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People Behind the Science

Emma Perry Carr (1880–1972)

mma Perry Carr was a U.S. chemist, In 1913, Carr introduced a departmental
Eteacher, and researcher internation- research program to train students through
ally renowned for her work in the field of collaborative research projects combining
spectroscopy. physical and organic chemistry. She achieved
Carr was born in Holmesville, Ohio, her renown in research into spectroscopy; un-
on July 23, 1880. After attending Coshoc- der her leadership, she, her colleague Dorothy
ton High School, Ohio, she spent a year Hahn, and a team of students were among the
(1898–1899) at Ohio State University and first Americans to synthesize and analyze the
then transferred to Mount Holyoke Col- structure of complex organic molecules using
lege for a further two years’ study. She absorption spectroscopy. Her research into
continued as a chemistry assistant for unsaturated hydrocarbons and far ultraviolet
three years and in 1905 finished her B.S. vacuum spectroscopy led to grants from the
at the University of Chicago. Between 1905 National Research Council and the Rock-
and 1908, Carr worked as an instructor at efeller Foundation in the 1930s. She served
Mount Holyoke, after which she returned as a consultant on the spectra for the Inter-
to Chicago and completed her Ph.D. in national Critical Tables, and during the 1920s
physical chemistry in 1910. From 1910 to and 1930s, she was three times a delegate to
1913, she was associate professor of chem- the International Union of Pure and Applied
istry at Mount Holyoke, after which she Chemistry. She was awarded four honorary
became professor of chemistry and head degrees and was the first to receive the Garvan
of the department, posts she held until her Medal, annually awarded to an American
retirement in 1946. woman for achievement in chemistry.

Source: © Research Machines plc 2006. All Rights Reserved. Helicon Publishing is a division of Research Machines.

SUMMARY

A chemical formula is a shorthand way of describing the composition Chemical reactions can also be classified as (1) combination,
of a compound. An empirical formula identifies the simplest whole (2) decomposition, (3) replacement, or (4) ion exchange. The first three
number ratio of atoms present in a compound. A molecular formula of these are redox reactions, but ion exchange is not.
identifies the actual number of atoms in a molecule. A balanced chemical equation describes chemical reactions and
The sum of the atomic weights of all the atoms in any formula is has quantitative meaning about numbers of atoms, numbers of mol-
called the formula weight. The molecular weight is the formula weight ecules, and conservation of atomic weights. The coefficients also de-
of a molecular substance. The formula weight of a compound can scribe the volumes of combining gases. At a constant temperature and
be used to determine the mass percentage of elements making up a pressure gases combine in small, whole number ratios that are given
compound. by the coefficients. Each volume at the same temperature and pressure
A concise way to describe a chemical reaction is to use formulas contains the same number of molecules.
in a chemical equation. A chemical equation with the same number of The number of atoms in exactly 12.00 g of C-12 is called Avo-
23
each kind of atom on both sides is called a balanced equation. A bal- gadro’s number, which has a value of 6.02 × 10 . Any substance that
anced equation is in accord with the law of conservation of mass, which contains Avogadro’s number of atoms, ions, molecules, or any chemi-
states that atoms are neither created nor destroyed in a chemical reac- cal unit is called a mole of that substance. The mole is a mea sure of a
tion. To balance a chemical equation, coefficients are placed in front of number of atoms, molecules, or other chemical units. The mass of a
chemical formulas. Subscripts of formulas may not be changed since mole of any substance is equal to the atomic mass of that substance.
this would change the formula, meaning a different compound. The mass, number of atoms, and mole concepts are generalized
One important group of chemical reactions is called oxidation- to other units. The gram-atomic weight of an element is the mass in
reduction reactions, or redox reactions for short. Redox reactions are grams that is numerically equal to its atomic weight. The gram-formula
reactions where shifts of electrons occur. The process of losing elec- weight of a compound is the mass in grams that is numerically equal
trons is called oxidation, and the substance doing the losing is said to be to the formula weight of the compound. The gram-molecular weight
oxidized. The process of gaining electrons is called reduction, and the is the gram-formula weight of a molecular compound. The relation-
substance doing the gaining is said to be reduced. Substances that take ships between the mole concept and the mass ratios can be used with a
electrons from other substances are called oxidizing agents. Substances chemical equation for calculations about the quantities of reactants and
that supply electrons are called reducing agents. products in a chemical reaction.

268 CHAPTER 10 Chemical Reactions 10-18