A catalyst is a substance that increases the rate (speed) of a chemical reaction but is recovered unchanged at the end of the reaction.

Every atom of each element must be accounted for.  You cannot start with 3-atoms of say carbon and end up with only 2-atoms of carbon. There must be 3-atoms (Law of Conservation of Matter).

The equation must be balanced so that all the atoms of each element are accounted for.

Example:

There are 2-atoms of hydrogen and 2-atoms of oxygen to start with but while there is 2-atoms of hydrogen in the product, there is only 1-atom of oxygen.  The chemical equation is not balanced.

A balanced equation end up with all the pieces it began with.

To balance the chemical equation we simply change the coefficient (number molecules) of each compound until it balances.  We can change any coefficient but we can never change the actual compound.

Now we have a balanced equation.  We are starting with 2-molecules of H₂ (total of 4-atoms) and 1-molecule of O₂(total of 2-atoms).  We end with 2-molecules of H₂O (4 -atoms of hydrogen and 2-atoms of oxygen).  The two sides have the same number of atoms of each element.  The chemical equation is balanced.

Calculation of Formula or Molecular Mass

To determine the molecular or formula mass of a compound (also referred to as the atomic mass unit; amu), multiply the number of atoms of a particular element in a compound by the atomic mass of that element, then add the partial molecular masses together to determine the total molecular mass of the compound (right click here to download the mass calculations tutorial program to practice formula mass).

Example:

Calculate the molecular mass of H₂O.

Molecular mass is a term which applies only to compounds that exist as molecules and are held together by covalent bonds. Formula mass is a term which applies to compounds that exist as ions and form ionic bonds. These ionic compounds are referred to as formulas.

Calculation of Percent Composition of Compounds

First determine the amu or mass of various elements in the compound. Determine the total amu or mass of the compound.

Divide the amu or mass of each element by the total mass and multiply by 100 (right click here to download the mass calculations tutorial program to practice formula mass).

GAS LAWS

Characteristics of Gases

• Expansion - gases expand indefinitely and uniformly to fill all the space in which they are placed.
• Indefinite Shape and Volume - gas has no definite shape or volume, but will fit the vessel in which it is placed.
• Compressibility - gases can be highly compressed.
• Low density - density of gases is very low and, therefore, measured in grams/liter (g/l)
• Diffusion - two or more different gases will normally mix completely and uniformly when in contact with each other.

Kinetic Theory

The kinetic theory states that heat and motion are related, that particles are in motion to some degree, and that heat is an indication of motion.

• Gases are composed of very small particles called molecules. The distance between particles is very great compared to the size of the molecule. The total volume of all the particles is a small fraction of the entire space occupied by the molecules. Therefore, the volume of a gas is mostly empty space.

Because of this large empty space, gases are

• very compressable
• low density

• No attractive forces exist between the molecules in a gas.

• The molecules are in a state of constant, rapid motion, with each other and the walls of the container.

• All collisions are perfectly elastic (no loss of energy in collision) therefore, the molecules do not slow down after collision.

• The average kinetic energy is proportional to the temperature in kelvins. The higher the temperature, the more motion.

The above conditions are called IDEAL GASES.

Pressure of Gases

Pressure is defined as force per unit area (force/area).

STP (standard temperature and pressure) - the conditions are 0 ^oC and 1 atm.

Units of pressure

• 14.7 psi = 1 atm
• 76.0 cm Hg = 1 atm
• 760 mm Hg = 1 atm
• 760 torr = 1 atm
• 1.013 x 10⁵ Pa = 1 atm

Boyle's Law

Boyles Law - at constant temperature, the volume is inversely proportional to the pressure.

As the pressure increases, the volume decreases proportionally.

As the pressure decreases, the volume increases proportionally.

Example:

If the pressure doubles, the volume halves.

If the volume halves, the pressure doubles.

Charles's Law

Charles Law - at constant pressures the volume is directly proportional to the temperature.

As the temperature increases, the volume increases proportionally.

As the temperature decreases, the volume decreases proportionally.

Example:

If the temperature doubles, the volume doubles.

If the temperature halves, the volume halves.

Gay-Lussac's Law

Gay-Lussac Law - at constant volume, the pressure is directly proportional to the temperature.

As the temperature increases, the pressure increases

As the temperature decreases, the pressure decreases

Example:

If the temperature doubles, the pressure doubles.

If the pressure halves, the temperature halves.

Dalton's Law of Partial Pressures

Daltons Law of Partial Pressures - each gas in a mixture exerts a partial pressure equal to the pressure it would exert if it was the only gas present.  The sum of all the partial pressures of all the gas is equal to the total pressure.

Example: A mixture of gases has the following partial pressures: H2 = 3 atm  N2 = 2 atm  O2 = 10 atm.  Calculate the total pressure of the system.

SOLUTIONS

A solution is a homogeneous mixture of two or more substances.

Some types of solutions are familiar.  For example, salt solution is salt dissolved in water.  A gaseous solution we all use is air which is a solution of various gases like oxygen, nitrogen, carbon dioxide, water and other gases.  It is possible to have solid solutions.  Metal alloys are solid solutions.  An example is brass which is a solution of copper and zinc.  Two liquids may dissolve to give a liquid solution.  Examples include gasoline, alcoholic beverages, vinegar, bleach etc.

A solution is generally composed of two substances.

The solute is the substance being dissolved and is in smaller quantity.

The solvent is the substance that dissolves the solute and is in larger quantity.

Properties of Solution

• Solutions are homogeneous mixtures. The properties are uniform throughout the mixture.
• In a solution, proportions of solute and solvent can be varied. That is, the amount of solute and solvent can be changed.
• The solute is not visible to the unaided eye. This is because the solute breaks down to ionic or molecular size.
• The solute does not settle out of solution.
• The solute can not be filtered out of solution.
• The solute can be separated from solution by changing temperature. That is, by freezing or boiling, or evaporation.

Factors That Affect Solubility

What determines if a solute will dissolve in a given solvent?

• The nature of solute and solvent. " LIKE DISSOLVES LIKE".  Water dissolves salts, but not oil or gasoline.
• Temperature:  the higher the temperature, more solute will dissolve in a given solvent (opposite for gases dissolved in liquids).
• Pressure:  pressure has little or no effect on the solubility of a solid or liquid, but the solubility of gases is greatly affected by pressure (Henry's Law).

Henry's Law

Henry's Law -   the solubility of a gas in a liquid is directly proportional to the applied pressure.

This means that higher the pressure, the more gas dissolves in a liquid.

Carbonated soft drinks are examples of a gas (carbon dioxide) dissolved in a liquid (water) at high pressure.  When a soft drink can is opened, the dissolved carbon dioxide escapes ( bubbles). If this can is left open for some time, the drink goes "flat".  All of the carbon dioxide has escaped.

What can be done to increase the rate at which a solute dissolves in a solvent?

Factors That Affect the Rate of Dissolution

• Particle size:  the smaller the particle size, the faster the rate of dissolution. Powders dissolve faster than large lumps.
• Temperature:  the higher the temperature, the faster is the rate of dissolution.  It is easier to dissolve sugar in a glass of hot tea than it is in a glass of iced tea.
• Rate of stirring:  stirring or agitation of the solute in a solvent causes it to dissolve faster.
• Concentration:   if a solution has some solute dissolved in it, the rate of dissolving additional solute will be slower.

Relative Terms for Expressing Solute Concentration

Solubility is the amount of solute that can be dissolved in a given amount of solvent.

Non-quantitaive terms: this means it does not contain any numbers but is based on relative strengths.

• Concentrated solution: the solution contains more solute than a dilute solution.
• Dilute solution: the solution contains less solute than concentrated solution.
• Unsaturated solution: the solution can dissolve more solute.
• Saturated solution: the solution contains the maximum amount of solute the solvent can dissolve.
• Supersaturated solution: the solution contains more solute than a saturated solution. This is very unstable condition and slight disturbance causes the excess solute to settle out.

Note: All of these are temperature dependent.  A solution that is saturated at 25^oC may be unsaturated at 45^oC.

Quantitative Terms for Expressing Solute Concentration

Solutions are made up of solute and solvent.  Concentration terms are basically a relationship between the amount of solute present and the amount of solution.  Terms like percent solution and parts per million (ppm),  each describe the concentration of a solute in a solution.  The particular concentration term to use is dependent on the eventual use of the solution.

Percent, parts per thousand (ppt), parts per million (ppm) and parts per billion (ppb) are useful for describing concentrations on a mass or volume basis.  Concentrated acids and bases are given in percent concentration.

Percent Concentration of Solute

The concentration of a solution can be expressed as the percentage of the solute based on the entire solution.  The solution is the total of the solute and the solvent.

The concentration can be expressed as the percent by mass (right click here to download the solution calculations tutorial program to practice percent solutions).

Example:  20 grams of NaCl is is mixed with 180 ml of water, what would be concentration as %NaCl by mass in this solution? First, write the formula for % mass which in this case would be the % NaCl since NaCl would be the solute and solutions are named after the solute. Next, make a table of all the data needed for the formula. % NaCl (w/w) =  ? mass water  =  180 g   (remember that the density of water = 1.0 g/ml) mass NaCl  =  20 g Next, plug the numbers into the formula.

When dealing with two liquids, the concentration can also be expressed as the percent by volume (click here and choose solutions to practice percent solution ).

 

Example:  200 ml of antifreeze is is mixed with 0.8 liters of water, what would be % concentration of antifreeze  by volume in this solution? First, write the formula for % volume which in this case would be the % anitfreeze since the antifreeze would be the solute and solutions are named after the solute. Next, make a table of all the data needed for the formula. % antifreeze (v/v)  =   ? volume water  =  800 ml  (remember 0.8 liters = 800 ml) volume antifreeze    =  200 ml Next, plug the numbers into the formula.

To make a percent solution, multiple the amount you wish to make by the percent solution and divide by 100.

Example:  Make 500 ml of a 20% ethanol solution. To make a percent solution, multiple the amount you wish to make by the percent solution and divide by 100. Amount needed is 500 ml and the concentration needed is 20%. Take 100 ml of ethanol and dilute it with water (400 ml) to make a total volume of 500 ml.  This will give 500 ml of a 20% ethanol solution.

Parts per Million (PPM)

The concentration can be expressed as parts by mass of solute per million parts of solution.

 

Example:  A drinking water sample has 15 mg of Pb per 500 ml of solution.  Calculate the ppm of Pb in the water. First, write the formula for ppm which in this case would be the ppm of Pb since lead would be the solute and solutions are named after the solute. Next, make a table of all the data needed for the formula. This is a dilute solution so assume the density of the solution is that of water. ppm Pb  =   ? mass Pb  =   15 mg mass solution  =   500 ml  =  500 g  =  500,000 mg Next, plug the numbers into the formula.