4.1 Writing and Balancing Chemical Equations

4.1 Writing and Balancing Chemical Equations

  • By the end of this section, you will be able to deduce chemical equations from narrative descriptions of chemical reactions.
  • Chemical equations can be written in a number of formats.
  • The use of symbols to represent individual atoms was introduced in the preceding chapter.
    • Chemical formulas that appropriately represent these species are created when atoms gain or lose electrons, or combine with other atoms to form a molecule.
    • One methane molecule, two diatomic oxygen molecule, and two water molecule are combined to produce one carbon dioxide molecule, one water molecule, and one methane molecule.
  • A chemical equation may be used to represent the reaction between methane and oxygen to yield carbon dioxide and water.
  • A coefficient of 1 is not usually omitted.
  • This example shows how common it is to use the smallest coefficients in a chemical equation.
    • Carbon dioxide and water are produced in a 1:2:1:2 ratio by methane and oxygen.
    • This ratio is satisfied if the numbers of these molecules are, respectively, 1-2-1-2, or 2-4-2-4, or 3-6-3-6, and so on.
  • The ratios between the number of reactants and the number of molecule of each species that form the products are the same and are given by the chemical reaction equation.
  • This is a requirement for the equation to be in line with the law.
    • It is possible to confirm it by summing the numbers of atoms on either side of the arrow and comparing them to each other.
    • The number of atoms for a given element is calculated using the coefficients of any formula containing that element by the element's subscript in the formula.
    • If an element appears in more than one formula on the same side of the equation, the number of atoms must be added together.
  • A balanced chemical equation can be derived from a description of a chemical reaction in Chapter 4.
    • Consider how the decomposition of water can yield hydrogen and oxygen.
  • The numbers of H atoms on the reactant and product sides of the equation are the same as the numbers of O atoms.
    • Changing the reactant formula from H2O to H2O2 will yield balance in the number of atoms, but it will also change the reactant's identity.
    • The O atom balance can be achieved by changing the coefficient for H2O to 2.
  • The H atom balance was upset by this change, but it is easy to reestablish by changing the H2 product to 2.
  • A balanced equation is needed for the reaction of nitrogen and oxygen.
  • Write the unbalanced equation first.

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  • Changes in coefficients are needed to balance nitrogen and oxygen atoms.
    • To balance the number of oxygen atoms, a reasonable first attempt would be to change the coefficients for the O2 and N2O5 to integers that will yield 10 O atoms.
  • The equation is balanced because the numbers of N and O atoms on either side are equal.
  • A balanced equation for the decomposition of ammonium nitrate is needed.
  • When balancing a chemical equation, fractions can be used as intermediate coefficients.
    • When balance is achieved, the equation's coefficients can be used to convert the fractional coefficients to the integers without upsetting the atom balance.
  • The coefficients are not the smallest possible to represent the relative numbers of reactant and product molecule.
  • This is an additional practice balancing equation.
  • A parenthetical abbreviation is used to indicate the physical states of reactants and products in chemical equations.
  • This equation shows the reaction that takes place when a metal is placed in water.
    • The solid sodium reacts with liquid water to produce hydrogen gas and a ionic compound.
  • The equation's arrow can be used to designate special conditions needed for a reaction.
    • The Greek letter Delta (D) may be used to indicate a reaction carried out by heating.
  • A lot of chemical reactions take place in aqueous media because of the abundance of water.
    • The chemical equations may be written with different levels of detail in mind.
    • Consider a reaction between ionic compounds in a solution.

  • The ionic compounds used as sources of Cl- and Ag+ are provided by these equations.
  • When carbon dioxide is dissolved in a solution of sodium hydroxide, it reacts to produce liquid water and carbonate.
    • Write balanced, complete, and net ionic equations.