Chemistry for Chemical Engineers

ChemistryforChemical
Engineers
Dr.AshleighJ.Fletcher

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Dr. Ashleigh J. Fletcher

Chemistry for Chemical Engineers

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Chemistry for Chemical Engineers
© 2012 Dr. Ashleigh J. Fletcher & bookboon.com
ISBN 978-87-403-0249-3

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Chemistry for Chemical Engineers

Contents

Contents
Quantifying systems

7

Atoms and bonding

11

he periodic table

20

Molecular structure

34

Mass and volume

39

he mole

42

Stoichiometry

44

Acid-base chemistry

46

Basic organic chemistry

54

Basic thermodynamics

67

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Chemistry for Chemical Engineers

Contents

Kinetic theory of gases

73

Physical properties of gases

78

Equilibria and kinetics

82

Efect of reaction conditions on the equilibrium position

89

Liquids and solutions

91

Colligative properties

97

Chemical reactions

101

Hess’s law and temperature dependence of equilibria

105

Material balances

114

Energy balances

119

Biography for Dr. Ashleigh Fletcher

125

360°
thinking

.

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Chemistry for Chemical Engineers

Two of the main distinctions between chemical engineers and other engineering disciplines are the topics
of mass and energy balances. Within these two topics there are a lot of underlying chemical principles that
help chemical engineers to perform calculations to determine what is happening in a system, allowing
better control of a process.
his book will outline the basic chemistry principles that are required by chemical engineers to understand
chemical reactions and relate them to the main themes of mass and energy balances. It does not serve
as a complete account of all the chemistry that is important for chemical engineering but should give a
grounding, which can be supplemented by reading further into the areas discussed, if required.

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Chemistry for Chemical Engineers

Quantifying systems

Quantifying systems
Working as a chemical engineer requires a capacity to interpret data and quantities provided from diferent
sources. It is essential that any quantities used or calculated are recorded correctly, as the inclusion or
omission of units changes the context dramatically. For example 7 is a purely numerical quantity, but
adding a unit, say kilograms so the measurement becomes 7 kg, conveys signiicantly more information. In
all working it is important to write down both numerical values and the corresponding units; as a result,
it is necessary to appreciate the relationship between certain units and have an ability to convert between

quantities. he properties that can be measured, such as time, length and mass, are known as dimensions
and can also be composed from multiplying or dividing other dimensions, for example velocity (length/
time). Units can be treated like algebraic variables when quantities are added, subtracted, multiplied or
divided but note that numerical values may only be added or subtracted if their units are the same. he
most common set of units that chemical engineers come into contact with are the seven fundamental
S.I. units of measurement, as deined in the International System of Units (the abbreviation S.I. comes
from the French for this classiication: Système Internationale d’Unités). he system was developed in
1960 and has been widely accepted by the science and engineering communities.
he table below shows the seven base units and their corresponding abbreviations, as chemical engineers
the most commonly used units will be those for amount of substance, mass, length, temperature and,
importantly, time.
Property

Unit

Abbreviated Notation

amount of substance

Mole

mol

electric current

Ampere

A

Length

Metre

m

luminous intensity

Candela

cd

Mass

Kilogram

kg

temperature

Kelvin

K

Time

Second

s

Base units of measurement according to the S.I. classiication

he seven units within the S.I. are referred to as base units, so for length that would be metre (m), but
these can be converted to other systems of measurement that represent the equivalent dimension, such
alternative units are referred also known as base units but not S.I., so for the example of length one
could use (t).

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Chemistry for Chemical Engineers

Quantifying systems

Sometimes, quantities are calculated from several dimensions, this is very common in chemical
engineering where lowrates, such as mass or volumetric lowrate are frequently used. In this case the
quantities are measured as mass/time (kg/s) and volume/time (m3/s); the corresponding units are a
composition of all the dimensions involved and are known as derived units.
Common derived units are listed in the table below. It should be noted that these dimensions have their
own unit and abbreviated notation, in addition to that from their derivation.
Equivalent property

Unit

Abbreviated notation

S.I. derived units

Volume

litre

l or L

0.001 m3 or 1000 cm3

Force

Newton

N

1 kg m/s2

Energy

kilojoule

kJ

103 N m

Pressure

bar

Bar

105 N/m2

Power

kilowatt

kW

1 kJ/s

Commonly used derived units

Note 1 N is deined as being equivalent to 1 kg m/s2 because a force of 1 N produces an acceleration of
1 m/s2 when applied to a mass of 1 kg. It is, therefore, useful to remember that 1J

1Nm

1kg m2 s-2

in order to simplify complex units generated in some equations.
he base units are not always the most useful mathematical representation of the numerical value
determined and may be necessary to use other methods to simplify the quantity. For example, 60 s can
be represented as 1 minute (1 min), similarly 0.000001 s could be represented as 10-6 s or 1 ms, the latter
unit (microseconds) and min are known as multiple units, and it is essential to be able to understand
not only the quantities involved in a system but also their level of scale. Chemical engineers must be
comfortable with the common preixes used with S.I. units and other units from around the globe.
Commonly used preixes are given below, with their names and numerical value.
Tera

(T)

1012

pico

(p)

10-12

Giga

(G)

109

nano

(n)

10-9

mega

(M)

106

micro

(m)

10-6

Kilo

(k)

103

milli

(m)

10-3

centi

(c)

10-2

deci

(d)

10-1

Common preixes in metric system

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Chemistry for Chemical Engineers

Quantifying systems

Converting units is an essential skill for all chemical engineers and the easiest method to use is fractional
representation. his keeps track of all numerical values and units throughout the conversion performed,
allowing those units that cancel to be easily identiied.
he equivalence between two expressions of a given quantity may be deined in terms of a ratio (expressed
here in common fraction notation):

1 cm
10 mm

1 centimetre per 10 millimetres

Ratios of this form are called conversion factors. Generally, when converting units, multiply by conversion
factor(s) as fractions with new units as the numerator (top) and old units as the denominator (bottom).
For example, convert 100 mm into cm:

Worked example – convert the gas constant from 8.314 J mol-1 K-1 to Btu lb-mol-1 ºC-1, using the following
conversions:
1 kJ = 0.9478 Btu; 1 kmol = 2.205 lb-mol; 1 K = 1 ºC
Firstly, write out the value given in fractional format:

8.314 J
mol K

hen write out each of the required conversions in the same format, making sure that the units match
and can cancel out in the working. For example, if the value to be converted has J on the top line, and

the conversion of 1 kJ = 0.9478 Btu is to be applied, it is irstly required that J is converted to kJ. To do
this, divide through by 1000 J and multiplying by kJ (as 1 kJ = 103 J = 1000 J). Ater this, kJ is now on
the top line:

kJ
8.314 J
1 kJ
×
= 8.314
1000 mol K
mol K 1000 J

It is then possible to use the conversion, 1 kJ = 0.9478 Btu, directly, to arrive at:

8.314

0.9478 Btu
kJ
0.9478 Btu
×
= 8.314
1000 mol K
1000 mol K
1 kJ

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