CHEM 1405 Concept Review: Introduction to Chemistry

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Matter: Anything that has mass and takes up space. Matter can be in the form of a pure substance or a mixture.

Pure Substance: Matter composed of only one type of molecule or one type of atom. (Elements or compounds)

Element: A pure substance composed of only one type of atom (i.e.-only oxygen or only iron)

Compound: A pure substance composed of only one type of molecule (i.e.-only H₂O molecules or only CO₂)

Mixture: Matter composed of two or more pure substances that are physically mixed but NOT chemically combined. (Examples: a tossed salad, cookie batter, or even a computer is a mixture of components.)

Homogenous Mixture: A mixture that is so completely and evenly mixed that there is no difference between one part of the mixture and another part. (Example: Maple syrup, salt water, air, and even metal alloys)

Heterogeneous Mixture: A mixture that is not evenly mixed and where the different components can be distinguished. (Example: Pulpy orange juice, chunky spaghetti sauce, this piece of paper which has an ink part and a paper part, etc.)

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Physical Properties: Characteristics that can be observed or measured without changing what the substance is.

Physical Change: A change in state or appearance that does not affect the composition or change what it is.

Chemical Property: the ability of a substance to react with other things or itself and become a different substance.

Chemical Change: a change where one substance reacts and becomes one or more different substances, each with different chemical and physical properties.

States of Matter Comparison

International System of Units (SI): The official system of measurement throughout the world, based largely on the metric system.

Comparative Measurements

*Measurement*	*Metric*	SI	*English*	*Measured Using…*
Length	meter (m)	meter (m)	foot (ft)	meter stick or ruler
Volume	liter (L)	cubic meter (m³)	gallon (gal)	graduated cylinder or beaker
Mass	gram (g)	kilogram (kg)	pound (lb)	scale or balance
Temperature	degree Celcius (˚C)	Kelvin (K)	degree Fahrenheit (˚F)	thermometer
Time	second (s)	second (s)	second (s)	atomic clock

Scientific Notation

A way of representing numbers (especially very large or very small numbers) in reference to a power of ten. The general form of a number in scientific notation is as follows:

M x 10ⁿ

Where “M” is a value greater than or equal to 1 and less than 10, and “n” is the power of ten.

To convert into scientific notation, follow these simple steps:

1. Decide where you would have to move the decimal point to make it a number between 1 and 10

Example: 1406 CH 2 img 1.png

2. Then count how many spaces you had to move the decimal point. (Note that in large numbers you won’t see a decimal point in the number so just place a decimal point after the “one’s place.”)

Example: 1406 CH 2 img 2.png

3. Write the new number as having the same number of significant figures as the old number and multiply it times 10 to the power of the number of spaces you moved. If the number was small (less than 1), give it a negative exponent. If the number was large, give it a positive exponent.

Example:

To convert from scientific notation back to normal notation, simply move the decimal point the number of spaces as the exponent of 10. If the exponent is negative, move the decimal in the direction that will make the number smaller. If the exponent is positive, move the decimal in the direction that will make the number bigger.

Significant Figures (Sig figs)

The numbers that reflect the degree of certainty to which a value is measured.

Measuring With Appropriate Significant Figures

To measure a value to the appropriate number of significant figures, one must measure all digits known with certainty PLUS one estimated digit. Digital instruments will do this for us, but for non-digital thermometers, meter sticks, graduated cylinders, etc., this means estimating between the smallest graduations (or markings) on the instrument. (i.e.-If the smallest marking is worth 0.1, you must estimate to the nearest 0.01)

Identifying the number of Significant Figures in a Measured Value

Whenever a numeric value is reported, it can be assumed that all non-zero numbers are significant. However, zeroes are sometimes needed simply to hold place value but not to indicate the degree of certainty of the value. When such zeroes are needed, we do not consider them significant. The three simple Rules of Zero below can help you determine whether a zero is considered “significant”.

Exact Numbers: Numbers with no uncertainty that are treated as having unlimited significant figures. These values typically come from accurate counting of discrete objects (4 atoms or 3 chairs), defined relationships.

(1 km = 1000 m), or from numbers as part of an equation (Circumference = 2πr).

The Rules of Zero (for Significant Figures)

1) Leading zeroes (zeroes before non-zero numbers) are NEVER significant.

EXAMPLE: 0.00000259 m (NOT significant)

2) Sandwiched zeroes (zeroes between non-zero numbers) are ALWAYS significant.

EXAMPLE: 10029 J (Significant)

3) Trailing zeroes (zeroes after non-zero numbers) are ONLY significant IF the value has a decimal point.

EXAMPLE: 12500 lbs (NOT significant) 12.500 lbs (significant)

Calculating with Significant Figures

In order to ensure that our calculated values continue to express the appropriate level of certainty corresponding to the measurements used in calculation, rules are used to determine how many significant figures a calculated value should have. They differ according to the mathematical process used in calculation.

Multiplication/Division Rule: The product or quotient of two or more measured values must have the same number of significant figures as the measured value with the fewest significant figures.

$LaTeX: 3.210\:m\times6.5\:m=21\:m^2$ $3.210\:m\times6.5\:m=21\:m^2$

4 S.F. 2 S.F. 2 S.F.

Addition/Subtraction Rule: The sum or difference of two or more measured values must have the same error as the measured value with the greatest error. The error of a measured value is in its estimated digit.

NOTE: For addition/subtraction, it doesn’t matter how many sig figs there are, but rather where the LAST sig fig is.

EXAMPLE: The estimated (uncertain) digit is bolded.

124.56 (The error is in the hundredths place)

+ 4.5 (The error is in the tenths place.)

129.1 (Answer must have its final significant figure (estimated digit) in the tenths place.)

Dimensional Analysis:

A method of problem solving that keeps track of units and cancels them using conversion factors.

Conversion Factor: A fraction where the numerator is equal (or at least directly related to) the denominator. Any time you can set one value equal to another, those values can be used as a conversion factor.

EXAMPLES: $LaTeX: 1\:inch=2.54\:cm\:\longrightarrow\:\left(\frac{2.54\:cm}{1\:inch}\right)\:or\:\left(\frac{1\:inch}{2.54\:cm}\right)$ $1\:inch=2.54\:cm\:\longrightarrow\:\left(\frac{2.54\:cm}{1\:inch}\right)\:or\:\left(\frac{1\:inch}{2.54\:cm}\right)$

Dimensional Analysis Example Problem: John Smith is 6.0 ft. tall. What is his height in meters?

1. Start with what is equal to the value you are looking for. The problem asks for height. What is equal to the man’s height? 6.0 ft.

2. Whatever unit is initially in the numerator must come down in the denominator of the next conversion factor so that it can cancel out. This also gives you a hint as to what conversion factor to use.

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3. Continue to add conversion factors until you get to the answer you are looking for.

$LaTeX: 6.0\:ft\times\left(\frac{12\:inches}{1\:ft}\right)\:\times\left(\frac{2.54\:cm}{1\:in}\right)\times\left(\frac{1\:m}{100\:cm}\right)=1.8\:m$ $6.0\:ft\times\left(\frac{12\:inches}{1\:ft}\right)\:\times\left(\frac{2.54\:cm}{1\:in}\right)\times\left(\frac{1\:m}{100\:cm}\right)=1.8\:m$

Density Formula:

$LaTeX: Density=\frac{mass}{volume}$ $Density=\frac{mass}{volume}$ or $LaTeX: D=\frac{m}{V}$ $D=\frac{m}{V}$

Temperature Scales

The Celsius scale is based upon the freezing (0˚C) and boiling (100˚C) points of water divided into 100 equal increments. The Kelvin scale is identical to the Celsius scale with regards to the size of increment but starts with its zero value (0 K) at absolute zero (the lowest temperature theoretically possible) instead of the freezing point of water. The equation for changing between the two scales is as follows:

$LaTeX: K=\:^\circ C+273.15$ $K=\:^\circ C+273.15$

The Fahrenheit scale is based on the same two temperature reference points of the freezing and boiling points of water, though those reference points are given different values (32˚F for freezing and 212˚F for boiling). Between the two points, there are exactly 180 increments, hence the use of the term “degrees”. The equations for changing between the Celsius and Fahrenheit scales are as follows:

$LaTeX: ^\circ C=\frac{5}{9}\left(^\circ F+32\:\right)$ $^\circ C=\frac{5}{9}\left(^\circ F+32\:\right)$ Or $LaTeX: ^\circ F=\frac{9}{5}\left(^\circ C\right)+32$ $^\circ F=\frac{9}{5}\left(^\circ C\right)+32$

Energy

Energy: the ability to do work or transfer heat Work: the action of a force applied through a distance

Kinetic Energy: the energy of motion Equation: $LaTeX: E_k=\frac{1}{2}mv^2$ $E_k=\frac{1}{2}mv^2$ (m is mass, v is velocity)

Thermal Energy (Heat): the energy of the motion of particles. This energy is actually a type of kinetic energy.

Potential Energy: any type of energy that is in a “stored” form, which includes gravitational potential energy, electrostatic potential energy, chemical potential energy, and more.

Chemical Potential Energy: the energy stored in the structure of atoms and molecules.

Law of Conservation of Energy: Energy is neither created nor destroyed, but only converted from one form to another.