L I D E. Chapter 4 Molecules, Compounds, and Chemical Reactions

Transcription

1 Chemistry in Focus 3rd edition Tro Chapter 4 Molecules, Compounds, and Chemical Reactions Molecules cause the behavior of matter. n all of science history, an exception to this rule has never been found. 3 Chemical Compounds and Chemical Formulas H O hydrogen atoms bonded to oxygen atom (though this image is likely a mixture of water with other substances)

2 4 Chemical Formulas ndicates the elements present in a compound and the relative number of atoms of each f we change the subscripts in a chemical formula, we no longer represent the same compound. H O is water. H O is hydrogen peroxide. 5 6 onic and Molecular Compounds lements with unstable electron configurations form compounds with other elements to gain stability. This can be accomplished by the transfer of electrons (ionic compounds) or by the sharing of electrons (molecular compounds).

3 7 onic Compounds Contain a metal and one or more nonmetals Metals and nonmetals are a good chemical match. Metals lose electrons. Nonmetals gain electrons. The result is an ionic bond. ubscripts represent the ratio of elements in the crystal lattice of the compound. 8 odium chloride onic compounds dissociate in water to form electrolyte solutions. uch solutions conduct electricity. 9 Molecular Compounds Contain only nonmetals lectrons in a bod are shared... but not always equally. The resulting bond is covalent. ubscripts represent the actual number(s) of each kind of atom in the molecule. Molecules can be VRY large and VRY complex protein molecules.

4 0 The bulk properties of molecular compounds depend on the molecules that compose them. The molecule is responsible for what we observe and experience. atoms, shape, structure, and bonds mall changes in a molecule can dramatically change the properties of the substance. Naming Compounds onic Name begins with the cation (metal) Followed by the base name of the anion (nonmetal) + ide NaBr is sodium bromide. MgBr is magnesium bromide. Names for ionic compounds O NOT contain prefixes to indicate the number of each type of atom.

5 3 4 5 Naming Compounds Molecular The more metallic element is written first and followed by the less metallic element. Prefixes are used to show the number of atoms of that element which are present. The mono-prefix is not used on the first element.

6 6 7 8 Formula Mass Formula mass of a compound is analogous to the atomic mass of an element. t is computed by summing the atomic masses of all the atoms in its formula. Molar Mass The formula mass of a compound in amu is numerically equivalent to its molar mass in grams per mole (g/mol). The molar mass of an element is the conversion factor between grams and moles of the element. t follows, then, that the molar mass of a compound serves as the conversion factor between grams and moles of that compound.

7 9 0 Chemical Formulas as Conversion Factors cientists, manufacturers, farmers, and many others may want to know how much of a particular element is in a certain substance. How much iron is present in a shipment of iron oxide? A chemical formula gives us equivalences between the elements in a particular compound and the compound itself. quivalence quivalences allow us to determine the amounts of constituent elements in a compound.

8 3 4 Chemical Reactions Compounds exist as a result of chemical reactions. xisting compounds can be transformed by further chemical reactions. These chemical reactions are represented by chemical equations. CH 4 + O CO + H O Reactants, the substances present prior to the reaction, are expressed to the left of the equation. Products, the substances formed by the reaction, are expressed on the right side of the equation. Chemical equations must be balanced; matter is conserved.

9 5 6 7 Balancing an quation Add coefficients to the reactants, and products make the number of atoms of each type of element on both sides of the equation equal. This changes the number of atoms and molecules involved in the reaction, but not the types of atoms and molecules. Balancing Guidelines f an element occurs in only one compound, balance that element first. f an element occurs as a free element, balance that element last. Change only coefficients, never subscripts. liminate fractions; use the lowest whole number ratio of coefficients.

10 Reaction toichiometry: Chemical quations as Conversion Factors As was the case with the subscripts within a chemical formula, the coefficients of a chemical equation can be used as conversion factors in chemical calculations, mathematical expressions that can answer the question, How much? Pizza and Molecules A balanced chemical equation is like a detailed recipe. Conversion factors are derived from the equivalences given in the recipe or balanced chemical equation.

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