Matter

I. Definition of Chemistry

See also: printout.

Chemistry is the study of matter and how it changes. Matter, in turn, is anything with volume and shape—anything that is composed of atoms.

There are five commonly accepted branches of chemistry:
  • Inorganic chemistry, which covers all elements and interactions except those of carbon; this branch is the focus of this class.
  • Organic chemistry, which deals with carbon compounds and their interactions.
  • Analytical chemistry, which measures natural and artificial materials.
  • Physical chemistry, which takes place at the subatomic level and combines nuclear physics and chemistry.
  • Biochemistry, the chemistry of biological systems.



II. History of the Atom

See also: Mr. Stine's atomic-theory page.

Democritus

The first atomic theory was proposed by Democritus (ca. 400 BC); the word atom comes from the Greek for "uncuttable," for Democritus's atoms were to be the smallest possible particles ("corpuscles") of matter. His atomic theory ultimately failed to challenge the reigning orthodoxy of Aristotle's four elements: air, fire, earth, and water.

Dalton

In the late eighteenth century, English schoolteacher John Dalton revived Democritus's dormant atomic theory. Though he produced only a scientific idea, not a formal theory, the five tenets of his atomic theory formed the basis of modern atomism:

  1. All matter is composed of atoms.
  2. Atoms are indestructible, spherical objects.
  3. Different elements are composed of different atoms.
  4. Atoms are rearranged, not destroyed, when reacted—now known as the law of conservation of mass or matter.
  5. Atoms combine to form compounds:
    1. Law of definite proportions. Compounds are composed of different atoms in fixed ratios with one another.
    2. Law of multiple proportions. Different ratios produce different compounds.

Thompson

In 1897, Cambridge physicist J. J. Thompson connected a battery to a gas-filled cathode-ray tube; the resulting greenish haze inside demonstrated the existence of the electron. From this result, Thompson posited what would become known as the plum-pudding or chocolate-chip cookie model of the atom:

plum_pudding_model.gif
Credit: nobelprize.org.

The plum-pudding model was not readily accepted until 1905, when, during his "annus mirabilis," the young Albert Einstein proved the existence of the atom in his Nobel Prize-winning paper on Brownian motion. The grains of pollen on the surface of the water were surely in constant motion because they were being jiggled by something!

Rutherford

In 1909, Thompson's former student Ernest Rutherford, now at the University of Manchester, conducted what would be known as the gold-foil experiment. Much as Einstein had reasoned that something had to account for the jiggling of grains of pollen suspended in water, Rutherford sought to understand why alpha particles fired at a thin sheet of gold foil were reflected back at angles much steeper than could be explained by Thompson's plum-pudding model. He wrote:

  • It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive centre, carrying a charge.

That "minute massive centre" was the atom's nucleus, around which its electrons orbited. The atom itself was mostly empty space, and the collisions Rutherford observed were the result of the alpha particles "bouncing off" the atom's electromagnetic field. Thus was born the solar-system model of the atom.

Later, the bell-jar experiment revealed the existence of positively-charged protons in the nucleus, held together by the massive and neutrally-charged neutrons.



Matter and Change
Physical vs. Chemical Properties
Matter can be classified into physical properties and chemical properties. Physical properties are used to describe matter. Physical properties are characteristics that can be observed or measured without changing the substance. These characteristics include mass, volume, boiling point, melting point, density, temperature, and color. Measuring of physical properties will not change the structure of the molecules.
Chemical properties are a substance’s ability to transform into different substances via a chemical change. They describe the potential to undergo a chemical change under certain conditions. Chemical properties can only be determined by changing the identity of a substance. Chemical properties include electronegativity, pH balance, flammability, enthalpy of formation, and heat of combustion.

Intensive vs. Extensive Properties
Physical properties of matter can be broken down into intensive properties and extensive properties. Intensive properties are not dependent on the amount of matter present. For example, the boiling point of water is 100°C no matter how much water is being boiled and the freezing point is 0°C for any amount of water used. Other examples of intensive properties are density, conductivity, color, and melting point. Extensive properties depend on the amount of matter present. For example, the mass of a substance increases as more of that substance is added. Other extensive properties include volume and energy.
Here is a link to a flow chart which catagorizes the different properties of matter.
Metals
A metal is an element that is a good conductor of heat and electricity. All metals are solids at room temperature (except mercury, a liquid) and have high melting points. They also have high tensile strength meaning they can resist breaking when pulled. Because of this, most metals are also malleable, meaning they can be turned into thin sheets, and ductile, meaning they can be drawn into wire. They also have metallic luster, or shininess. Metals generally have high densities, electric conductivities, and thermal conductivities.
Nonmetals
A nonmetal is an element that is a poor conductor of heat and electricity. Many nonmetals are gases at room temperature. At room temperature, Bromine is a liquid and carbon, phosphorus, selenium, sulfur and iodine are all solid. These solids are generally brittle and not malleable or ductile. Nonmetals also have high electronegativities and low mellting points.
Metalloids
Separating metals and non metals on the periodic table are metalloids. A metalloid is an element that has characteristics of metals and nonmetals. All metalloids are solids at room temperature. Some of their characteristics are between those of metals and nonmetals. For example, metalloids are not as malleable as metals but are more malleable than nonmetals. Also, they are semiconductors, which means they are not as conductive as metals but more conductive than nonmetals.





VI. Density

Density = Mass / Volume

That's really it:

external image mathtex.cgi?%5Crho=%5Cfrac%7Bm%7D%7BV%7D

If you must know, common units in chemistry for density are kilograms per liter (kg/L) and grams per milliliter (g/mL). You've been dealing with density since middle—nay, elementary—school; by now you know that density's only redeeming quality is that it allows you to make density columns. From eighth grade, you know that density is an intensive property (see above) that can be useful in determining the identity of an unknown substance.

If the external image mathtex.cgi?%5Crho looks like an upside-down and backwards external image mathtex.cgi?d, that's fine too.

Specific Gravity

Specific gravity is the relationship of the density of a substance to the density of water at a given temperature and pressure, typically 4°C and 1 atmosphere, respectively.

external image mathtex.cgi?SG=%5Cfrac%7B%5Crho_%7Bsubstance%7D%7D%7B%5Crho_%7BH_2O%7D%7D

Change


Physical vs. Chemical Changes
A physical change is a change in the appearance of a substance while a chemical change is a change in the connectivity of atoms within a substance.
external image matter_intro_2_240.gif
A change that does not involve changing the identity of a substance is called a physical change. Physical changes do not change the chemical nature or molecular build up of a substance. Any change of state—solid to liquid, liquid to gas, gas to liquid, liquid to solid—is a physical change because it changes the state of the substance, not the identity. For example, water has the same molecular structure as ice, H2O; meaning going from water to ice is a physical change. Examples of physical changes are boiling, melting, freezing, cutting, and breaking.
A change that involves changing the identity of a substance is a chemical change. When a chemical change occurs, a new substance is created from the original substance reacted. For example, combusting methane creates carbon dioxide and water.

CH4 + 2O2 CO2 + 2H2O
The substances that react in a chemical change are called the reactants. The substances formed by a chemical change are called the products. Because the products are different from the reactants, the identity of the original substance was changed making this reaction a chemical change. Examples of chemical changes include combusting, burning, and rusting.


Indicators of a Chemical Change
A Chemical Change is a process that results in a difference in molecular or atomic composition. It can also be called a chemical reaction. This occurs when one or more substances are converted into different substances. A chemical reaction looks like this:
C + O2 à CO2
(Carbon plus Oxygen yields Carbon Dioxide)
The substances to the left of the arrow are called reactants and the substances to the right of the arrow are called products.
There are six signs that can easily indicate when a chemical change has occurred.
Gas is Produced
The bubbles that form when an antacid is dropped into water is one example. Also, bubbles of carbon dioxide gas form when vinegar is mixed with baking soda.
Temperature Change
A temperature change indicates that energy in the form of heat has been absorbed or released. BUT this is not necessarily a sign because some physical changes also release heat.
Color Change
Leaves changing color in autumn are a chemical change.
Light is Produced
When light is produced it is the release of energy that creates the light. Fireworks are an example of a chemical change that produces light. BUT this is not necessarily a sign because some physical changes also release light.
Formation of a Precipitate
There are many chemical reactions that take place between two liquids or aqueous solutions. When a solid forms in the products, a chemical change has occurred.
Odor Forms
When food spoils, it undergoes a chemical change and a new (smelly) odor forms.

Reading the Periodic Table

The current periodic tables hosts 109 elements, arranged in order of increasing atomic number (the number of protons found in the nucleus). On each ‘card’ for each element is also its atomic mass, which is an average weight of the protons, neutrons, and electrons of the specific element. Each element on the table also has a specific symbol to identify it as, for example, the symbol for sodium is Na.
untitled.gif
The elements on the periodic table are also arranged by their characteristics (metals, nonmetals, metalloids, and noble gases), families/groups, and period. The vertical columns on the table are known as groups/families. Elements of the same group/family often time have very similar characteristics (such as electron configuration). The horizontal rows on the table are periods. The d-block (transition metals) of similar periods have similar characteristics.

Classification of Matter

States of Matter

You can melt it, freeze it, cut it, or boil, but a substance’s chemical properties won’t change. Take the element lead as an example: Assume you have a bar of lead, which has a density of 11.34 g/cm. Now break the bar in half then find the mass and the volume. The density should be the same, this is because you aren’t changing the chemical characteristics of the element (including boiling point, melting point, and density). Breaking the bar of lead was only a physical change; it did not disturb the identity of the element.

nnbv.gif

Another example of a physical change is changing a substances state of matter; sold, liquid, gas, and plasma are all different states of matter that substances go be in, depending on the amount of energy it has/gains.

When matter is brought to temperatures near absolute zero the molecules eventually stop moving entirely; this state is know as the Bose-Einstein Condensate. The discovery of this state of matter was made by Satydenra Nath Bose with assistance from Albert Einstein in 1924.

Solid is the state of matter where electrons are packed closely together, preventing little to no movement past one another. In this state the molecules are packed tightly together, making the substance take a definite shape. Think of trying to fit an ice cube into a narrow graduated cylinder. Since the substance(s) is/are solid it won’t change it’s shape to regardless of whether or not it would fit into the container.

An energy level up from a solid state of matter is liquid. Think of the same ice cube, now having been sitting in a beaker by the window all day. The energy add to the ice cube from the sun coming through the window causes it to melt, the molecules now move past each other and assume the shape of their container (in this case it is the beaker).

The beaker of water is now placed on a hot plate. After a few minutes the temperature of inside the beaker reads 100 degrees Celsius. At this point water vapor begins to escape from the open beaker, leading us to the next state of matter; gas. By placing the beaker on the hot plate, more energy is added to the system and the molecules move even further away from each other and at faster speeds. Like liquids, gases can assume the shape of a given container.

The next state of matter was discovered in the 1920s: plasma. Plasma is a state of matter that is reached when a gas is heated enough so that each molecule is stripped of it’s electrons; resulting in ionized gas. Real world examples of plasma include lightening, neon signs, and the earths upper atmosphere (ionosphere).


Mixtures & Pure Substances

Matter can be divided into one of two major groups: pure substances or mixtures. A pure substance has a constant composition. Imagine a bowl of sugar; if you take a sample from the bowl, it will have the same characteristics (such as melting point, density, etc) as well as the same composition (each will have the chemical composition of C12H22O11).

“Regardless of the method of separation, a pure compound will always contain the same elements, in the same proportion by mass.” - Dalton

Imagine that same bowl of sugar, now add 2 cups of water to it and stir. The sugar should dissolve in the water and create what is called a solution. Solutions (or homogeneous mixtures) are a type of mixture that can be separated without breaking the chemical bonds present (you can separate the water and sugar by heating the solution until all the water turns into vapor). Homogeneous mixtures are uniform in composition. Now imagine mixing together salt (NaCl) and water, the result is called a heterogeneous mixture.

Heterogeneous mixtures do not maintain the same composition in samples, because they do not have the same concentration throughout the solution (ex; sat sits at the bottom of a container after being mixed with the water. Taking a sample from the top will result in a lower concentration of salt while taking a sample from the bottom yields the opposite result).

Energy

In chemistry, "energy" is defined as the ability to do work. Basically, work makes something happen. Things like picking up a ball, throwing a ball, and catching a ball all involve work, and therefore, all require energy to do. Energy comes in many different forms, such as light, heat, movement, etc. and is measured with a unit called "Joules." Chemistry involves two types of energy: kinetic and potential.

KINETIC ENERGY--- This is the energy of movement. Any time something is in motion, it possesses kinetic energy. The ball being picked up, caught, and thrown in the above example has kinetic energy. The equation to find the amount of energy (in joules) an object in motion has is: (1/2)mv^2 where "m" is the mass of the object in "kg" and "v" is the velocity of the object in "m/s." The relevance of kinetic energy in chemistry is to the motion of molecules. In fact, the temperature of a substance is just a measure of the average kinetic energy of all of its molecules. Therefore, in chemistry, kinetic energy is the same thing as heat. As molecules move faster, temperature increases. There are three different ways in which a molecule has movement/kinetic energy. They are:


  • Linear Movement---- This is basically the whole molecule moving from one point to another.
  • Vibrational Movement---- This is intramolecular movement; when a molecule vibrates, it's because its bonds are lengthening and shortening.
  • Rotational Movement---- Another movement solely within the molecule; a bond (or more than one) is spinning, so part of the molecule is rotating around and around.

A complete lack of molecular movement/kinetic energy, 0 K (-273 C), is called absolute zero. It is theoretical and cannot be reached.


POTENTIAL ENERGY--- Potential energy is stored energy; it is the energy an object possesses while stationary at one position, the energy of an object being acted upon solely by gravity. The equation to find the joules of potential energy an object has is: mgh where "m" is mass in "kg," "h" is the heigh of the object in meters, and "g" is the acceleration due to gravity, which is 9.8 m/s^2. In chemistry, potential energy is the energy stored in chemical bonds, it is the energy that holds the separate atoms in a molecule together. In chemical reactions, when bonds are broken, energy is released. When bonds are formed, energy is absorbed.

Enthalpy--- Enthalpy (H) is a way to measure energy changes in a system. Enthalpy is calculated by adding together the system's internal energy (U) and the Work done by the system. Work=pressure*volume. H= U+PV Usually, the pressure or the volume of the system doesn't change, so the change in enthalpy is often equal to the change in heat released or absorbed by the system.

Endothermic and Exothermic Reactions
There are two types of chemical reactions: endothermic and exothermic.
Exothermic
There are many different chemical reactions that release energy in the form of heat or light. These are exothermic reactions. Whenever energy is released the reaction can be classified as exothermic. An exothermic reaction may produce heat and may even be explosive. It occurs when energy is released to the surroundings from the system. The reactants of the reaction have more energy than the products, therefore energy is released to the surroundings in order for the products to form. The products of an exothermic reaction are more stable than the reactants. Exothermic reactions feel hot to the touch; the temperature goes up.
The graph of an exothermic reaction looks like this:
external image exo.gif
Ea is the energy required to be released to the surroundings for the products to form.

Endothermic
There are other chemical reactions that must absorb energy. Whenever energy is absorbed the reaction can be classified as endothermic. An endothermic reaction occurs when energy is absorbed from surroundings to the system. The reactants of the reaction have less energy than the products, therefore energy is absorbed by the system in order for the products to form. The products of an endothermic reaction are less stable than the reactants. Endothermic reactions feel cold to the touch; the temperature goes down.
The graph of an endothermic reaction looks like this:
external image endo.gif
Ea is the energy required to be absorbed in order for the products to form.