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1.3: Chemical Equations

  • Page ID
    6974
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    Learning Objectives

    • Describe ionic, covalent, and hydrogen bonding
    • Explain synthesis, decomposition, exchange, and combustion reactions
    • Describe the energy flow through chemical reactions

    Combination and Decomposition

    Ionic Bond, Covalent Bond, Hydrogen Bond, Molecular Weight, and Mole

    When the outermost energy level of an atom is not completely filled by electrons, the unfilled spaces or extra electrons in the outer energy level, are critical as to whether it is easier for the atom to gain or lose electrons. The most chemically stable configuration for any atom is to have its outermost shell of electrons filled. For example, an atom of sodium, Na, has one electron in its outermost energy level. This level is filled when eight electrons occupy this space. For this reason, sodium atoms find it easier to give up one electron in order to have a full outermost shell. An atom of chlorine, Cl, has seven electrons in its outermost energy level. This level is filled when eight electrons occupy this space. Chlorine finds it easier to accept an electron in order to fill the level with electrons. Sodium and chloride combine to form salt. Sodium gives up one electron to chlorine; therefore, sodium loses an electron to acquire a completed outermost energy level and chlorine accepts the electron in order to have a completed outermost shell.

    The valence, or combining capacity, of an atom is the number of extra or missing electrons in its outermost electron shell. Hydrogen has a valence of 1 (one unfilled space, or one extra electron), oxygen has a valence of 2 (two extra electrons), chlorine has a valence of 1 (one extra electron), and magnesium has a valence of 2 (two extra electrons).

    Atoms achieve the full complement of electrons in their outermost energy shells by combining to form molecules, which are made up of atoms of one or more elements. A molecule that contains at least two different kinds of atoms, such as sodium chloride (salt), is called a compound. In sodium chloride, the omission of a subscript indicates that there are one atom of sodium and one atom of chloride. In the compound, water (H2O) the subscript 2 indicates that there are two atoms of hydrogen and the absence of a subscript for oxygen indicates that there is one atom of oxygen. Molecules hold together because the valence electrons of the combining atoms form attractive forces, called chemical bonds, between the atomic nuclei. For this reason, valence may also be viewed as the bonding capacity of an element. Because energy is required for chemical bond formation, each chemical bond possesses a certain amount of potential chemical energy.

    Atoms form bonds in one of two ways. They either gain or lose electrons from their outer electron shell, or they share outer electrons. When atoms have gained or lost outer electrons, the chemical bond is called an ionic bond. When outer electrons are shared, the bond is called a covalent bond. The types of bonds that are actually found in molecules range from highly ionic to highly covalent.

    Ionic Bonds

    Atoms are electrically neutral when the number of positive charges (protons) equals the number of negative charges (electrons). When an isolated atom gains or loses electrons, the balance is upset. If the atom gains electrons, it acquires a negative charge because electrons are negatively charged. Such a negatively or positively charged atom is called an ion.

    For example, sodium (Na) has 11 protons and 11 electrons, with one electron in its outer electron shell. Sodium tends to lose the single outer electron. It is considered an electron donor. When sodium donates an electron to another atom, it is left with 11 protons and 10 electrons. Sodium is positively charged, and it is called a sodium ion. It is written as Na+. Chlorine (Cl) has a total of 17 electrons, 7 of them are in the outer electron shell. This outer shell can hold 8 electrons. For this reason, chlorine tends to pick up an electron that has been lost by another atom. It is considered to be an electron acceptor. By accepting an electron, chlorine totals 18 electrons. But it has only 17 protons in its nucleus. The chloride ion for this reason has a charge of -1, and it is written as Cl-.

    The opposite charges of the sodium ion (Na+) and chloride ion (Cl-) attract each other. The attraction, an ionic bond, holds the atoms together, and a molecule of table salt is formed. An ionic bond is an attraction between ions of opposite charge that holds them together to form a stable molecule. An ionic bond is an attraction between atoms in which one atom loses electrons and another atom gains electrons. Strong ionic bonds have limited importance in living cells. Weaker ionic bonds formed in aqueous (water) solutions are important in biochemical reactions in living organisms.

    An atom whose outer electron shell is less than half-filled will lose electrons and form positively charged ions, called cations, such as potassium (K+), calcium (Ca+2), and sodium (Na+). When an atom’s outer shell is more than half-filled, the atom will gain an electron and form negatively charged ions, called anions, such as iodide ion (I-), sulfide (S-2), chloride (Cl-).

    Covalent Bonds

    A covalent bond is a chemical bond formed by two atoms sharing one or more pairs of electrons. Covalent bonds are stronger and more common in living organisms that are true ionic bonds. In the hydrogen molecules, H2, two hydrogen atoms share a pair of electrons. Each hydrogen atom has one electron in the outer orbit. In order to have a full shell, one additional electron is needed. In the hydrogen molecule, the one hydrogen atom with one electron shares an electron with the other hydrogen atom so that a shared pair of electrons actually orbit the nuclei of each atom. The outer electron shells of both atoms are filled. Atoms that share one pair of electrons form a single covalent bond. Atoms that share two pairs of electrons form a double covalent bond, expressed as two single lines (=). A triple covalent bond is expressed as three single lines, and it occurs when atoms share three pairs of electrons.

    Methane (CH4) has covalent bonding between atoms of hydrogen and carbon. The outer electron shell of the carbon atom can hold eight electrons but has only four electrons. Each hydrogen atom can hold two electrons but has only one electron in the outer orbit. In the methane molecule, the carbon atom gains four hydrogen electrons to complete its outer shell, and each hydrogen atom completes its outer shell by sharing one electron from the carbon atom. Each outer electron of the carbon atom orbits the carbon nucleus and a hydrogen nucleus. Each hydrogen electron orbits its nucleus and the carbon nucleus.

    Elements like hydrogen and carbon, whose outer electron shells are half-filled, form covalent bonds easily. In living organisms, carbon almost always forms covalent bonds. Covalent bonds are formed by the sharing of electrons between atoms. Ionic bonds are formed by attrition between atoms that have lost or gained electrons and are positively or negatively charged.

    Hydrogen Bonds

    Another type of bond is the hydrogen bond where a hydrogen atom that is covalently bonded to one oxygen or nitrogen atom is attracted to another oxygen or nitrogen atom. These bonds are weak and do not bind atoms into molecules. They serve as bridges between different molecules or between various portions of the same molecule.

    When hydrogen combines with atoms of oxygen or nitrogen, the relatively large nucleus of the larger oxygen or nitrogen atoms has more protons and attracts the hydrogen electron more strongly than does the small hydrogen nucleus. In a molecule of water (H2O), the electrons tend to be closer to the oxygen nucleus than to the hydrogen nuclei. As a result, the oxygen portion of the molecule has a slightly negative charge, and the hydrogen portion of the molecule has a slightly positive charge. When the positively charged end of one molecule is attracted to the negatively charged end of another molecule, a hydrogen bond is formed. This attraction can also occur between hydrogen and other atoms of the same molecule, especially in large molecules. Oxygen and nitrogen are the elements most frequently involved in hydrogen bonding.

    Hydrogen bonds are weaker than ionic or covalent bonds. Hydrogen bonds are formed and broken relatively easily. This property accounts for the temporary bonding that occurs between certain atoms of large and complex molecules, like proteins and nucleic acids.

    Molecular Weight and Moles

    Bond formation results in the creation of molecules. Molecules are discussed in terms of units of measure called molecular weight and moles. The molecular weight of a molecule is the sum of the atomic weights of the atoms that make up the molecule. To relate the molecular level to the laboratory level, a unit called a mole is used.

    One mole of a substance is its molecular weight expressed in grams so that one mole of water (H2O) weighs 18 grams because the molecular weight of water is 18, or...

    • H2O: The atomic number of Hydrogen (2) is 1 and Oxygen (16) is 16; [(2 x 1, H) + 16, O] = 18

    Chemical Reactions

    Chemical energy occurs whenever chemical bonds between atoms are formed or broken during chemical reactions. A chemical reaction that absorbs more energy than it releases is called an endergonic reaction, meaning that energy is stored in the chemical bonds that are formed as potential energy. A chemical reaction than releases more energy than it absorbs is called an exergonic reaction meaning that energy is released from the chemical bonds, kinetic energy (chemical bonds are broken releasing the stored potential energy).

    Synthesis Reactions

    When two or more atoms, ions, or molecules combine to form new and larger molecules, the reaction is called a synthesis reaction. To synthesize means to put together, and a synthesis reaction forms new bonds. This process most often requires that energy be added to the system.

    • Synthesis reaction: (A, atom, ion, or molecule) + (B, atom, ion, or molecule) + energy → (AB, new molecule)

    The combining substances, A and B are called the reactants, and the substance formed by the combination, AB, is the product. The arrow indicates the direction in which the reaction proceeds.

    Pathways of synthesis reactions in living organisms are collectively called anabolic reactions, or simply anabolism. The combining of sugar molecules to form starch and of amino acids to form proteins are examples of anabolism. Anabolic reactions, in general, require an input of energy. The energy is stored in the newly formed chemical bond as potential energy (chemical energy).

    Decomposition Reactions

    The reverse of a synthesis reaction is a decomposition reaction. To decompose means to break down into smaller parts, and in a decomposition reaction, chemical bonds are broken. Decomposition reactions split large molecules into smaller molecules, ions, or atoms. A decomposition reaction occurs:

    • (AB, new molecule) → (A, atom, ion, or molecule) + (B, atom, ion, or molecule) + energy

    Decomposition reactions that occur in living organisms are collectively called catabolic reactions, or catabolism. Catabolism occurs when sucrose is broken down into simpler sugars like glucose during digestion. Bacterial decomposition of petroleum is another example of decomposition reactions. Energy is released with the chemical bonds are broken down.

    Exchange Reactions

    Chemical reactions are based on synthesis and decomposition. Many reactions, such as exchange reactions, are actually part of synthesis and part decomposition. An exchange reaction works:

    • (AB, molecule) + (CD, molecule) → (AD, molecule) + (BC, molecule)

    Combustion Reactions

    Combustion reactions are types of chemical reactions where compounds and oxidants react to produce heat and new products. A common combustion reaction is the reaction between oxygen and hydrocarbons to yield water and carbon dioxide:

    • (Oxygen) + (Hydrocarbon) → (Carbon Dioxide) + (Water) + Heat
      • For example:
        • 2H2 + O2 → 2H2O + heat
        • CH4 + 2O2 → CO2 + 2H2O + heat

    It is also common for a combustion reaction to release light and produce a flame. However, it is not necessary. For a combustion reaction to be initiated, the activation energy for the reaction must be overcome. Often combustion reactions are started with a flame, which provides the heat to initiate the reaction. Once combustion begins, the heat that is produced sustains the reaction until the reactants are used.

    In order to recognize combustion reactions, oxygen will be on the reactant side of the equation and the release of heat will be on the product side of the equation. Sometimes the fuel molecule contains oxygen. For example, the combustion of ethanol:

    • C2H5OH + 3O2 → 2CO2 + 3H2O

    Combustion is an exothermic reaction so that it releases heat. However, sometimes the reaction proceeds so slowly that a temperature change is not noticeable. The signs that a combustion reaction occurs is the presence of oxygen as a reactant and carbon dioxide, water, and heat as products. Inorganic combustion reaction may not form the same types of products; however, they are recognizable by the reaction of oxygen. The surest method of recognizing a combustion reaction is that the products contain carbon dioxide and water.

    Combustion does not always proceed to completion or 100-percent efficiency. The reactions are prone to limiting reactants. Two types of combustion reactions exist:

    • Complete Combustion—known as clean combustion where oxidation of the reactants (hydrocarbons) produces only carbon dioxide and water. The burning of candle wax, where the heat from the wick vaporizes the wax (hydrocarbon), is an example. The reaction results from the oxygen in the air so that carbon dioxide and water are the products. All of the wax burns so that nothing remains once the candle is consumed. The water vapor and carbon dioxide dissipate into the atmosphere.
    • Incomplete Combustion—incomplete combustion is the oxidation of a hydrocarbon that is incomplete or dirty. The products are carbon monoxide and carbon (soot) in addition to carbon dioxide. An example would be the combustion of coal, where soot and carbon monoxide are the products. Fossil fuels oxidize incompletely, and they release waste products.

    Energy Flow in Chemical Reactions

    Chemical bonds represent stored chemical energy, and chemical reactions ultimately result in net absorption or release of energy. Reactions that release energy are called exergonic reactions. These reactions yield products with less energy than the initial reactants, along with energy that can be harvested for use. Catabolic and oxidative reactions are exergonic for the most part.

    The products of energy-absorbing, endergonic, reactions contain potential energy in their chemical bonds which is more than the energy that the reactants contained. Anabolic reactions are energy-absorbing reactions. For example, the energy released when fuel molecules are broken down (oxidized) is captured in ATP molecules and used to synthesize complex biological molecules the body needs to sustain life.

    3.5 Factors that Influence the Rate of Chemical Reactions

    For atoms and molecules to react, they must collide with enough force to overcome the repulsion between their electrons. Interaction between valence shell electrons cannot occur long distance. The force of collisions depends on the speed of the particles. Solid, forceful collisions between rapidly moving particles in which valence shells overlap are much more likely to cause a reaction than are collisions in which the particles graze each other.

    Increasing the temperature of a substance increases the kinetic energy of its particles and the force of their collisions. Chemical reactions proceed more quickly at higher temperatures.

    Chemical reactions progress more rapidly when the reacting particles are present in high numbers, or concentrations, because the chance of collisions is greater. As the concentrations of the reactants declines, the reaction slows. Chemical equilibrium eventually occurs unless additional reactants are added or products are removed from the reaction site.

    Smaller particles move faster than large particles and tend to collide more frequently and more forcefully. The smaller the reacting particles, the faster a chemical reaction proceeds at a given temperature and concentration.

    Chemical reactions in nonliving systems can be speeded up by heating. However drastic increases in body temperature are life-threatening because important biological molecules are destroyed. At normal body temperature, most chemical reactions would proceed at too slow of a pace to maintain life, if catalysts were not present. Catalysts are substances that increase the rate of chemical reactions without becoming chemically changed or part of the product. Biological catalysts are called enzymes.

    Review Questions

    1. What is a covalent bond?
    2. What is an ionic bond?
    3. Describe hydrogen bonding?
    4. Describe synthesis, decomposition, exchange, and combustion reactions.
    5. Explain exergonic and endergonic reactions.

    Chapter Quiz

    1. The ___________ or combining capacity of an atom is the number of extra or missing electrons in its outermost electron shell.
      1. Valence
      2. Atomic number
      3. Exchange rate
      4. Nucleus
    2. ___________ is the oxidation of a hydrocarbon that is incomplete or dirty. The products are carbon monoxide and carbon (soot) in addition to carbon dioxide.
      1. Complete combustion
      2. Incomplete combustion
      3. Reduction
      4. Endergonic
    3. ___________ is where a hydrogen atom that is covalently bonded to one oxygen or nitrogen atom is attracted to another oxygen or nitrogen atom. These bonds are weak and do not bind atoms into molecules.
      1. Ionic bonding
      2. Covalent bonding
      3. Hydrogen bonding
      4. Temporary bonding
    4. __________ is an attraction between ions of opposite charge that holds them together to form a stable molecule.
      1. Ionic bonding
      2. Covalent bonding
      3. Hydrogen bonding
      4. Temporary bonding
    5. ___________ are stronger and more common in living organisms than are true ionic bonds. In the hydrogen molecules, H2, two hydrogen atoms share a pair of electrons.
      1. Ionic bonds
      2. Covalent bonds
      3. Hydrogen bonds
      4. Temporary bonds
    6. ___________ represent stored energy.
      1. Chemical bonds
      2. Hydrogen bonds
      3. Covalent bonds
      4. Ionic bonds
    7. When two or more atoms, ions, or molecules combine to form new and larger molecules, the reaction is called a ___________.
      1. Decomposition reaction
      2. Exchange reaction
      3. Combustion reaction
      4. Synthesis reaction
    8. ___________ are substances that increase the rate of chemical reactions without becoming chemically changed or part of the product.
      1. Products
      2. Reactants
      3. Catalysts
      4. Chemical bonds
    9. For a combustion reaction to be initiated, the ___________ energy for the reaction must be overcome.
      1. Kinetic
      2. Potential
      3. Activation
      4. Decomposition
    10. Reactions that release energy are called ___________ reactions.
      1. Endergonic
      2. Synthesis
      3. Exergonic
      4. Substitution

    This page titled 1.3: Chemical Equations is shared under a not declared license and was authored, remixed, and/or curated by John Rowe (ZTC Textbooks) .

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