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Physical and Chemical Properties

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Title: Physical and Chemical Properties


1
Physical and Chemical Properties
  • All substances have properties that we can use to
    identify them. For example we can identify a
    person by their face, their voice, height, finger
    prints, DNA etc.. The more of these properties
    that we can identify, the better we know the
    person. In a similar way matter has properties -
    and there are many of them. There are two basic
    types of properties that we can associate with
    matter. These properties are called Physical
    properties and Chemical properties
  • Physical propertiesProperties that do not change
    the chemical nature of matter
  • Chemical propertiesProperties that do change the
    chemical nature of matter

2
  • Examples of physical properties are color,
    smell, freezing point, boiling point, melting
    point, infra-red spectrum, attraction
    (paramagnetic) or repulsion (diamagnetic) to
    magnets, opacity, viscosity and density. There
    are many more examples. Note that measuring each
    of these properties will not alter the basic
    nature of the substance.
  • Examples of chemical properties are heat of
    combustion, reactivity with water, pH, and
    electromotive force.
  • The more properties we can identify for a
    substance, the better we know the nature of that
    substance. These properties can then help us
    model the substance and thus understand how this
    substance will behave under various conditions.

3
Conservation of Mass in Chemical Reactions
  • Democritus (460-370 BC) and somewhat later John
    Dalton (1766-1844) were the first to consider
    matter at its most microscopic form. They both
    came up with the concept of the "atom" as being
    the smallest unit of matter and thus being
    undivisible. This observation has an important
    and fundamental consequence mass is neither
    created nor destroyed during the course of a
    chemical reaction. How do we come to this
    conclusion? We know that chemical reactions take
    place at the atomic/molecular level. That is
    molecules and atoms interact with one another
    during a chemical reaction. If atoms are
    indivisible then they cannot be destroyed during
    a chemical reaction. If atoms cannot be destroyed
    then the mass of reactants must equal the mass of
    the products in a chemical reaction. e.g.,
  •  Reactants -------gt Products
  • Mass of Reactants Mass of Products

4
  • This can be visualized by considering the
    formation of water from oxygen and hydrogen
    molecules
  • Note that the hydrogen and oxygen atoms simply
    rearrange themselves but are not destroyed.
    Therefore mass is conserved.
    Iron Oxygen
    -----gt Rust
    100 g ?g ------gt 143g
    mass
    reactants mass products
    mass products 143g mass
    reactants 100
    mass of oxygen mass oxygen 43 g

5
Elements, Compounds and Mixtures
  • All substances have mass and therefore must be
    composed of atoms. These atoms and how they
    assemble themselves in the substance determines
    their chemical and physical properties.
    Substances can be classified according to how
    these atoms are assembled and is known as
    Classification of Matter All matter falls into
    one of three categories elements, compounds or
    mixtures. Furthermore, mixtures can be classified
    as homogeneous or inhomogeneous. The scheme looks
    something like the diagram next slide

6
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7
  • This classification depends upon how we try and
    separate matter into its basic components. This
    separation is called the "process". There are two
    processes a physical and a chemical process.
  • Physical process a process using physical
    properties
  • Chemical process a process using chemical
    properties
  • If we have a sample of matter and can find a
    physical process such as evaporation, magnets,
    color etc. to separate it then the sample is a
    "mixture". Furthermore if the sample is a mixture
    of solids and liquids (e.g., sand and water) etc.
    or two or more liquids that don't mix (e.g., oil
    and vinegar) then the mixture is "inhomogeneous".
    Otherwise the sample is a "homogeneous" mixture.

8
  • If there is no physical process that will
    separate the sample then the sample is a "pure"
    substance. If a chemical process such as
    combustion or oxidation breaks the substance down
    to its constituent atoms then the substance is a
    "compound"(e.g., salt, sugar, water). Otherwise
    the substance is an "element" (e.g., copper
    penny, aluminum foil). Compounds are made up of
    molecules or salts. Elements are made up of
    single types of atoms.

9
Density
  • Density is a physical property of matter. Most
    commonly density refers either to the mass per
    unit volume (mass density) or the number of
    objects (e.g., atoms, molecules) per unit volume
    (number density). We will focus out attention on
    mass density. The mass density has the units
    mass/volume. Since volume has the units length
    "cubed" then the SI unit of mass density is
    kg/m3. More common units of density are g/ml or
    g/l. Substances have different densities. In fact
    the density of a substance can often be used to
    help identify it. In the next slide there is a
    table of densities of common materials

10
An important example is water. The above table
states that liquid water has a mass of 1 g in
every ml. Thus 2 ml of water has a mass of 2 g
etc.. Table sugar is more dense than water by
about 60 percent. Density does not depend upon
size. For example the water in a swimming pool
has the same density a glass of that swimming
pool water. Calculations with density are
straight forward and involve the formula for
density namely Dm/V, where Ddensity, m
mass and V volume.
11
  • Example 1 What is the volume of a nugget of gold
    that has a mass of 3.45 g? The density of gold
    can be looked upon as a conversion factor from
    mass to volume i.e.,
  • Example 2 A light substance is found to weigh 23
    g and to have a volume of 0.192 liters. What is
    the substance?
  • Based upon this result we would guess that this
    substance might be balsa wood.
  • Example 3 What is the mass of 1 liter of sugar?

12
Compounds
  • There are two basic types of compounds. They are
    distinguished by by the manner in which the atoms
    bind to one another in the compound. These two
    types are called "molecular" compounds and
    "salts" (or equivalently "ionic" compounds)
  • Molecular compoundsThese compounds are made up
    of molecules whose atoms bind to one another
    through "covalent" bonds.
  • SaltsThe atoms in salts are held together with
    "ionic" bonds. Unlike molecules, salts always
    form solids in a regular array called a
    "crystalline solid".

13
  • A bond is the "glue" that holds atoms together.
    In compounds this glue can either be covalent or
    ionic.
  • Covalent bondsThe electrons are shared between
    atoms. Therefore this sharing of electrons
    provides the glue.
  • Ionic bondsIonic bonds occur due to the mutual
    attraction between atoms with positive and
    negative charges i.e., ions.

14
Examples of Molecules
Acetaldeyhde (top)
N-hexane (top)
Taxol (left)
15
An Example of a Salt
Sodium Chloride (NaCl)
16
Energy and Chemical Reactions
  • When matter undergoes transformations that change
    its chemical and physical properties then that
    transformation was brought about by a chemical
    reaction. On the other hand chemical reactions
    can only take place if there is sufficient energy
    to make the reaction proceed. Therefore energy is
    a prerequisite for chemical reactions.

17
  • Energy can come in many forms e.g., heat, work,
    light, kinetic, potential, chemical etc..
    Moreover, energy can itself transform among these
    various forms. For example a ball at the edge of
    a table has zero kinetic energy and positive
    potential energy. If the ball drops it will have
    zero potential energy and positive kinetic energy
    the instant it hits the floor. However the sum of
    the potential and kinetic energy is the same
    throughout the ball's dropping history. Therefore
    energy has neither been created or destroyed but
    has transformed from potential to kinetic energy.

18
  • Molecular of chemical energy can mean several
    things Chemical bonds are a source of energy,
    the movement of molecules in space is kinetic
    energy, the vibrations and rotations of molecules
    is another source of chemical energy. All of
    these forms of chemical energy contribute in one
    way or another to chemical reactions.
  • The units of chemical reactions are
    straightforward and is given in the diagram
    below
  • There are many other units for energy including
    electron volt (ev), erg, kjoule (kJ) etc.

19
Specific Heat and Heat Capacity
  • Specific heat is another physical property of
    matter. All matter has a temperature associated
    with it. The temperature of matter is a direct
    measure of the motion of the molecules The
    greater the motion the higher the temperature
  • Motion requires energy The more energy matter
    has the higher temperature it will also have.
    Typically this energy is supplied by heat. Heat
    loss or gain by matter is equivalent energy loss
    or gain.

20
  • With the observation above understood we can now
    ask the following question by how much will the
    temperature of an object increase or decrease by
    the gain or loss of heat energy? The answer is
    given by the specific heat (S) of the object. The
    specific heat of an object is defined in the
    following way Take an object of mass m, put in x
    amount of heat and carefully note the temperature
    rise, then S is given by
  • In this definition mass is usually in either
    grams or kilograms and temperatture is either in
    kelvin or degres Celcius. Note that the specific
    heat is "per unit mass". Thus, the specific heat
    of a gallon of milk is equal to the specific heat
    of a quart of milk. A related quantity is called
    the heat capacity (C). of an object. The relation
    between S and C is C (mass of obect) x
    (specific heat of object).

21
  • A table of some common specific heats and heat
    capacities is given below

22
  • Consider the specific heat of copper , 0.385 J/g
    C. What this means is that it takes 0.385 Joules
    of heat to raise 1 gram of copper 1 degree
    Celsius. Thus, if we take 1 gram of copper at 25
    C and add 1 Joule of heat to it, we will find
    that the temperature of the copper will have
    risen to 26 C. We can then ask How much heat
    will it take to raise by 1 C 2g of copper?.
    Clearly the answer is 0.385 J for each gram or
    2x0.385 J 0.770 J. What about a pound of
    copper? A simple way of dealing with different
    masses of matter is to determine the heat
    capacity C as defined above. Note that C depends
    upon the size of the object as opposed to S that
    does not.
  • Example 1 How much energy does it take to raise
    the temperature of 50 g of copper by 10 C?

23
  • Example 2 If we add 30 J of heat to 10 g of
    aluminum, by how much will its temperature
    increase?
  • Thus, if the initial temperature of the aluminum
    was 20 C then after the heat is added
    the temperature will be 28.3 C.

24
Daltons Atomic Theory
  • Democritus first suggested the existence of the
    atom but it took almost two millennia before the
    atom was placed on a solid foothold as a
    fundamental chemical object by John Dalton
    (1766-1844). Although two centuries old, Dalton's
    atomic theory remains valid in modern chemical
    thought
  • . Dalton's Atomic Theory
  • 1) All matter is made of atoms. Atoms are
    indivisible and indestructible.
  • 2) All atoms of a given element are identical in
    mass and properties
  • 3) Compounds are formed by a combination of two
    or more different kinds of atoms.
  • 4) A chemical reaction is a rearrangement of
    atoms.

25
  • Modern atomic theory is, of course, a little more
    involved than Dalton's theory but the essence of
    Dalton's theory remains valid. Today we know that
    atoms can be destroyed via nuclear reactions but
    not by chemical reactions. Also, there are
    different kinds of atoms (differing by their
    masses) within an element that are known as
    "isotopes", but isotopes of an element have the
    same chemical properties.
  • Many heretofore unexplained chemical phenomena
    were quickly explained by Dalton with his theory.
    Dalton's theory quickly became the theoretical
    foundation in chemistry.

26
Composition of the Atom
  • Atoms have a definite structure. This structure
    determines the chemical and physical properties
    of matter. This atomic structure was not fully
    understood until the discovery of the neutron in
    1932. The history of the discovery of atomic
    structure is one of the most interesting and
    profound stories in science. In 1910 Rutherford
    was the first to propose what is accepted today
    as the basic structure of the atom. Today the
    Rutherford model is called the "planetary" model
    of the atom. In the planetary model of the atom
    there exists a nucleus at the center made up of
    positively charged particles called "protons" and
    electrically neutral atoms called "neutrons".
    Surrounding or "orbiting" this nucleus are the
    electrons. In elements the number of electrons
    equals the number of protons.

27
  • The picture above greatly exaggerates the size of
    the nucleus relative to that of the atom. The
    nucleus is about 100,000 times smaller than the
    atom. Nevertheless, the nucleus contains
    essentially all of the mass of the atom. In order
    to discuss the mass of an atom we need to define
    a new unit of mass appropriate to that of an
    atom. This new unit of mass is called the "atomic
    mass unit" or amu. The conversion between the amu
    and gram is
  • 1 amu 1.67x10-24 g

28
  • The mass, in amu, of the three particles is given
    in the table below
  • Note that the mass of an electron is about 2000
    times smaller than that of the proton and
    neutron. Also note that the mass of the proton
    and neutron is close to 1 amu. This is a useful
    fact to remember. If the number of electrons does
    not equal the number of protons in the nucleus
    then the atom is an ion
  • cation number of electrons lt number of protons
  • anion number of electrons gt number of protons

29
Rutherfords Planetary Model of the Atom
  • By 1911 the components of the atom had been
    discovered. The atom consisted of subatomic
    particles called protons and electrons. However,
    it was not clear how these protons and electrons
    were arranged within the atom. J.J. Thomson
    suggested the"plum pudding" model. In this model
    the electrons and protons are uniformly mixed
    throughout the atom

30
  • Rutherford tested Thomson's hypothesis by
    devising his "gold foil" experiment. Rutherford
    reasoned that if Thomson's model was correct then
    the mass of the atom was spread out throughout
    the atom. Then, if he shot high velocity alpha
    particles (helium nuclei) at an atom then there
    would be very little to deflect the alpha
    particles. He decided to test this with a thin
    film of gold atoms. As expected, most alpha
    particles went right through the gold foil but to
    his amazement a few alpha particles rebounded
    almost directly backwards.

31
  • These deflections were not consistent with
    Thomson's model. Rutherford was forced to discard
    the Plum Pudding model and reasoned that the only
    way the alpha particles could be deflected
    backwards was if most of the mass in an atom was
    concentrated in a nucleus. He thus developed the
    planetary model of the atom which put all the
    protons in the nucleus and the electrons orbited
    around the nucleus like planets around the sun.

32
Isotopes and Atomic Symbols
  • Atomic Symbols
  • The atom of each element is made up of
    electrons, protons and neutrons. All atoms of the
    same neutral element have the same number of
    protons and electrons but the number of neutrons
    can differ. Atoms of the same element but
    different neutrons are called isotopes. Because
    of these isotopes it becomes necessary to develop
    a notation to distinguish one isotope from
    another - the atomic symbol.)

33
  • The atomic symbol has three parts to it
  • 1. The symbol X the usual element symbol
  • 2. The atomic number A equal to the number of
    protons (placed as a left subscript)
  • 3. The mass number Z equal to the number of
    protons and neutrons in the isotope (placed as a
    left superscript

34
  • Examples 1
  • Consider two isotopes of gallium, one having the
    37 neutrons and the other having 39 neutrons.
    Write the atomic symbols for each isotope.
    Solution
  • Example 2
  • How many neutrons does the isotope of copper with
    mass number Z 65 have?
  • Solution From the periodic table we see that
    copper has an atomic number of 29. Since Z is the
    number of protons plus the number of neutrons,
    then No. neutrons 65 - 29 36

35
The Mass
  • The standard for every unit must be defined.
    Length is an example. The basic unit of length is
    the meter which was defined in 1983 as equal to
    the distance traveled by light in a vacuum in
    1/299,792,458 of a second. Mass must also be
    defined. The definition of mass today is the amu
    (atomic mass unit). The amu is defined in the
    following way the mass of one atom of the
    carbon-12 isotope is EXACTLY 12 amu.
  • mass of one carbon-12 atom 12 amu

36
  • All other masses are measured relative to this
    carbon-12 standard. For example, suppose we do an
    experiment and find that the isotope bromine-81
    has a mass that is 6.743 times that of carbon-12.
    Then the mass of bromine-81 would be given by

37
Atomic Weights
  • Most elements can be found on earth (with the
    exception of those elements that too unstable and
    thus must be synthesized in the laboratory).
    Since all elements have isotopes then we must
    consider how much of one isotope of an element
    exists versus another isotope of the same
    element. These are called the "natural"
    abundances on earth.

38
  • Natural Abundances
  • Suppose we go to a cave and mine element "X".
    After careful analysis we find that in our sample
    of element X there exists three isotopes Xa, Xb
    and Xc. Moreover, we find that out of every 100
    atoms the various isotopes are distributed as
    follows, and their masses are given.

39
  • Then the average mass (atomic weight) is given
    by
  • The atomic weight of each element is included
    along with the element symbol in the periodic
    table. It is important to note that no one atom
    has a mass equal to that of the atomic weight.
    Remember the atomic weight represents that
    average mass of the atoms.
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