CHEM 121 Overview

Part 2

BALANCED CHEMICAL EQUATIONS• A balanced chemical equation is one in which the number of

atoms of each element in the reactants is equal to the number of atoms of that same element in the products.

• A reaction can be balanced by applying the law of conservation of matter.

• Coefficients (in red below) are written to the left of each reactant or product in order to achieve balance.

2 H2 (g) + O2 (g) → 2 H2O (l)

IONIC EQUATIONS• MOLECULAR EQUATIONS

– In a molecular equation, each compound is represented by its formula.

• TOTAL IONIC EQUATIONS– In a total ionic equation, all soluble ionic substances are

represented by the ions they form in solution. Substances that do not dissolve or that dissolve but do not dissociate into ions are represented by their formulas.

NaCl (aq) = Na+ (aq) + Cl− (aq)Na2S (aq) = 2 Na+ (aq) + S2− (aq)

Na 3PO4 (aq) = 3 Na+ (aq) + PO43− (aq)

IONIC EQUATIONS EXAMPLE• Write the following molecular equation in total ionic and net ionic forms.

Soluble substances are indicated by (aq) after their formulas and insoluble solids are indicated by (s) after their formulas.

BaCl2 (aq) + Na2S(aq) BaS(s) + 2NaCl(aq)

• In total ionic form, all substances except the insoluble BaS will be written in the form of the ions they form:

Ba2+(aq) + 2Cl-(aq) + 2Na+(aq) + S2-(aq) BaS(s) + 2Na+(aq) + 2Cl-(aq)

• In net ionic form, all spectator ions are dropped. Both the Na+ and Cl- ions are spectator ions because they appear on both sides of the equation. The net ionic equation is:

Ba2+(aq) + S2-(aq) BaS(s)

THE MOLE AND CHEMICAL EQUATIONS

• The mole concept can be applied to balanced chemical equations and used to calculate mass relationships in chemical reactions.

• Balanced equations can be interpreted in terms of the mole concept and the results used to provide factors for use in factor-unit solutions to numerical problems.

THE LIMITING REACTANT– The limiting reactant present in a mixture of reactants is the

reactant that will run out first, and thus, it determines the amount of product that can be produced.

– A useful approach to solving limiting reactant problems is to calculate the amount of product that could be produced by each of the quantities of reactant that are available. The reactant that gives the least amount of product is then the limiting reactant.

REACTION YIELDS• The amount of product calculated in the examples is

called the theoretical yield. The amount of product actually produced is called the actual yield. These two quantities are used to calculate the percentage yield using the following equation:

100 yeildltheoretica

yieldactual yield%

OXIDATION NUMBERS• Oxidation numbers (also called oxidation states) are

positive or negative numbers assigned to elements in chemical formulas according to a set of rules. The term oxidation number is abbreviated O.N.– Rule 1: The O.N. of any uncombined element is 0. – Rule 2: The O.N. of a simple ion is

equal to the charge on the ion. – Rule 3: The O.N. of group IA and IIA elements when they are in

compounds are always +1 and +2, respectively. – Rule 4: The O.N. of hydrogen is +1.

– Rule 5: The O.N. of oxygen is -2. – Rule 6: The algebraic sum of the oxidation numbers of all atoms in

a complete compound equals zero. – Rule 7: The algebraic sum of the O.N. of all the atoms in a

polyatomic ion is equal to the charge on the ion.

CHANGES IN STATE• Changes in state are often accomplished by adding or removing heat

from a substance.• Changes in state caused by adding heat to a substance are classified as

endothermic (heat in) processes. • Changes in state caused by removing heat are classified as exothermic

(heat out) processes.

Add Heat Give off Heat

SOLVENT & SOLUTE• SOLVENT OF A SOLUTION

– The solvent of a solution is the substance present in the largest amount in the solution

• SOLUTE OF A SOLUTION– A solute of a solution is any substance present in an amount less than that of

the solvent. A solution may contain more than one solute.• SOLUBLE SUBSTANCE

– A substance that dissolves to a significant extent in a solvent without stating how much actually will dissolve.

• INSOLUBLE SUBSTANCE– This is a term used to describe a

substance that does not dissolve to a significant extent in a solvent.

• IMMISCIBLE• Two liquids that do not mix.

DEGREES OF SATURATION• A saturated solution is a solution that contains the maximum

amount possible of dissolved solute in a stable situation under the prevailing conditions of temperature and pressure.

• A supersaturated solution is an unstable solution that contains an amount of solute greater than the solute solubility under the prevailing conditions of temperature and pressure.

• An unsaturated solution is a solution that contains an amount of solute less than the amount required to form a saturated solution under the prevailing conditions of temperature and pressure.

Crystallization

1. A seed crystal is added to a supersaturated solution2. Crystallization is initiated (crystal growth)3. Crystallization is completed and purified compound can be separated from solvent.

THE SOLUTION PROCESS• The solution process involves interactions between solvent

molecules (often water) and the particles of solute. • An example of the solution process for an ionic solute in water:

– NaCl– BaBr2

– Na2SO4

THE SOLUTION PROCESS (continued)• An example of the solution process for a polar solute in water:

– Sucrose– Ethanol– Vitamins

MOLARITY• The molarity of a solution expresses the number of moles of

solute contained in one liter of solution.• The mathematical calculation of the molarity of a solution

involves the use of the following equation:

• In this equation, the number of moles of solute in a sample of solution is divided by the volume in liters of the same sample of solution.

solution of liters

solute of molesM

SOLUTION PREPARATION - Dilution• A quantity of solution with a concentration greater

than the desired concentration is diluted with an appropriate amount of solvent to give a solution with a lower concentration. This type of problem is made simpler by using the following equation:

(Cc)(Vc) = (Cd)(Vd)

• In this equation, Cc is the concentration of the concentrated solution that is to be diluted, Vc is the volume of concentrated solution that is needed, Cd is the concentration of the dilute solution, and Vd is the volume of dilute solution.

SPONTANEOUS PROCESSES

• Spontaneous processes are processes that take place naturally with no apparent cause or stimulus.

ENTROPY• Entropy is a measurement or indication of the disorder or

randomness of a system. • The more disorderly or mixed up a system is, the higher its

entropy.

EXOTHERMIC & ENDOTHERMIC DIAGRAMS• The difference between endothermic and exothermic reactions

is clearly indicated by the following energy diagrams. • In exothermic reactions, the energy is lost as the reaction

occurs. The products have less energy than the reactants. • The reverse is true for endothermic reactions which gain energy

and cause the products to have more energy than reactants.

CHEMICAL EQUILIBRIUM• All chemical reactions can (in principle) go in both directions and

products, located to the right of the arrow, can react to form reactants, located to the left of the arrow. This condition is indicated by the use of a double arrow pointing in both directions as shown below:

H2(g) + I2(g) 2HI(g)

• When the reaction rate toward the right is equal to the reaction rate toward the left, the reaction is said to be in a state of equilibrium.

FACTORS THAT INFLUENCE THE POSITION OF EQUILIBRIUM

• According to Le Châtelier's principle, the position of equilibrium shifts in response to changes made in the equilibrium.

• The factors that will be considered are:– concentrations of reactants and products– reaction temperature– catalysts

• In general, Le Châtelier's principle predicts a shift away from the side to which something is added and toward the side from which something is removed.

ARRHENIUS ACIDS & BASES• ARRHENIUS ACID

– An Arrhenius acid is any substance that provides hydrogen ions, H+, when dissolved in water.

• ARRHENIUS BASE– An Arrhenius base is any substance that provides hydroxide ions,

OH-, when dissolved in water.

• EXAMPLES OF AN ARRHENIUS ACID AND BASE– HNO3 is an acid: HNO3(aq) H+ (aq) + NO3

- (aq)– KOH is a base: KOH(aq) K+ (aq) + OH- (aq)

BRØNSTED ACIDS & BASES• BRØNSTED ACID

– A Brønsted acid is any hydrogen-containing substance that is capable of donating a proton (H+) to another substance.

• BRØNSTED BASE– A Brønsted base is any substance capable of accepting a proton

from another substance.

• EXAMPLE OF A BRØNSTED ACID AND BASE– HNO2(aq) + H2O(l) H3O+ (aq) + NO2

-(aq)– In this reaction, HNO2 behaves as a Brønsted acid by donating a

proton to the H2O. The H2O behaves as a Brønsted base by accepting the proton.

CONJUGATE ACIDS & BASES

• CONJUGATE ACIDS AND BASES– The base formed (NO2

-) when a substance (HNO2) acts as a Brønsted acid is called the conjugate base of the acid. Similarly, the acid formed (H3O+) when a substance (H2O) acts as a Brønsted base is called the conjugate acid of the base.

• CONJUGATE ACID-BASE PAIRS– A Brønsted acid (such as HNO2) and its conjugate base (NO2

-) form what is called a conjugate acid-base pair.

– The same name is given to a Brønsted base (such as H2O) and its conjugate acid (H3O+).

THE SELF-IONIZATION OF WATER• Pure water does not contain only H2O molecules. In addition,

small but equal amounts of H3O+ and OH- ions are also present.• The reason for this is that in one liter of pure water 1.0 x 10-7

moles of water molecules behave as Brønsted acids and donate protons to another 1.0 x 10-7 moles of water molecules, which act as Brønsted bases. The reaction is:

• As a result, absolutely pure water contains 1.0 x 10-7 mol/L of both H3O+ and OH-.

• The term neutral is used to describe any water solution in which the concentrations of H3O+ and OH- are equal.

• The equilibrium expression is:

H2O (l) + H2O (l) ⇆ H3O+ (aq) + OH− (aq)

22

-3

OH

OH OHK

THE ION PRODUCT OF WATER (continued)• The equilibrium expression can be rearranged to give:

• Because the concentration of water is essentially constant, the product of K multiplied by the square of the water concentration is equal to another constant designated as Kw, and called the ion product of water. The equation then becomes:

• Because the molar concentration of both H3O+ and OH- in pure water is 1.0 x 10-7, the numerical value for Kw can be calculated:

-3

22 OH OHOHK

-3W OH OHK

1427-3W 100.1100.1OH OHK

THE ION PRODUCT OF WATER (continued)

• ACIDIC SOLUTION– An acidic solution is a solution in which the concentration of H3O+ is

greater than the concentration of OH-. It is also a solution in which the pH is less than 7.

• BASIC OR ALKALINE SOLUTION– A basic or alkaline solution is a solution in which the concentration of

OH- is greater than the concentration of H3O+. It is also a solution in which the pH is greater than 7.

1427-3W 100.1100.1OH OHK

THE pH CONCEPT• It is often the practice to express the concentration of H3O+ in

an abbreviated form called the pH rather than to use scientific notation.

• It is also a common practice to represent the H3O+ ion by the simpler H+ ion.

• The pH notation is defined below, using H+ in place of H3O+:

pH = -log[H+], or in alternate form [H+]= 1x10-pH

• Or for pOHpOH = -log[OH-], or in alternate form [OH-]= 1x10-pOH

CLASSIFICATION OF HOUSEHOLD PRODUCTS

Weak BasesWeak Acids

NEUTRALIZATION REACTIONS

• In neutralization reactions, an acid reacts with a base to produce a salt and water. The following are typical neutralization reactions involving the base sodium hydroxide, NaOH, which is also known commercially as lye.

• Reaction with hydrochloric acid:

NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)• The salt produced in this reaction is sodium chloride,

commonly called table salt.• Reaction with nitric acid:

NaOH(aq) + HNO3(aq) → NaNO3(aq) + H2O(l)

• The salt produced in this reaction is sodium nitrate.

ANALYZING ACIDS AND BASES• The analysis of acid solutions to

determine the amount of acid they contain is an important procedure done in many laboratories.

• An acid-base titration is one commonly-used method of analysis.

• When a titration is done, an accurately-measured volume of acid is put into a flask using a pipet.

• A few drops of indicator solution is added, then a base solution of known concentration is carefully added from a buret until all the acid has been reacted (equivalence point).

• The point at which all the acid has reacted is shown by a color change (endpoint) in the indicator.

• The concentration of the base and the volume required in the titration allow the concentration of acid to be determined.

BUFFERS• Buffers are solutions with the ability to resist changing pH when

acids (H+) or bases (OH-) are added to them.• Many useful buffers consist of a solution containing a mixture of a

weak acid and a salt of the acid (e.g. acetic acid and sodium acetate).

• Any added acid (H+ ions) react with the anion from the salt, which also happens to be the conjugate base of the weak acid.

• Any added base (OH- ions) react with the nonionized weak acid.

• The buffer capacity is the amount of acid (H+) or base (OH-) that can be absorbed by a buffer without causing a significant change in pH.

C2H3O2− (aq) + H+ (aq) ⇌ HC2H3O2 (aq)

HC2H3O2 (aq) + OH− (aq) ⇌ C2H3O2− (aq) + H2O (l)

COMPOUND FORMULAS• A compound formula consists of the symbols of the elements

found in the compound. Each elemental symbol represents one atom of the element. If more than one atom is represented, a subscript following the elemental symbol is used.

COMPOUND FORMULAS EXAMPLES• Carbon monoxide, CO

– one atom of C– one atom of O

• Water, H2O – two atoms of H – one atom of O

• Ammonia, NH3 – one atom of N– 3 atoms of H