Transcript Units - Chemistry at Winthrop University

Introduction to
General Chemistry
Part 2:
Units
Ch. 1
Qualitative and Quantitative Analysis
• In chemistry, the scientific method is used to investigate
scientific phenomena & acquire new knowledge
• Empirical evidence is gathered which supports or refutes a
hypothesis
• Empirical evidence is either quantitative or qualitative
– Quantitative data is numerical, and results can be
measured
– Qualitative data is NOT numerical, but consists of
observations and descriptions
Quantitative and Qualitative Analysis
A+B
Quantitative data
• How much C is formed?
• How efficient is the
reaction?
• What is the rate of the
reaction?
C
Qualitative data
• What color is it?
• Is it solid, liquid, gas?
• How does it smell?
Units
• Quantitative measurements are represented by a:
NUMBER and a UNIT
• A unit is a standard against which a physical quantity is
compared physical quantity
– Temperature is measured in Co, Ko,or Fo
– Currency is measured in $USD
– Distance is measured in meters, miles, ft, etc.
– Time is reported in seconds, minutes, hr, etc.
• Internationally accepted system of measurements is called
the SI unit system
SI Unit System: The Units of Physical Science
Greek Prefixes
• Prefixes indicate powers of 10
– ex. k= 103; 5 kg = 5 x (103)g
A Review of Scientific Notation
Scientific notation indicates a factor (F) multiplied by a power (n) of 10
F x 10n
• Important: All integers end with a decimal point, even though
it is not commonly written (1  1. )
• For all non integers, simply shift the decimal right n places
2.5 x 105 = 250000.
1.8773 x 108 = 187730000.
• For negative exponents, shift the decimal left. All values less
than 1 have negative exponents.
7.141 x 10-2 = .07141
3.867 x 10-7 = .0000003867
Convert to standard notation
• 3.4912 x 104
• 8.971 x 10-3
• 6.50 x 100
Convert to scientific notation
• 15
• 125.3
• 0.003003
Group Work
Convert the following values to grams in both standard and
scientific notation.
– 421.4 kg
– 1170.1 mg
– 481 µg
Why Are Units Important? Example #1
• In 1999, NASA lost the $125M
Mars Orbiter System.
• One group of engineers failed to
communicate with another that
their calculated values were in
English units (feet, inches,
pounds), and not SI units.
• The satellite, which was
intended to monitor weather
patterns on Mars, descended
too far into the atmosphere and
melted.
Why Are Units Important? Example #2
• In 1983, an Air Canada Plane
ran out of fuel half way through
its scheduled flight. Why?
• Airline workers improperly
converted between liters and
gallons.
• Luckily, no one died.
Derived SI Units: VOLUME
• Many measured properties have units that are combinations of
the fundamental SI units
• Volume: defines the quantity of space an object occupies; or the
capacity of fluid a container can hold
– expressed in units of (length)3 or Liters (L)
– 1 L is equal to the volume of fluid that a cube which is 10 cm on
each side can hold
V = (10 cm)3 = 1000 cm3
1 L = 1000 cm3
10
cm
1000 mL = 1000 cm3
10 cm
10 cm
mL = cm3
Derived SI Units: DENSITY
• All matter has mass, and must therefore occupy space. Density
correlates the mass of a substance to the volume of space it
occupies.
• Density = mass per unit volume (mass/volume). Different
materials have different densities.
Would a 20-gallon
filled with bricks
same mass as an
volume of feathers?
container
have the
equivalent
NO!
g
 feather  0.025 3
cm
g
 brick  1.90 3
cm
THE DENSITY OF WATER IS 𝟏
𝒈
𝒄𝒎𝟑
𝒐𝒓 𝟏
𝒈
𝒎𝑳
Group Work
• A cubic container that is 25 cm on each side is filled with
ethanol. The density of ethanol is 0.79 g/mL.
– What is the volume of ethanol in the cube in mL?
– What is the volume in L?
– What is the mass of ethanol in kg?
Give answers in scientific notation!!
Derived SI Units: ENERGY
• What is Energy?
– Energy is defined as the capacity to perform “work”
• How do we define work?
• Work is defined as the action of applying a force
acting over some distance. Work can not be done
if no energy is available.
• In SI units, we use the unit Joule (J) to represent energy.
𝑘𝑔 𝑚2
𝐽=
𝑠2
Conservation of Energy
Energy is never created or destroyed, merely converted between
forms and transferred from place to place. The total energy of
the universe is finite.
Forms of Energy
• Energy comes in many forms and can be converted from one
form to another. Some examples are given:
• Chemical Energy
– Energy stored in chemical bonds (e.g. gasoline, coal, etc.)
that can be released by chemical reaction, typically
combustion (fire)
• Heat Energy (thermal energy)
– Heat is defined as energy flow between bodies of matter
resulting from collisions of molecules or random motions
of electrons.
Forms of Energy
• Mass Energy
– Energy and mass are interchangeable. During a fusion
reaction (e.g. stars), mass is lost and energy is formed.
This mass appears as energy according to the following:
𝐄 = ∆𝐦𝐜 𝟐
where m is the change in mass (in kg), c is the speed of
light, and E is the energy released (J). This is the basis of
nuclear power.
• Kinetic Energy
– Energy of motion (e.g. a moving car). An object with mass
m, moving at a velocity V (meters/sec) has kinetic energy:
𝟏
𝐄𝐤 = 𝐦𝐕 𝟐
𝟐
Forms of Energy
• Potential Energy
– Potential energy corresponds to energy that is stored as a
result of the position of mass in a field.
• If a mass m is held at a height h (meters) above the
ground, assuming a gravitational acceleration of 9.8
m/s2 (g), its potential energy is:
𝐄𝐏 = 𝐦𝐠𝐡
– If the object is dropped, it loses potential energy. However,
it speeds up as it falls, so its kinetic energy increases
equally (conversion).
Forms of Energy
• Electrical Energy
– Energy resulting from electric current, the movement of
electrons through a conductive circuit. Electrical energy is
a type of potential energy. For a charge q (coulombs, C)
moving across a voltage, V
𝐄𝐞𝐥𝐞𝐜 = 𝐪𝐕
• Light/Radiation
– The energy of a wave of light is calculated as the product
of Planck’s constant, h (J s), and the wave frequency, ν
(1/s)
𝐄 = 𝐡𝐯
Power
• It is often necessary to express the rate of energy usage. This
is called power.
𝐞𝐧𝐞𝐫𝐠𝐲
𝐏𝐨𝐰𝐞𝐫 =
𝐭𝐢𝐦𝐞
• Typically, we speak in terms of energy per second. In SI units,
a joule per second (J/s) is known as a watt (W).
Temperature
• Temperature: a measure of the tendency of a substance to
lose or absorb heat. Temperature and heat are not the same.
• Heat always flows from bodies of higher temperature to those
of lower temperature
– The stove top is ‘hot’ because the surface is at a much
higher temperature than your hand, so heat flows rapidly
from the stove to your hand
– Ice feels ‘cold’ because it is at a lower temperature than
your body, so heat flows from your body to the ice, causing
it to melt
Temperature
• When performing calculations in chemistry, temperature
must always be converted to Kelvin (oK) units (unless
otherwise stated).
• The lowest possible temperature that can ever be reached
is 0oK, or absolute zero. At this temperature, all molecular
motion stops.
• To convert temperatures to the Kelvin scale:
oK
: oC + 273.15