Chem 20: Chapter 6 - Notes

Acids vs. Acidic

Acids vs. Acidic

Acids:

The terms “Acid” and “Base” refer to a chemical substance.

• a solution may be an Acid, Base, or Neutral Compound

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Acidic:

The terms “Acidic” and “Basic” refer to the properties of a solution, and exist on a continuum

Acid

“Acid” describes the chemical compounds that:

• can ionize in water to form hydrogen and hydronium ions.

• pH less than 7

• can be neutralized by bases

Acid describes a molecule

Acidic

“Acidic” → the ability to release hydrogen ions.

• a solution may be more or less acidic, or more or less basic

• in some cases, a neutral solution may have acidic or basic properties, even with a pH of 7.

Acidic describes a property on a continuum

Alkalinity

The property of a solution that indicates its basic nature, also called basicity.

Concentration

Amount of solute dissolved in a specific amount of solvent

• found by multiplying given concentration by mole ratio (R/G)

Dilute

To make a solution less concentrated by adding more solvent

• a solution that is less concentrated than another

Concentrated

A solution that has a high amount of solute relative to solvent

Classical Names of Acids

• hydro____mic acid

• ___ic acid

• ___ous acid

based on the anion

IUPAC Names of Acids

• aqueous hydrogen ____ (anion)

Early Theories of Acids

• Sir Humphry Davy (early 1800s)

• Justus von Liebig (1838)

• Svante Arrhenius (1903)

• Modified Arrhenius (1957)

Svante Arrhenius Theory

Theory:

• acids are substances that ionize in aqueous solutions to form hydrogen ions

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Problems:

• a H+ ion is very small (a single proton) and unlikely to exist on its own in polar water

Modified Arrhenius Theory

Theory:

• H+ ion bonds to a water molecule to form a hydronium ion, H3O+(aq)

  • it is hydronium that gives acid their acidic properties!

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Problems:

• some compounds can potentially

  produce H3O+ or OH- ions

  • cannot be classified as acid or bases using MAT

Ionization

a neutral atom or molecule is converted to an ion

• molecules become charged ions

Arrhenius Acids

substance that ionizes into positive hydrogen ions and negative ions in solution

• water causes the acid molecules to ionize, resulting in acidic properties

General Formula of Arrhenius Acids

HX(s/l/g) → H+(aq) + X-(aq)

• ex) HCl(g) → H+(aq) + Cl-(aq)

Dissociation (SA, NI, SB)

compound comes apart into charged ions

• dissociation equation for bases are the same for ionic compounds, except that the negative ion will always be OH-(aq)

Arrhenius Base

ionic hydroxide compound that dissociates into positive ions and negative hydroxide ions in solution.

• basic properties are the result of the presence of the hydroxide ion

General Formula of Arrhenius Bases

MOH(s) → M+(aq) + OH-(aq)

• ex) Mg(OH)2(s) → Mg2+ (aq) + 2 OH-(aq)

Modified Arrhenius Acids

a substances that ionizes and bonds to a water molecule to form hydronium ions and negative ions in solutions.

• water causes the acid molecules to ionize, resulting in acidic properties

General Formula of Modified Arrhenius Acids

HX(s/l/g) + H2O(l) → H3O+(aq) + X-(aq)

• ex) HCl(g) + H2O(l) → H3O+(aq) + Cl-(aq)

Use H+(aq) if:

• writing dissociation equations

• asked for Arrhenius definition

Use H3O+(aq) if:

• focusing on behavior of the acid in water

• pH calculations

• asked for Modified Arrhenius definition

• analyzing neutralization reaction

Modified Arrhenius Bases

a base is any substance that increases the hydroxide ion concentration in solution.

• ionic compound that contains hydroxide ion → strong base

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other compounds can react with water to produce hydroxide ions indirectly, still classified as bases under Modified Theory

Neutralization

a reaction between hydronium and hydroxide ions to produce water

• a form of double replacement reaction

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• H3O+(aq) + OH-(aq) → 2 H2O(l)

Neutralization (Arrhenius Perspective)

• strong acid would have already ionized to produce H3O+ before reacting with the base.

  • reacting species from acid → hydronium, H3O+ (aq)

  • reacting species from base → hydroxide, OH-(aq)

• Group 1 Cation (Na+) and Group 17 Anion (Cl-) are spectators

pH scale

system for communicating hydronium ion concentration

• pH value is a # without units, between 0-14.

  • can, but not often exceed range

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pH refers to power of hydrogen (hydronium) ions

• measure of the activity of hydrogen (hydronium) ions in a solution and acidity.

Amount of pH Change

• movement of one number → 10 fold change

  • 7-9 → 10 × 10 = 10²

pH → S.D & pOH → S.D

# of SIG DIGS for the concentration = # decimal places of the pH

Strong Acids Vs. Weak Acids

• NOT defined by their concentration.

  • They are defined by their ability to ionize.

Strong Acids

• ionize completely (100%)

• extremely good conductors of electricity

• lower pH → more acidic

• if volume of soln = volume of H2O(l)

Weak Acids

• ionize partially (<50%)

• not good conductors of electricity

• higher pH → less acidic

Concentration

how many moles of the acid were dissolved into a given volume of water

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concentrated acid → more moles of acid per liter

diluted acid → less moles of acid per liter

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expressed numerically using molar concentration

Strength

how completely the acid ionizes in solution

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strong acid → ionizes completely

weak acid → ionizes partially

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expressed numerically using pH

Monoprotic

there is only one mole of hydrogen ions to react

• ex) HCl(aq), HNO3(aq), HI(aq)

Polyprotic

more than one hydrogen ion to react

• ex) H2SO4(aq), H3PO4(aq)