•Chemical Formulas and Equations

• The products of chemical research have substantially increased food supplies but have also increased the possibilities of pollution. Balancing the benefits and hazards of the use of chemicals requires a knowledge of chemistry and a knowledge of the alternatives.

• Chemical Formulas

• Introduction– Empirical formula

• The simplest whole number ratio of elements in a compound

• Ionic compounds are always shown as empirical formulas.

– Molecular Formula• The actual numbers of atoms in a molecule.

– Structural Formula• Show the relative arrangements of atoms in a molecule

• The name, molecular formula, sketch, and structural formula of some common molecules. Compare the kinds and numbers of atoms making up each molecule in the sketch to the molecular formula.

• If you know the name of an ingredient, you can write a chemical formula, and the percent composition of a particular substance can be calculated from the formula. This can be useful information for consumer decisions.

• Molecular and Formula Weights– The formula weight of a compound is the sum of all of

the atomic weights of the atoms in a chemical formula.– The formula weight of an ionic compound is found by

adding up all of the atomic weights of the atoms in the compound.

– The molecular weight is the formula weight of a molecule

• Percent Composition of Compounds

• Mass percent is the ratio of the atomic weight of all the atoms of a certain element to the total formula of molecular weight of the substance.

• Chemical Equations

• Introduction– Chemical reactions occur when bonds between the

outermost parts of atoms are formed or broken– Chemical reactions involve changes in matter, the

making of new materials with new properties, or energy changes.

– Chemical Reactions are described using a shorthand called a chemical equation

• Balancing equations– Chemical equations show the conversion of reactants (the

molecules traditionally shown on the left of the arrow) into products (the molecules traditionally shown on the right of the arrow).

• A + sign shows that the compounds on the left of the arrow combine to form the product or products on the right of the arrow.

• The arrow is read as “reacts to yield”

• Example– C + O2 CO2

• This reads carbon plus oxygen reacts to yield carbon dioxide

• The charcoal used in a grill is basically carbon. The carbon reacts with oxygen to yield carbon dioxide. The chemical equation for this reaction, C + O2 CO2, contains the same information as the English sentence but has quantitative meaning as well.

– An equation is balanced when the total number of atoms of each element occur the same number of times on both sides of the equation.

– Law of conservation of mass.• Atoms are neither created nor destroyed in a chemical reaction.

• They may, however, be combined differently or converted into energy.

– When balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but you may not change the subscripts.

• Changing the subscripts changes the compound.

• The meaning of subscripts and coefficients used with a chemical formula. The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.

• Tanks like these grow larger as they are filled with natural gas, then collapse back to the ground as the gas is removed. Why do you suppose the tanks are designed to inflate and collapse? One reason is to keep the gas under a constant pressure. The height of each tank varies with the amount of gas inside, so more gas means a greater volume rather than a greater pressure. A rigid gas tank with a constant volume would be under very high pressure when full and very low pressure when nearly empty, which would make it difficult to pump gas into or out of the tank.

– There are four basic steps to balancing a chemical equation.• Write the correct formula for the reactants and the

products in an unbalanced equation.• Inventory the number of each kind of atom on both

sides of the unbalanced equation.• Determine where to place coefficients in front of

formulas to balance the equation.• Take another inventory to determine if:

– The numbers of atoms on both sides of the equation are now balanced.

– The coefficients are in the lowest possible whole number ratios.

– When balancing equations remember the following:• Atoms are neither lost nor gained during a chemical

reaction.• A correct formula of a compound cannot be changed

by altering the number or placement of subscripts.• A coefficient in front of a formula multiplies

everything in the formula by that number.

– A general approach to balancing reactions is:• Look first to formulas of compounds with the most

atoms and try to balance the atoms or compounds they were formed from or decomposed to.

• Polyatomic ions that appear on both sides of the equation should be balanced as independent units with their charge.

• Try both the “Crossover technique” and the use of “fractional coefficients”

• Compare the numbers of each kind of atom in the balanced equation with the numbers of each kind of atom in the sketched representation. Both the equation and the sketch have the same number of atoms in the reactants and in the products.

– Conventions• gas (g)• Liquid (l)• Aqueous solution (aq)• Escaping gas ()• Solid formation ()• Change of temperature ()

• One of two burners is operating at the moment as this hot air balloon ascends. The burners are fueled by propane (C3H8), a liquified petroleum gas (LPG). Like other forms of petroleum, propane releases large amounts of heat during the chemical reaction of burning.

• Hydrocarbons are composed of the elements hydrogen and carbon. Propane (C3H8) and gasoline, which contain octane (C8H18) are examples of hydrocarbons. Carbohydrates are composed of the elements of hydrogen, carbon, and oxygen. Table sugar, for example, is the carbohydrate C12H22O11. Generalizing, all hydrocarbons and carbohydrates react completely with oxygen to yield CO2 and H2O.

• Oxidation-reduction reactions– An oxidation reduction reaction is one in which electrons

are transferred between atoms.– Oxidation is the loss of electrons– Reduction is the gain of electrons– Oxidizing agents are substances which take electrons

away from other atoms.• An oxidizing agent is reduced when it oxidizes another atom

– Reducing agents are substances which donate electrons to other substances.

• A reducing agent is oxidized in the process of reducing another atom.

• Oxidizing agents take electrons from other substances that are being oxidized. Oxygen and chlorine are commonly used, strong oxidizing agents.

• Types of chemical reactions– Combination reactions

• This is a synthesis reaction where several atoms or molecules combine to form one or more new compounds.

• The combining substances can be elements, compounds, or combinations of these two.

• Example

– 2 Mg (s) + O2 (g) 2 MgO (s)

• Magnesium and oxygen in this reaction combine to form magnesium oxide.

• Rusting iron is a common example of a combination reaction, where two or more substances combine to form a new compound. Rust is iron (III) oxide formed on these crews from the combination of iron and oxygen under moist conditions.

– Decomposition reactions• A decomposition reaction is one in which a compound

is broken down– Into elements.– Into Simpler compounds– Into both elements and simpler compounds

• In a decomposition reaction there usuall need to be some sort of an input of energy to cause the decomposition to proceed.

» Example »

2HgO (s) 2 Hg (s) + O2

• Mercury (II) oxide is decomposed by heat, leaving the silver-colored element mercury behind as oxygen is driven off. This is an example of a decomposition reaction, 2 HgO 2 HG + O2 . Compare this equation to the general form of a decomposition reaction.

– Replacement reactions• A replacement reaction is one where an atom or a

polyatomic ion is exchanged for another tom or polyatomic ion.

• These types of reactions occur as some elements have a greater ability to hold or attract electrons to themselves.

• Elements that have the least ability to hold electrons are the most reactive.

• A metal will replace any element that occurs above it in the activity series.

• Metal ions above hydrogen in the activity series will replace hydrogen as hydrogen ionizes from acids in solution.

• Example– Zn (s) + H2SO4 (aq) ZnSO4 + H2

• The activity series for common metals, together with some generalizations about the chemical activities of the metals. The series is used to predict which replacement reactions will take place and which reactions will not occur. (Note that hydrogen is not a metal and is placed in the series for reference to acid reactions.)

• This shows a reaction between metallic aluminum and the blue solution of copper (II) chloride. Aluminum is above copper in the activity series, and aluminum replaces the copper ions from the solution as copper is deposited as a metal. The aluminum loses electrons to the copper and forms aluminum ions in solution.

– Ion exchange reactions• An ion exchange reaction is a reaction that takes place

when the ion of one compound interacts with the ions of another compound forming.

– A solid that comes out of the solution (precipitates)– A gas– Water

• Example• 3Ca(OH)2 (aq) + Al2(SO4)3 (aq) 3CaSO4 (aq) + 2Al(OH)3

• Information from chemical equations

• Introduction.– Information from a balanced chemical equation tells us

information about:• Atoms

• Molecules

• Atomic weights.

– The coefficients in the balanced reaction is the number of atoms or molecules involve in the reaction.

– In 1808 Gay-Lussac determined that gases combine in small, whole number volumes when the temperature and pressure were held constant.

• This is the Law of Combining Volumes.

– Avogadro proposed an explanation for the law of combining volumes in 1811.

• It was proposed that gases at the same temperature contained the same number of molecules.

– This had two implications for the coefficients in a balanced equation

• The coefficients represent the number of molecules of each substance

• It also represents the ratios of the combining volumes.

• Reacting gases combine in ratios of small, whole-number volumes when the temperature and pressure are the same for each volume. (A) One volume of hydrogen gas combines with one volume of chlorine gas to yield two volumes of hydrogen chloride gas. (B) Two volumes of hydrogen gas combine with one volume of oxygen gas to yield two volumes of water vapor.

• Avogadro's hypothesis of equal volumes of gas having equal numbers of molecules offered an explanation for the law of combining volumes.

• Units of measurement used with equations.– We use a mole concept to bring together the concepts of

counting numbers and atomic weights of elements.– The mole is derived from the following information.

• Atomic weights are an average of the relative masses of all of the isotopes of the given element.

• The number of C-12 atoms in exactly 12.00 g of C12 is 6.02 X 1023.

– This called Avogadro’s number.

• An amount of a substance that contains Avogadro’s number of atoms, ions, molecules, or any other chemical unit is called a mole.

• A mole of C-12 atoms is defined as having a mass of exactly 12.00 g, a mass that is equal to its atomic weight.

• The mole concept for (A) elements, (B) compounds, and (C) molecular substances. A mole contains 6.02 X 1023 particles. Since every mole contains the same number of particles, the ratio of the mass of any two moles is the same as the ratio of the masses of individual particles making up the two moles.

– The gram atomic weight of an element is the mass in grams of one mole of an element that is numerically equal to its atomic weight.

– The gram formula weight of a compound is the mass in grams of one mole of the compound that is numerically equal to its formula weight.

• The gram formula weight of a compound is the sum total of all the individual atomic weight in the formula.

– The gram molecular weight is the gram formula weight of a molecular compound.

• Quantitative uses of equations– A balanced chemical equation can be used to interpret

the:• Molecular ratio of reactants to products.• Mole ratio of the reactants to products.• Mass ratio of the reactants to products.

– The molecular ratio leads to the concept of the mole ratio since any number of molecules can react as long as they are in the correct ratio

– Since 6.02 X 1023 molecules is the number of particles in a mole, the coefficients therefore represent the number of moles involved in a chemical reaction.

– The gram formula weight of a compound is the mass in grams of one mole that is numerically equal to its formula weight.

• The equation also describes the mass ratios of the reactants to products.