When a chemist encounters a new reaction, it does not usually come with a label that shows the balanced chemical equation. Instead, the chemist must identify the reactants and products and then write them in the form of a chemical equation that may or may not be balanced as first written. Consider, for example, the combustion of -heptane (C7H16), an important component of gasoline:
You cannot change subscripts in a chemical formula to balance a chemical equation; you can change only the coefficients. Changing subscripts changes the ratios of atoms in the molecule and the resulting chemical properties. For example, water (H2O) and hydrogen peroxide (H2O2) are chemically distinct substances. H2O2 decomposes to H2O and O2 gas when it comes in contact with the metal platinum, whereas no such reaction occurs between water and platinum.
The simplest and most generally useful method for balancing chemical equations is “inspection,” better known as trial and error. We present an efficient approach to balancing a chemical equation using this method.
The assumption that the final balanced chemical equation contains only one molecule or formula unit of the most complex substance is not always valid, but it is a good place to start. Consider, for example, a similar reaction, the combustion of isooctane (C8H18). Because the combustion of any hydrocarbon with oxygen produces carbon dioxide and water, the unbalanced chemical equation is as follows:
These are all ways of stating the information given in the balanced chemical equation, using the concepts of the mole, molar or formula mass, and Avogadro’s number. The ratio of the number of moles of one substance to the number of moles of another is called the . For example, the mole ratio of H2O to N2 in is 4:1. The total mass of reactants equals the total mass of products, as predicted by Dalton’s law of conservation of mass: 252 g of (NH4)2Cr2O7 yields 152 + 28 + 72 = 252 g of products. The chemical equation does , however, show the rate of the reaction (rapidly, slowly, or not at all) or whether energy in the form of heat or light is given off. We will consider these issues in more detail in later chapters.
In a chemical reaction, one or more substances are transformed to new substances. A chemical reaction is described by a chemical equation, an expression that gives the identities and quantities of the substances involved in a reaction. A chemical equation shows the starting compound(s)—the reactants—on the left and the final compound(s)—the products—on the right, separated by an arrow. In a , the numbers of atoms of each element and the total charge are the same on both sides of the equation. The number of atoms, molecules, or formula units of a reactant or product in a balanced chemical equation is the coefficient of that species. The mole ratio of two substances in a chemical reaction is the ratio of their coefficients in the balanced chemical equation.
In addition to providing qualitative information about the identities and physical states of the reactants and products, a balanced chemical equation provides information. Specifically, it tells the relative amounts of reactants and products consumed or produced in a reaction. The number of atoms, molecules, or formula units of a reactant or a product in a balanced chemical equation is the of that species (e.g., the 4 preceding H2O in ). When no coefficient is written in front of a species, the coefficient is assumed to be 1. As illustrated in , the coefficients allow us to interpret in any of the following ways:
What information can be obtained from a balanced chemical equation? Does a balanced chemical equation give information about the rate of a reaction?
Consistent with the law of conservation of mass, the numbers of each type of atom are the same on both sides of and . (For more information on the law of conservation of mass, see .) As illustrated in , each side has two chromium atoms, seven oxygen atoms, two nitrogen atoms, and eight hydrogen atoms. In a , both the numbers of each type of atom and the total charge are the same on both sides. and are balanced chemical equations. What is different on each side of the equation is how the atoms are arranged to make molecules or ions. . In this reaction, and in most chemical reactions, bonds are broken in the reactants (here, Cr–O and N–H bonds), and new bonds are formed to create the products (here, O–H and N≡N bonds). If the numbers of each type of atom are different on the two sides of a chemical equation, then the equation is unbalanced, and it cannot correctly describe what happens during the reaction. To proceed, .
Here is a balanced chemical equation for the process of aerobic respiration. You only need to memorise this for the Higher Tier GCSE paper, however I am sure that you really want a grade “A” so why not memorise it.
The starting material (left) is solid ammonium dichromate. A chemical reaction (right) transforms it to solid chromium(III) oxide, depicted showing a portion of its chained structure, nitrogen gas, and water vapor. (In addition, energy in the form of heat and light is released.) During the reaction, the distribution of atoms changes, but the of atoms of each element does not change. Because the numbers of each type of atom are the same in the reactants and the products, the chemical equation is balanced.
Please make sure that you have memorized the word equation for photosynthesis, and the balanced chemical equation if you are doing the higher papers. You can find these equations and an explanation of how to balance chemical equations here.
While we have long appreciated the structure of ecosystems, the importance of ecosystem functioning has lagged behind somewhat. This module aims to redress the balance by exploring the use of modern tools which allow us to thoroughly integrate measures of ecological structure and functioning. Aspects of the Metabolic Theory of Ecology, body-size relationships, stable isotope analysis and DNA bar- coding will all be covered in relation to topics such as photosynthetic and chemosynthetic primary production; the impacts of invasive species; aquatic-terrestrial linkages and cross ecosystem boundary subsidies; biogeochemistry and nutrient dynamics; plankton dynamics and organismal physiology in a changing world.