Extending this symbolism to represent both the identities and the relative quantities of substances undergoing a chemical (or physical) change involves writing and balancing a chemical equation. When atoms gain or lose electrons to yield ions, or combine with other atoms to form molecules, their symbols are modified or combined to generate chemical formulas that appropriately represent these species. Write and balance chemical equations in molecular, total ionic, and net ionic formats.Īn earlier chapter of this text introduced the use of element symbols to represent individual atoms.Derive chemical equations from narrative descriptions of chemical reactions.If an element appears in more than one formula on a given side of the equation, the number of atoms represented in each must be computed and then added together.By the end of this section, you will be able to: Note that the number of atoms for a given element is calculated by multiplying the coefficient of any formula containing that element by the element’s subscript in the formula. It may be confirmed by simply summing the numbers of atoms on either side of the arrow and comparing these sums to ensure they are equal. This is a requirement the equation must satisfy to be consistent with the law of conservation of matter. Balancing EquationsĪ balanced chemical is equation has equal numbers of atoms for each element involved in the reaction are represented on the reactant and product sides. Regardless of the absolute number of molecules involved, the ratios between numbers of molecules are the same as that given in the chemical equation. One mole of methane molecules and 2 moles of oxygen molecules react to yield 1 mole of carbon dioxide molecules and 2 moles of water molecules.įigure 2. One dozen methane molecules and two dozen oxygen molecules react to yield one dozen carbon dioxide molecules and two dozen water molecules.One methane molecule and two oxygen molecules react to yield one carbon dioxide molecule and two water molecules.Likewise, these coefficients may be interpreted with regard to any amount (number) unit, and so this equation may be correctly read in many ways, including: 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 (Figure 2). Methane and oxygen react to yield carbon dioxide and water in a 1:2:1:2 ratio. Realize, however, that these coefficients represent the relative numbers of reactants and products, and, therefore, they may be correctly interpreted as ratios. It is common practice to use the smallest possible whole-number coefficients in a chemical equation, as is done in this example. The relative numbers of reactant and product species are represented by coefficients (numbers placed immediately to the left of each formula).Plus signs (+) separate individual reactant and product formulas, and an arrow (\rightarrow) separates the reactant and product (left and right) sides of the equation.The substances generated by the reaction are called products, and their formulas are placed on the right sight of the equation.The substances undergoing reaction are called reactants, and their formulas are placed on the left side of the equation.This example illustrates the fundamental aspects of any chemical equation: The reaction between methane and oxygen to yield carbon dioxide in water (shown at bottom) may be represented by a chemical equation using formulas (top). The chemical equation representing this process is provided in the upper half of Figure 1, with space-filling molecular models shown in the lower half of the figure.įigure 1. Consider as an example the reaction between one methane molecule (CH 4) and two diatomic oxygen molecules (O 2) to produce one carbon dioxide molecule (CO 2) and two water molecules (H 2O). The preceding chapter introduced the use of element symbols to represent individual atoms. Write and balance chemical equations in molecular, total ionic, and net ionic formats.By the end of this section, you will be able to:
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