Chapter 1 & 2 Notes ­15.notebook

1

September 21, 2015

Jul 22­5:37 PM

Chemistry is the study of substances in terms of

Composition what is it made of

Structure how is it put together

Properties what characteristics does it have

Jul 22­5:37 PM

Reactions and Energies

Physical how does it change form but stays the same substance – small energies (joules)

Chemical how does it behave with other substances and changes to new substances – larger energies (kilojoules)

Nuclear how does its nucleus behave – really large energies (megajoules); these involve transmutations – converting one element into another

Jul 22­5:43 PM

Scientific Method

• Making observations

o Facts obtained by observing and measuring events in nature

• Writing a hypothesis

o A statement that explains the observations

• Doing experiments

o Procedures that test the hypothesis

Jun 24­3:43 PM

Law:

Theory:

Jun 24­3:41 PM Jun 24­3:43 PM

Activities of Science

• Assembling important facts and observations.

• Organizing the information and seeking regularities, or patterns, in it.

• Trying to explain why the regularities exist ­ construct models.

• Communicating the findings and their explanations to others. Verifying data!

Chapter 1 & 2 Notes ­15.notebook

2

September 21, 2015

Jun 24­3:44 PM

SOME KEY IDEAS

Conservation Laws

initial(i) = final (f)

Mass: mass of reactants = mass of products (Lavoisier)

Energy: energy is neither created or destroyed, only changes form

Charge: electrons lost (oxidation) = electrons gained (reduction)

Jun 24­3:45 PM

MATTER

Matter ­

anything that occupies space and has mass.

Mass ­

the quantity of matter possessed by a body or a measure of its inertia; can be measured two ways:

1) gravitational mass (weight) ­ the result of mass and the pull of gravity. Mass and weight are not the same ­ weight varies as gravitational attraction varies. The terms are used interchangeably since the earth's gravitational attraction is essentially constant. Weight is a force due to gravity and is measured with a scale. Mass is an inherent property of matter and is independent of gravity. Mass is measured using a balance.

Let’s compare balances and scales on the Earth and the Moon. We will weigh a 5 kilogram ball using each instrument first on Earth and then on the Moon. But first, what is the numeric equality relating mass (kilograms) and weight (pounds)?

1.0 kg = ______ lb

Sep 9­10:01 AM

On Earth:

balance = scale =

on Moon:

balance = scale =

Aug 5­12:51 PM

Most of the mass of everyday objects is in protons and neutrons, and a little bit more in electrons. Identifying the mass of an object basically comes down to saying how many nucleons are in it. A nucleon is either a proton or a neutron. A person having a mass of 75 kg has about 50, 000, 000, 000, 000, 000, 000, 000, 000, 000 nucleons. How would we express this in scientific notation? (Note: don’t confuse this figure with the national debt you and your children and probably your grandchildren will have to pay).

Aug 2­12:24 PM

2) inertial mass ­

a measure of the resistance to a force that tends to change the motion of a body.

"A body at rest tends to remain at rest. A body in motion tends to continue moving in a straight line at a constant speed ­ even when an outside force is pushing or pulling on it." (Newton's First Law of Motion). All bodies with mass have this resistance to forces that tend to change their motion. The greater the force needed to overcome this resistance the greater the mass of the body.

Aug 2­12:25 PM

Substance ­

refers to any particular variety of matter that always has the same properties and composition, regardless of how and where a sample of it is obtained. Samples of a substance are said to be homogeneous ­ the properties and composition are the same throughout the sample.

Properties ­

the characteristics by means of which one kind of matter may be distinguished from any other kind of matter. There are two major types of properties:

1) physical properties ­ those characteristics that may be observed without changing the chemical composition of a substance ­ e.g. color, taste, odor, density, melting point.

2) chemical properties ­ those characteristics that describe how a substance interacts (or fails to interact) with other substances to produce new substances ­ e.g. activity, combustibility.

In addition, there are intensive and extensive properties:

extensive properties ­ depend on how much of a particular sample is on hand; examples include volume, weight and mass

intensive properties ­ properties that do not depend on the size of the sample; examples include melting point, boiling point and density

Chapter 1 & 2 Notes ­15.notebook

3

September 21, 2015

Aug 2­12:26 PM Aug 2­12:27 PM

Elements ­

substances which cannot be decomposed into simpler substances by means of an ordinary chemical change; e.g. gold, iron. All atoms of a given element have the same number of protons (atomic number) in their nucleus.

• the elements on the Periodic Table are arranged according to increasing atomic number.•• the Periodic Table consists of 18 vertical columns termed groups or families; the elements within a group have similar chemical properties (because they have the same number of electrons in their outermost shell (level) ­ called valence electrons.•• the Periodic Table consists of 7 horizontal rows called periods. The period number indicates the level (shell) in which an atom’s valence electrons are found.•• most elements are metals; the metalloids or semimetals occur along the “steps” separating the metals from the nonmetals ­ these include B, Si, As, and Te.•• most elements in nature occur as monatomic atoms ­ that is they occur as single atoms bonded to each other in a sample of that element; a few exist as diatomic molecules ­ two atoms bonded together ­ these are H2, N2, O2, F2, Cl2, Br2, and I2; in addition phosphorous occurs as a tetratomic molecule (P4) and sulfur as an octatomic molecule (S8).•• most elements occur as solids at room temperature and pressure. The only two liquids are mercury (Hg) and bromine (Br2); the elements which are gases include H2, N2, O2, F2, Cl2, and the Noble gases (Group 18 ­ He, Ne Ar, Kr, Xe, Ar and So.

Aug 2­12:29 PM

II) Compounds ­

substances made up of two or more elements chemically combined, e.g. water, carbon dioxide. Compounds can be decomposed by ordinary chemical means into simpler substances. For example, heating sugar will decompose it to water and carbon.

Another way in which a compound like a salt (consisting of a metal chemically combined with a nonmetal) can be decomposed is by a process called electrolysis. If we pass an electric current through a molten (melted) salt, it will break the compound into the elements that make it up; one element at the positive electrode (designated the anode) and the other at the negative electrode (designated the cathode)

Can you write a chemical equation representing this reaction?

Oct 4­8:28 AM

The elements in a compound are combined in a definite proportion by mass. This is a statement of the Law of Constant Composition. To determine the percentage composition by mass of the elements in a compound, we first determine the mass of the compound and then the percent of each element is the mass each element contributes over the total mass times 100. Water, for example has a total mass of 18 amu (atomic mass units). The percentage of hydrogen = 2/18 X 100 = 11.2% and oxygen = 16/18 X 100 = 88.8%. Another way of saying tis is that in 100. grams of water we have 11.2 g of hydrogen and 88.8 g of oxygen.

Jul 22­5:49 PM

Problem:

Determine the percentage by mass of each of the elements in any sample of carbon dioxide:

Jul 22­5:48 PM

Mixtures ­

consist of two or more substances physically combined, each of which retains its individual properties. Hence, the properties of a mixture are intermediate between those of its components. Mixtures may be homogeneous ­ i.e. a solution of salt water, or they may be heterogeneous ­ having nonuniform composition throughout ­ e.g. soil, concrete.

• the composition of a mixture is not fixed by mass (as in the case of compounds).•• the components of a mixture are held together by physical means; the elements in a compound are held together as a result of a chemical reaction ­ chemical bonds•• the components of a mixture can be separated by means; those of a compound require chemical methods

Chapter 1 & 2 Notes ­15.notebook

4

September 21, 2015

Aug 2­12:35 PM

Physical methods of separation include:

distillation ­ separation on the basis of difference in boiling points.

Aug 2­12:38 PM

filtration ­separation of a liquid from a solid (on basis of size)

centrifugation ­ separation on the basis of difference in density

extraction ­ separation based on difference in solubility

chromatography ­ separation on the basis of adherence to a mobile vs stationary phase

in illustration on the next page, for example, component A has a greater adherence for the stationary column while component B has a greater adherence to the mobile solvent

Oct 4­8:31 AM

Matter can undergo two kinds of changes:

1) physical changes ­ those that produce new physical properties without altering the chemical properties ­ no new substance is formed; e.g. melt ice, magnetize iron.

Note that for most substances, the solid phase is the densest phase. The particles making up the substance are held together by “electrostatic” (meaning plus – minus attractions) forces of attraction in a crystalline arrangement. This means the particles can only move (i.e. vibrate) in fixed positions in the crystal. In the liquid phase the forces of attraction are a little weakened and the substance is a little less dense. The particles are still hold to each other but now they can “move” around each other (i.e. they can also rotate). In the gas phase the forces of attraction are not effective and the particles are separated and fly around randomly. The gas phase is the least dense. Can you think of any exceptions to the phase­density relation described here?

Oct 4­8:35 AM

Oct 4­8:35 AM

H2 and O2 gases H2O gas H2O liquid

Key hydrogen

oxygen

Aug 2­12:39 PM

2) chemical changes ­ those that produce new substances that differ in chemical properties and composition from the original substances, e.g. iron rusts, milk sours, paper burns.

Chapter 1 & 2 Notes ­15.notebook

5

September 21, 2015

Aug 13­1:24 PM

The Nitrogen Cycle

Nitrogen is an essential component of proteins and the genetic material that makes up DNA (deoxyribonucleic acid), and a constant supply is vital for all living organisms. Although 78% of the earth’s atmosphere is composed of nitrogen gas (N2), plants and animals cannot use this nitrogen directly. Atmospheric nitrogen must be converted to other nitrogen compounds before it can be absorbed through the roots of plants. This change is achieved by nitrogen fixation, a process carried out by specialized bacteria that have the ability to transform atmospheric nitrogen into ammonia (NH3). Some nitrogen­fixing bacteria live in soil. Others live in nodules on the roots of leguminous plants such as peas, beans, clover, and alfalfa. The ammonia produced in root nodules is converted into a variety of nitrogen compounds that are then transported through the plant as needed.

Aug 13­1:26 PM

Aug 13­1:27 PM

Another means by which atmospheric nitrogen is converted to a usable form is lightning. The electric discharges in lightning cause nitrogen and oxygen in the atmosphere to combine and form oxides of nitrogen, which in turn react with water in the atmosphere to form nitric acid (HNO3). The nitric acid, which reaches the earth’s surface dissolved in rainwater, reacts with substances in soil and water to form nitrates (NO3

­) which are directly absorbed through plant roots. Compared to biological fixation, lightning accounts for only a small fraction of the usable nitrogen in soil.

Although some trees and grasses can absorb ammonia produced by nitrogen­fixing bacteria directly from the soil, most plants can only use nitrogen that is in the form of nitrates. The transformation of ammonia into nitrates is carried out by specialized soil bacteria in a process known as nitrification.

Plants convert ammonia or nitrates that they get from the soil or root nodules into proteins and other essential nitrogen­containing compounds. Animals get their essential nitrogen supplies by eating plants. When plants and animals die and decompose, the nitrogen­containing compounds in their tissue are broken down by decomposers; ammonia is eventually formed and returned to the soil. Nitrogen is also returned to the soil in animal wastes. Both urine and feces have a high content of nitrogen­containing compounds. In this way, nitrogen is continually cycled through food chains.

Aug 13­1:29 PM

Not all the ammonia and nitrates that are formed in soil by the process just described become available for plants. Both ammonia and nitrates are very soluble in water. As rainwater percolates downward through the ground, these compounds are leached out of the topsoil. They are often carried away in runoff into nearby streams, rivers, and lakes, where they are recycled through aquatic food chains. Another process that removes nitrates from the soil is denitrification, in which bacteria carry out a series of reactions that convert nitrates back to nitrogen gas.

In a few locations around the world, nitrates have accumulated in large mineral deposits. In Chile, for example, as mountain streams originating in the Andes Mountains flowed across dry, hot desert toward the sea over thousands of years, much of the water evaporated, leaving behind huge deposits of sodium nitrate. Sodium nitrate is called saltpeter and countries have fought wars over it.

In the natural environment, a balance is maintained between the amount of nitrogen removed from the atmosphere and the amount returned. However, because most soils contain an insufficient amount of nitrogen for maximum plant growth, farmers frequently apply synthetic inorganic fertilizers containing ammonia and nitrates. As a result of runoff from fertilized farmland, the extra nitrogen­containing compounds reaching rivers and lakes may upset natural balance, sometimes with damaging consequences for the environment. The production of these fertilizers is one of the most important chemical processes in history and will be discussed later in the year as well as the water pollution resulting from fertilizer runoff.

An alternative to the use of synthetic fertilizer for replenishing nitrogen in the soil is to plant a nitrogen­fixing crop, such as clover, and plow it back into the soil. Another method is to spread manure and allow the natural soil bacteria to degrade it and release nitrogen­containing compounds that plants can absorb.

Sep 9­11:24 AM

Demonstration

Hydrogen

Symbol: HAtomic number: 1 Boiling point (K): 20.4 Atomic mass: 1.00795 Melting point (K): 13.4

Density: 0.0899 g/L

Of every 1,000 atoms in the Earth’s crust, about 150 are hydrogen. Because of its low atomic mass, hydrogen constitutes less than one percent of the mass of the Earth’s crust. However, it appears to be the most abundant element in the universe. More than eighty­eight percent of all atoms in the universe are hydrogen, and about ninety percent of the mass of the Sun is hydrogen. On Earth, hydrogen occurs in thousands of compounds, from water and hydrocarbons to acids and proteins.Hydrogen is almost nonexistent in the elemental form in nature. Because of its low density, hydrogen molecules do not remain in the atmosphere for very long – they simply rise and drift into space. Any unreacted hydrogen produced in the laboratory today may be thousands of miles away by tomorrow.

Its small size and mass make hydrogen unique among the elements in properties and reactions. Hydrogen was discovered in 1776 by Henry Cavendish when he treated active metals with dilute nonoxidizing acids. Other chemists soon began experimenting with hydrogen and amazed themselves and others with its properties. Pilatro de Rozier filled his lungs with hydrogen and set fire to the gas coming out of his mouth. When he repeated the experiment with a mixture of air and hydrogen, an explosion resulted. Cavallo, a Frenchmen, filled soap bubbles with hydrogen and ignited them. Joseph Black, Chair of Chemistry at Edinburgh, astounded his dinner guests by filling a calf’s bladder with hydrogen and allowing it to float to the ceiling.

Sep 9­11:24 AM

Reaction:

Symbols:

Names:

Chemical or physical reaction?

Masses:

Reaction of hydrogen gas in “air”:

Is this a physical or chemical change? Explain.

Chapter 1 & 2 Notes ­15.notebook

6

September 21, 2015

Sep 8­11:43 AM