Concepts

Question 1

<p>
Describe the concept of <strong>experimental uncertainty</strong>, including the terms <strong>absolute uncertainty</strong> and <strong>relative uncertainty</strong>.</p>

Answer 1

<ul>
<li>
All experimental measurements include some uncertainty. Evaluation of the uncertainty helps determine whether the experiment was done correctly</li>
<li>
The uncertainty of &plusmn;1 in the last digit of a measurement is assumed: if this uncertainty is different from &plusmn;1, it is written as part of the number. For example: 23.45 &plusmn;0.05 indicates an uncertainty of &plusmn;5 in the last digit.</li>
<li>
The absolute uncertainty divided by the value of the number: This may be expressed as a percent by multiplying by 100 percent.</li>
</ul>

Question 2

<p>
Describe the concept of <strong>experimental errors</strong>, including the terms <strong>accuracy </strong>and <strong>precision</strong>.</p>

Answer 2

<ul>
<li>
All experiments involve uncertainty and error. Uncertainty may be expressed as the <strong>precision </strong>of an experiment by determining how clos replicate experimental results are to each other.</li>
<li>
<strong>Accuracy </strong>is a measure of how close the experimental result is to the true value. Often we do not know the true value. True values are best obtained by experiments that use independent methods to measure the same phenomenon.</li>
</ul>

Question 3

<p>

| Explain the <strong>mole </strong>concept.</p>

Answer 3

<ul>
<li>
The mole is a number, just as a dozen represents 12 and a ream represents 500. The mole is 6.022 x 10²³ . Eggs are fairly large so a dozen represents a reasonablye quantity. Atoms and molecules are extremely small so that large number represents easily measurable quantities that are useful in the lab.</li>
<li>
Chemical equations tell us how reactions occur.They can represent atoms and molecules on the atomic scale, and can represent moles of one substance reacting with another on the mole or lab scale.</li>
<li>
Simple factor--label calculations allowfor many necessary calculations in chemistry, using the mole scale.</li>
</ul>

Question 4

<p>
The concept of an <strong>acid </strong>is very important in chemistry. What three theories concern the nature of acids?</p>

Answer 4

<ul>
<li>
<strong>Arrhenius Theory</strong>: A substance that increases H⁺ in an aqueous solution</li>
<li>
<strong>Bronsted Theory</strong>: any substance that donates protons</li>
<li>
<strong>Lewis Theory</strong>: an elecotron pair acceptor</li>
</ul>

Question 5

<p>

| Explainn the concept of an <strong>activated complex</strong>.</p>

Answer 5

<p>
When molecules collide in a chemical reaction the reactants transform into the products. This transformation coincides with the conversion of the reactants&#39; kinitic energy to potential energy and then back to kinetic energy of the products. The structure of the atoms when the potential energy is at a maximum is called the <strong>activated complex</strong>.</p>

Question 6

<p>

| What concept includes <strong>allotropes</strong>?</p>

Answer 6

<ul>
<li>
Many distinct molecules are made of two or more elements.</li>
<li>
An allotrope is a molecule containing only one element, which differs from other allotrpes of the same element.</li>
<li>
For example: the three allotrpes of carbon, graphite, diamond, and C₆₀ (buckminsterfullerene).</li>
<li>
Not all elements have allotropes.</li>
</ul>

Question 7

<p>

| What is an <strong>amorphous </strong>substance?</p>

Answer 7

<ul>
<li>
<strong>Amorphous </strong>means without structure.</li>
<li>
Amorphous substances have no long-range crystal structure.</li>
<li>
Glass is an example of an amorphous substance.</li>
</ul>

Question 8

<p>

| <strong>Amphoteric </strong>and <strong>amphiprotic </strong>substances have the same underlying concept. Explain.</p>

Answer 8

<p>
<strong>Amphoteric </strong>and <strong>amphiprotic </strong>substances can act as both a conjugate acid and a conjugate base.</p>

Question 9

<p>

| What is the concept behind the <strong>Arrhenius Theory</strong>?</p>

Answer 9

<ul>
<li>
The <strong>Arrhenius Theory</strong> describes the nature of acids and bases.</li>
<li>
Acids increase the hydrogen ion concentration of solutions and bases increase the hydroxide ion concentration of solutions.</li>
</ul>

Question 10

<p>

| What is the concept behind the <strong>atomic number</strong>?</p>

Answer 10

<ul>
<li>
The <strong>atomic number</strong> is the number of an element in the periodic table.</li>
<li>
It is also a number representing the number of protons in the nucleus of an atom.</li>
<li>
The atomic number defines the identity of an element.</li>
</ul>

<p>
The basic idea is that each element must fall in some sort of order. Mendeleev originally based the order on atomic masses. It was later found that the order should be based on the number of protons in the nucleus of an element.</p>

Question 11

<p>

| What is an <strong>atomic orbital</strong>?</p>

Answer 11

<ul>
<li>
The <strong>atomic orbital</strong> is the region in space, outside the nucleus, that has a high probability of containing an electron</li>
<li>
The orbital structure of the elements</li>
<li>
An orbital within an element</li>
<li>
Can be <em>s, p, d,</em> or orbital</li>
<li>
Atomic orbitals have specific shpaes and sizes as defined by quantum numbers</li>
</ul>

Question 12

<p>

| What is the concept behind the <strong>atom</strong>?</p>

Answer 12

<ul>
<li>
The <strong>atom </strong>was named for the smallest indivisible particle in nature.</li>
<li>
Discoveries in physics in the late 1800s showed this was not true.</li>
<li>
The <strong>atom </strong>is the fundamental particle defining one of the 117 elements.</li>
</ul>

Question 13

<p>

| What is the concept behind<strong> Avogadro&#39;s Principle</strong> or<strong> Law</strong>?</p>

Answer 13

<ul>
<li>
<strong>Avogadro&#39;s Principle</strong> or<strong> Law</strong> states that there is a direct relationship between the moles of a gas and the volume of that gas.</li>
<li>
This concpet indicated that hydrogen and oxygen were diatomic elements and subsequently led to the correct assignment of atomic masses.</li>
</ul>

Question 14

<p>
Like the concept of an acid, the concept of what a base is depends on the theory used. What theories describe a <strong>base</strong>?</p>

Answer 14

<ul>
<li>
<strong>Arrhenius Theory:</strong> any compound which increases the hydroxide concentration of a solution</li>
<li>
<strong>Bronsted Theory:</strong> a substance that is a proton acceptor</li>
<li>
<strong>Lewis Theory:</strong> a <strong>base </strong>is an electron pair donor</li>
</ul>

Question 15

<p>
What concept states that only a few three-dimensional <strong>basic structures</strong> exist for chemical molecules?</p>

Answer 15

<ul>
<li>
The shapes of chemical molecules may be linear, triangular planar, tetrahedral, trigonal bipyramidal, or octahedral.</li>
<li>
Additional shapes are derived from these <strong>basic structures</strong>.</li>
</ul>

Question 16

<p>

| What is <strong>bond order</strong>?</p>

Answer 16

<ul>
<li>
<strong>Bond order</strong> is the average number of bonds per atom covalently bonded to a central atom.</li>
<li>
This may be related to bond strength, bond vibrational frequency, and bond length.</li>
<li>
Bond order = ( total &sigma; and &pi; bonds)/( # atoms bonded to central atom)</li>
</ul>

Question 17

<p>

| What is a <strong>bonding electron pair</strong> and a <strong>nonbonding electron pair</strong>?</p>

Answer 17

<ul>
<li>
This concept views the sharing of a pair of electrons with opposite spins as the basis of the covalent bond joining two atoms together.</li>
<li>
<strong>Bonding electron pairs</strong> are pairs of electrons with opposite spins that are part of a bond. These can participate as sigma bonding electrons or pi bonding electrons.</li>
<li>
Bonding electron pairs can be used to evaluate bond order, bond strength, vibrational frequencies, and molecular geometry.</li>
<li>
<strong>Nonbonding electron pairs</strong>, as opposed to bonding pairs, do not contribute to attractive forces that hold a molecule together. Nonbonding electrons have zero density along the internuclear axis. They are often called &quot;lone pairs.&quot;</li>
<li>
<strong>Nonbonding electron pairs</strong> occupy space and affect the geometry of the molecule. They may also be donated if the substance acts as a Lewis base.</li>
</ul>

Question 18

<p>

| What are the concepts underlying the <strong>Bronsted-Lowry Theory</strong>?</p>

Answer 18

<ul>
<li>
Many bases, particularly nitrogen bases, are not adequately described using the <strong>Bronsted-Lowry Theory</strong>. This theory treats acids as proton (H⁺) donors and bases as proton acceptors. This eliminates the need to consider the hydroxide ion (OH^-) other than the result of an acid process.</li>
<li>
This theory introduces the concept of conjugate acid-base pairs. Each acid has a conjugate base and each base has a conjugate acid. For example: Hydrofluoric acid, HF (conjugate acid), and the fluoride ion, F^- (conjugate base).</li>
<li>
Relative amounts (or concentrations) of conjugate acid compared to the conjugate base in a system at equilibrium indicate the position of equilibrium (also related to the value of the equilibrium constant K_eq).</li>
<li>
Relative strengths of the conjugate acid to conjugate base are related to the position of equilibrium.</li>
</ul>

Question 19

<p>

| What is the nature of a <strong>catalyst</strong>?</p>

Answer 19

<ul>
<li>
A <strong>catalyst </strong>is a substance that speeds up the rate of a chemical reaction by providing an alternate reaction pathway with a lower activation energy.</li>
</ul>

<p>
- The action of catalysts is viewed in several different ways.</p>

<p>
- One view has the catalyst alter the shape of a reactant resulting in a slightly more reactive species.</p>

<p>
- Another view has the catalyst holding the reactant in a preferred position for interaction with another molecule to increase the number of effective collisions.</p>

Question 20

<p>

| What is the concept behind a <strong>chain reaction</strong>?</p>

Answer 20

<ul>
<li>
The basic concept of a <strong>chain reaction</strong> is that one molecular or atomic event provides additional reactants that result in an increased reaction rate.</li>
<li>
Two examples of <strong>chain reactions</strong> are common:</li>
</ul>

<p>
- First is a nuclear reaction that produces more neutrons than were needed to initiate it and therby causing more reactions than in the preceding step. For example: One neutron is needed for the fission of U-235 while four neutrons are released when the fission occurs.</p>

<p>
- A second example involves some free-radical reactions. In these situations, one reaction produces several free radicals that in turn produce more free radicals than are consumed.</p>

Question 21

<ul>
<li>
What is the concept behind the word <strong>system </strong>as used in chemistry?</li>
<li>
How does it relate to the <strong>universe </strong>and the <strong>surroundings</strong>?</li>
</ul>

Answer 21

<ul>
	<li style="text-align: center;">
		To study something we need to define exactly what is being studied. In chemistry and other sciences the <strong>universe </strong>encompasses everything. the universe is divided into two parts. The <strong>system </strong>is everything being studied and the <strong>surroundings </strong>is everything else.</li>
	<li style="text-align: center;">
		We can write:</li>
</ul>
<p style="text-align: center;">
	Universe = system + surroundings</p>
<ul>
	<li style="text-align: center;">
		Use of the term <strong>system </strong>is usually encountered in thermodynamics and thermochemistry.</li>
</ul>

Question 22

<p>

| What is the concept behind the term <strong>colligative property</strong>?</p>

Answer 22

<ul>
<li>
A <strong>colligative property</strong> is any one of several physical properties of solutions that change depending upon the relative amount of solute particles present in the solution.</li>
<li>
Changes in melting and boiling points of solutions are colligative properties in that they change when solutes are added to pure substances.</li>
<li>
Changes in vapor and osmotic pressure are two other colligative properties.</li>
</ul>

Question 23

<p>

| What is the concept behind a <strong>chemical compound</strong>?</p>

Answer 23

<ul>
<li>
Chemists envision a <strong>chemical compound</strong> as a substance that has a fixed ratio (by either mass or atoms) of two or more different atoms.</li>
<li>
Chemical compounds have definite physical properties that can be used to identify the compound.</li>
<li>
Chemical compounds also have characteristic chemical properties that indicate what reactions they participate in and what reactions they do not.</li>
</ul>

Question 24

<p>

| Discuss the concept of <strong>concentration</strong>.</p>

Answer 24

<ul>
<li>
<strong>Concentration </strong>is a concept that allows scientists to quantitatively express precisely how much material is mixed with another.</li>
<li>
There are a variety of units for concentration: molarity (<em>M</em>), molality (<em>m</em>), mole fraction (<em>X</em>), weight (mass), percent (w/w%), and so on.</li>
<li>
Concentration can be used for mixtures in solid, liquid, and gas phases.</li>
</ul>

Question 25

<p>

| Explain the concept of the <strong>conjugate acid-base pair</strong>.</p>

Answer 25

<ul>
<li>
The <strong>conjugate acid-base pair</strong> concept develops the idea that weak acids and bases are always paired with their corresponding bases and acids.</li>
<li>
The conjugate acid of any base has one more H+ than the base.</li>
<li>
With this concept, the relative strengths of the conjugate acid and conjugate base are inversely related and can be expressed by the equation:</li>
</ul>

<p>
K_a &times; K_b = K_w</p>

Question 26

<p>

| Describe the concept of a <strong>covalent bond</strong>.</p>

Answer 26

<ul>
<li>
The basic idea of a<strong> covalent bond</strong> is that sharing a pair of electrons imparts stability to a pair of atoms; this stability is called a <em>bond</em>.</li>
<li>
The sharing of electrons enables different atoms, including some metals of moderat electronegativity, to obtain a statble electron configuration without the formation of ions.</li>
<li>
Stable electron configurations are often those that mimic the electron configuration of the noble gases.</li>
</ul>

Question 27

<p>

| Explain the concpets behind <strong>random error</strong> and <strong>determinate (systematic) error</strong>.</p>

Answer 27

<ul>
<li>
All experimental measurements have error associated with them.</li>
<li>
<strong>Random error</strong> is associated with the error of estimating the last digit of a measurement, usually in the process of extending the scale by one digit. This error cannot be avoided, and if a series of replicate measurements is made, the error of the average is close to zero.</li>
<li>
<strong>Determinate (systematic) error</strong> is due to procedural or instrumental errors that can be corrected and eliminated. This includes improper use of instruments, poor experimental technique, incorrect calculations, and/or incorrectly calibrated instruments. Once corrected, all errors should be random.</li>
</ul>