ionic bonding (Zumdahl & Zumdahl, 2007, p.346).

Bonding Across the Periodic Table

On the left side of the periodic table, atoms gain stability by formation of ions. This is far easier than for these atoms to gain electrons. They are therefore said to have low electronegativity. When they lose their valence electrons, they gain a net positive charge because they posses more protons relative to the electrons. The charge carried by these cations depends on the number of electrons lost. For examples, metals in group 1 will carry a net charge of positive one and those in two a charge of positive two (Zumdahl & Zumdahl, 2007, p.346).

Elements on the right side of the periodic table have high electron affinity and therefore tend to gain electrons from the surrounding so as to obtain a stable configuration. In the process, they obtain a negative charge creating an anion. Electron affinity increases as one moves to the right across the periodic table.

Bonding is facilitated by the difference in charge between the two species of ions. These are brought together by electrostatic force to create a strong bond. The number of atoms coming together depends on charges borne by each ion. For example, one sodium cation will bond with one chloride ion to form sodium chloride molecule, which is represented as Na+Cl. Similarly, a magnesium ion will couple with two nitrate ions to form magnesium nitrate, Mg (NO2)2.

On the right side of the periodic table, elements exhibit reluctance to lose their valence electrons due to their high electron affinity. Therefore, the energy required to remove an electron is enormous as compared to that needed to gain an electron. Due to the need for electron pairing the atoms, there is need to share their unpaired electrons to form stable entities, molecules.

Each atom has to donate a single unpaired electron for a true covalent bond to be formed. However, in some instances one atom is required to donate both electrons. In this case, the bond is referred to as a dative covalent bond. Bonding can occur between atoms of the same element or it can occur between dissimilar atoms. In the latter case, charge distribution within the molecule is skewed with the more electronegative atom possessing a partial negative charge and the less electronegative one having a partial positive charge.

For example, two chlorine atoms combine covalently to form chlorine molecules, Cl+Cl Cl2.This is the stable independent unit of chlorine gas. Similarly, nitrogen combines with hydrogen in a ratio of 1:3 to form ammonia, NH3.

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Conclusion

It is important to understand the process of bonding in detail as this provides a forum for the design and utilization of various chemical methods in the separation of compounds and mixtures to yield pure substances. Separation of ionic and covalent compounds utilizes different methods based on the type of bonds.

References

Ramsden, E. (2001). Key science chemistry. New York, NY: Nelson Thornes.

Tillery, B., Enger, E. & Ross, F. (2007). Integrated science. 4 Edn. New York, NY: McGraw-Hill Higher Education.

Zumdahl, S. S. & Zumdahl, S. A. (2007). Chemistry: Media enhanced edition. 7Edn. New Jersey, NJ: Cengage Learning.

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