ACIDS AND BASES

CHEMISTRY: ACIDS AND BASES

INTRODUCTION

  • Many chemical reactions in nature involve acids and bases or are sensitive to their presence.

  • Understanding acids, bases, and the reactions involving them is essential for the TEAS exam.

  • Definition and key concepts include:

    • Acids

    • Bases

    • Buffer systems

    • pH and its measurement

ACIDS

  • Definition: An acid is a substance that produces hydrogen ions, H extsuperscript{+}, in an aqueous solution.

  • Examples:

    • Hydrochloric Acid (HCl):

    • Strong acid that completely dissociates in water.

    • Reaction:

      • HCl<br>ightarrowH++Cl−HCl <br>ightarrow H^+ + Cl^-

    • Each H extsuperscript{+} ion is reactive and bonds to water to form hydronium ion:

      • H++H<em>2OightarrowH</em>3O+H^+ + H<em>2O ightarrow H</em>3O^+

    • Overall Reaction:

      • HCl+H<em>2OightarrowH</em>3O++Cl−HCl + H<em>2O ightarrow H</em>3O^+ + Cl^-

    • Everyday examples:

    • Citric acid in fruits (e.g., lemons).

    • Acetic acid in vinegar (weak acid, does not fully dissociate).

    • Hydrochloric acid in human stomachs aids protein digestion.

BASES

  • Definition: A base is a substance that produces hydroxide ions, OH extsuperscript{-}, in an aqueous solution.

  • Examples:

    • Sodium Hydroxide (NaOH):

    • Strong base that completely dissociates in water.

    • Reaction:

      • NaOH<br>ightarrowNa++OH−NaOH <br>ightarrow Na^+ + OH^-

    • Ammonia (NH3):

    • Acts as a base by accepting a proton from water:

      • NH<em>3+H</em>2O<br>ightleftharpoonsNH4++OH−NH<em>3 + H</em>2O <br>ightleftharpoons NH_4^+ + OH^-

    • Characteristics:

      • Ammonia is a weak base (not all molecules accept protons).

pH SCALE

  • Definition of pH: A logarithmic scale based on the concentration of hydrogen ions in a solution, calculated using:

    • pH=−extlog[H+]pH = - ext{log}[H^+]

  • Scale:

    • Ranges from 0 to 14:

    • pH < 7: Acidic

    • pH = 7: Neutral

    • pH > 7: Basic

  • Human blood pH: Approximately 7.4, maintained through buffers.

BUFFER SYSTEMS

  • Definition of a Buffer: A solution of a weak acid and its conjugate base, or a weak base and its conjugate acid that helps resist pH changes.

  • Function:

    • Buffers neutralize added acids or bases, maintaining stable pH levels in biological systems.

  • Example in human body: Carbonic Acid (H extsubscript{2}CO extsubscript{3}) and Bicarbonate (HCO extsubscript{3} extsuperscript{-})

    • Reaction of CO extsubscript{2} with water:

    • CO<em>2+H</em>2O<br>ightleftharpoonsH<em>2CO</em>3CO<em>2 + H</em>2O <br>ightleftharpoons H<em>2CO</em>3

    • Dissociation of carbonic acid:

    • H<em>2CO</em>3<br>ightleftharpoonsH++HCO3−H<em>2CO</em>3 <br>ightleftharpoons H^+ + HCO_3^-

  • Buffer Action:

    • If a base is added, it neutralizes with H extsuperscript{+} ions provided from the weak acid, maintaining constant pH.

    • If an acid is added, bicarbonate acts as a proton acceptor, preventing significant pH changes.

NEUTRALIZATION REACTIONS

  • Definition: A chemical reaction in which an acid and a base react to form water and a salt.

  • Core Reaction:

    • The H extsuperscript{+} from the acid combines with OH extsuperscript{-} from the base, forming water.

  • Example Reaction:

    • HCl+NaOH<br>ightarrowH2O+NaClHCl + NaOH <br>ightarrow H_2O + NaCl

  • Result: Water and sodium chloride (a salt) are formed.

  • When equal moles of acid and base are mixed, the resulting solution is neutral (pH = 7).

PRACTICE PROBLEMS

  1. Which of the following substances is an example of an acid?

    • A. HI

    • B. CO extsubscript{2}

    • C. NH extsubscript{3}

    • D. KCl

  2. Which of the following substances are examples of a base? (Select all that apply.)

    • A. KOH

    • B. HCl

    • C. NH extsubscript{3}

    • D. HI

    • E. NaOH

  3. Orange juice, stomach acid, and coffee are all acids. Which of the following is the pH level for these substances?

    • A. A pH less than 2

    • B. A pH less than 7

    • C. A pH at about 7

    • D. A pH above 7

  4. Which of the following statements for this reaction is false? H<em>2PO</em>4−ightleftharpoonsH++HPO42−H<em>2PO</em>4^- ightleftharpoons H^+ + HPO_4^{2-}

    • A. The reaction is reversible.

    • B. H extsubscript{2}PO extsubscript{4} extsuperscript{-} is acting as an acid.

    • C. H extsubscript{2}PO extsubscript{4} extsuperscript{-} and HPO extsubscript{4} extsuperscript{2-} could be used to make a buffer.

    • D. This is a neutralization reaction.

  5. Predict the outcome of the neutralization reaction:

    • H<em>2SO</em>4+Ca(OH)2<br>ightarrow…H<em>2SO</em>4 + Ca(OH)_2 <br>ightarrow …

SUMMARY

  • Understanding acids and bases, their properties, and how they interact is crucial for chemical and biological processes.

  • Knowledge of pH, buffers, and neutralization reactions are fundamental to maintain balance within living organisms.