Writing Chemical Formulae

6 Key excerpts on "Writing Chemical Formulae"

• 

  eBook - ePub

  General Chemistry for Engineers

  • Jeffrey Gaffney, Nancy Marley(Authors)
  • 2017(Publication Date)
  • Elsevier

    (Publisher)

  chemical equation is the symbolic representation of a chemical reaction using symbols of the elements and chemical formulas. These equations are written according to the following format:

  1.  

  The reactants , the substances that are present before the chemical change takes place, are written on the left side of the equation and the products , the species formed from the chemical reaction, are written on the right side of the equation. The reactants and products are separated by an arrow which indicates the direction of the reaction. If there are multiple products and reactants, a plus sign (+) is placed between them.

  A + B → C + D

  2.  

  Small whole numbers are placed in front of the symbols and formulas to show the number of units (atoms, molecules, ions, moles) that react or are produced. When no number is shown, one unit is implied.

  2A + 3B → 2C + D

  3.  

  The physical state of each substance can be indicated by the following symbols: s = solid state, l = liquid state, g = gaseous state, aq = aqueous solution. The symbols are written in parentheses following the symbol or formula of the substance.

  A s + B l → C g + D s

  Chemical reactions involve change. Elements are combined into compounds, compounds are transformed into new compounds or are decomposed back into elements. During chemical reactions, bonds are broken and new bonds are formed. However, the number of atoms of each element must be present before and after any chemical reaction. The principle that atoms are neither created nor destroyed in a chemical reaction is known as the conservation of mass . The conservation of mass is represented in a chemical equation by a process called “balancing the equation.” This is done by adjusting the coefficients so that the number of atoms of each element on the right side of the equation equals the number of atoms of the same element on the left side of the equation. For example, consider the chemical equation for the combustion of methane (CH4 ). Writing the reaction as shown in the previous Case Study where a  = 1 and b

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  eBook - ePub

  Chemistry

  Concepts and Problems, A Self-Teaching Guide

  • Richard Post, Chad Snyder, Clifford C. Houk(Authors)
  • 2020(Publication Date)
  • Jossey-Bass

    (Publisher)

  6 Chemical Equations

  Now that you are familiar with atoms, symbols, molecules, formulas, and nomenclature, let's look at what happens when we mix substances together. The most important result of your efforts with this chapter will be your ability to write a balanced chemical equation that represents the reaction between two or more different substances that produces at least one new substance.

  Chemical equations are the chemist's shorthand. They show at a glance what substances have been mixed together and what new substance(s) have been produced. Chemists are able to predict the products of a mixture of substances even though they may never have actually mixed the substances in the laboratory. This is very important to research chemists trying to prepare new products that are useful and beneficial to mankind.

  You will learn how to complete and balance several kinds of chemical equations and how chemists recognize whether or not a reaction does indeed occur when substances are mixed.

  You will discover that some things remain unchanged during a chemical reaction. Chemists are more concerned with the things that change, so you will learn a scheme that shows what does change during certain chemical reactions.

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  eBook - ePub

  The Science For Conservators Series

  Volume 1: An Introduction to Materials

  • The Conservation Unit Museums and Galleries Commission(Author)
  • 2008(Publication Date)
  • Routledge

    (Publisher)

  Owing to the practical difficulties of depicting molecules accurately, more symbolic ways of representing them are used by scientists. These are much more useful than the ones we have just seen for discussing chemical reactions. Consequently they are the ones most frequently found in chemistry books and in conservation texts.

  B1 Molecular formulae

  molecular formula

  The first of these more symbolic models is the molecular formula. It tells concisely how many atoms of which elements are contained in each molecule of a compound.

  subscript

  The molecular formula for methane is CH4 . Comparing this combination of two letters and a number with the pictures in figures 3.2 and 3.3 shows you immediately that it describes the molecule as having one carbon atom and four hydrogen atoms. The elements present are identified by their symbols (see page 33 in Chapter 2 ), and there is another convention to show how many of each sort of atom is present. Although the information might have been written CHHHH, the convention is to represent any number of atoms greater than one by a little numeral at the bottom right-hand corner of the element’s symbol (a subscript). Thus in CH4 the letter C stands for one atom of carbon and H4 for four atoms of hydrogen. (The symbols must always be written carefully; Co is the symbol for cobalt but CO is the molecular formula for carbon monoxide.)

  Exercises

  In order to help you become familiar with writing molecular formulae it may be useful for you to do the following: 1  Write in the appropriate numbers in the blank spaces in the following examples;

  a  the molecular formula for carbon dioxide is CO2 . Each molecule contains _____ atom(s) of carbon and _____ atom(s) of oxygen.

  b  the molecular formula for sulphuric acid is H2 SO4 . Each molecule contains _____ atom(s) of hydrogen, _____ atom(s) of sulphur and _____ atom(s) of oxygen.

  c  the molecular formula of acetone is C3 H6

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  eBook - ePub

  Barron's Science 360: A Complete Study Guide to Chemistry with Online Practice

  • Mark Kernion, Joseph A. Mascetta(Authors)
  • 2021(Publication Date)
  • Barrons Educational Services

    (Publisher)

  PART III

  USING ATOMS AND MOLECULES

  Passage contains an image

  4 CHEMICAL FORMULAS

  WHAT YOU WILL LEARN

  Upon completing this chapter, you will be able to: •Articulate and apply the Laws of Definite Composition, Multiple Proportions, and Conservation of Mass •Write chemical formulas for common chemical substances from their names •Write the names of common chemical substances from their formulas •Write the names of common acids from their formulas and vice versa •Write a simple balanced reaction equation using chemical formulas including phase information

  Laws of Definite Composition and Multiple Proportions

  Near the end of the eighteenth century, the French chemist Joseph Proust argued that the chemical elements combined only in particular ways by mass to produce chemical compounds. As chemical knowledge grew, this concept became generally accepted and is known today as the Law of Definite Composition (sometimes referred to as the Law of Definite Proportions). This idea was one of the foundations of the atomic theory. In the early nineteenth century, chemist John Dalton theorized that all matter is comprised of atoms (small, discrete bits of matter). This is the atomic theory.

  The Law of Definite Composition, with its specificity in describing the particular masses of the elements that combine to form compounds, naturally hints at the particulate nature of matter. John Dalton himself developed the Law of Multiple Proportions, embellishing the idea that matter is specific, discrete, and particular. Dalton noted that when two elements combined to form two different compounds, they did so in such a way that the ratio of the masses of the second element, when combined with equal amounts of the first, would always turn out to be small whole numbers. Both of these laws then pointed to the fixedness or discreteness (or the atomic nature) of matter.

  Examples of both laws can be seen in the familiar compounds carbon monoxide and carbon dioxide. The masses of carbon and oxygen required to make 100.0 grams of both compounds are shown:

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  eBook - ePub

  Understanding Medicinal Plants

  Their Chemistry and Therapeutic Action

  • Bryan Hanson(Author)
  • 2013(Publication Date)
  • Routledge

    (Publisher)

  Figure 3.1.

  The most useful information scientists have learned about the elements is contained in the periodic table of elements. The periodic table provides a means of organizing a large amount of information gathered over more than 100 years. It provides the scientist with both a way to remember things and a means of drawing analogies. It also reveals important information about how the elements combine with one another, which enables the periodic table to tell us about compounds as well as the individual elements.

  FIGURE 3.1. Definitions related to composition and objects.

  Let’s take a brief tour around the periodic table (see Table 3.1 ) before learning how to apply it to understand bonding in molecules. In each box in the periodic table is the symbol used to represent that atom. (I have also provided a list of the symbols with the full name and atomic number of each element in Table 3.2 ) Usually the symbol is chosen in such a way as to help us remember the name of the element, although for some elements the symbol’s origin lies in its linguistic history, not its current name. Also in the box is the atomic number. The atomic number is equal to the number of protons in the atom’s nucleus, which gives the atom its unique identity. This information will be useful to us later.

  Where did the periodic table come from? Chemists, their predecessors (the alchemists), and physicists have been making observations of the elements and their compounds for hundreds of years. These observations include such things as the visual appearance of the elements, their melting points, their atomic masses, and their densities. They also include chemical reactivity, such as acidic or basic character, the ability to react with water, or the formula of the compound formed between a particular element and oxygen or chlorine, for example.4

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  Eco-facts and Eco-fiction

  Understanding the Environmental Debate

  • William H. Baarschers(Author)
  • 2013(Publication Date)
  • Routledge

    (Publisher)

  atoms that are all of the same kind. A chunk of the element iron contains only iron atoms and a piece of the element carbon contains only carbon atoms.

  This "handful" of elements does not explain the tremendous variety of chemicals we recognize around us, some nine million of them. There is a lot more to it. The next step is the notion that atoms can combine into larger particles, molecules. Molecules can be built up from atoms of the same kind, or they can contain many different kinds of atoms, atoms of different elements. A collection of molecules represents an amount that we can see and weigh, of substances we call chemical compounds. All the molecules of the compound water are built from one atom of oxygen and two atoms of hydrogen. All the molecules of the compound cholesterol consist of twenty-seven atoms of carbon, forty-six atoms of hydrogen and one atom of oxygen.

  We can use a simple form of shorthand to describe these molecules. We use the first, or the first and another letter of the Latin name of each element to describe it. For example, we use C for carbon, Co for cobalt, and Cd for cadmium. Then we add subscript numbers to indicate how many atoms there are of each kind. The combination leads to the idea of chemical formulae. So, in chemistry shorthand, water becomes H2 O and cholesterol is C27 H46 0. In the case of water this formula is enough to describe the molecule, for cholesterol it is not. For cholesterol we have to add a drawing of the geometry of the molecule, the arrangement of all the atoms in space. Such a drawing that represents the architecture of a molecule is called a structural formula. A properly drawn structural formula is a sufficient and complete description of that molecule.

  Berzelius (1779-1848), a Swedish chemist, divided chemicals into two large categories. One contained those chemicals that were directly derived from living things, either from plants or animals. He postulated that a "vital force" was essential for the formation of these chemicals from living organisms. Naturally, Berzelius called these chemicals "organic chemicals." All other chemicals, not derived from living organisms, were therefore

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