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Unit 6: Quantifying Chemical Reactions—Stoichiometry and Moles

Section 1: Introduction

Chemists make many detailed observations about how matter behaves. To represent this information, they use formulas that summarize chemical reactions with numbers and symbols. As Figure 6-1 shows, chemical formulas make it possible to describe complex processes very concisely.

Chemical Formula for Photosynthesis

Figure 6-1. Chemical Formula for Photosynthesis

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Chemical Formula for Photosynthesis

Figure 6-1. Chemical Formula for Photosynthesis

Photosynthesis is the process by which plants turn carbon dioxide from the atmosphere into carbohydrates (plant tissue), using light as an energy source and releasing oxygen as a byproduct. Photosynthesis is one of the most important biological processes on Earth: It makes air breathable, and supplies most of the energy required for all life forms.

In this unit, we will learn how to use chemical formulas to summarize what we know. Important types of chemical formulas include:

  • Empirical formulas, which describe the ratios in which elements combine to form compounds;
  • Molecular formulas, which show the numbers of different atoms in these compounds; and
  • Lewis structures, which symbolize the bonds between the atoms from the formulas that give compounds their unique properties.

Using the process of combustion analysis (burning a material to break it into its component parts so that we can determine its makeup), we will see how chemists use formulas to summarize laboratory experiments. To describe activities such as combustion analysis accurately, chemists use specific terms to measure mass and volume. We will learn about moles, Avogadro's rule and number, atomic mass, molecular mass, and how to find mass percentages for molecules and mixtures. Finally, we'll want to see what happens when we put different compounds together in chemical reactions. We'll look at how we balance a chemical equation, equilibrium reactions, limiting reagents, and yields.

Quantifying chemical processes accurately is critically important in the chemical industry and related fields, such as pharmacology (developing and manufacturing drugs). To produce substances that consistently have the required qualities, and to do so efficiently and cost-effectively, chemists need to know exactly how much of every reagent (ingredient) goes into a reaction, and how much of what types of substances will be produced. When scientists know exactly what processes will happen in a reaction and how much of each reagent they will need, they can avoid ending up with large quantities of expensive materials left over. They can also avoid making too much product, as renowned neurologist and author Oliver Sacks did in a boyhood chemistry experiment. (See Stinking Up the House sidebar.)

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