Acids and Bases

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• Salt: Substance formed when hydrogen in an acid is substituted by a metal ion

• For a group 1 metal M: $M^{+}$

Acid	Salt
$HCl$	$MCl$
$HN{O}_3$	$MN{O}_3$
$H_{2}S{O}_4$	$MHS{O}_4$
$H_{2}S{O}_4$	$M_{2}S{O}_4$

• An acid is a proton donor
• A base is a proton acceptor

The Role of Water

• Acid-base reactions occur in aqueous solutions
• Water can accept or donate protons; it can act as an acid or a base $HCl+H_{2}O\to C{l}^{-}+H_3{O}^{+}$ $H_{2}O+N{H}_3\rightleftharpoons O{H}^{-}+N{H}_4^{+}$
• $H_3{O}^{+}$ is the hydronium ion
• Acid-base reactions are proton transfer reactions but the proton $H^{+}$ is never found on its own
• It is always bonded to water (dative bond) in an aqueous solution
• I.e. $H_{(aq)}^{+}$ exists as $H_3{O}_{(aq)}^{+}$

Theories

Humphrey Davy Acid-Base Theory

• Acids are substances that contain hydrogen
• Not all acids contain oxygen
• Flaw: Not all hydrogen-containing substances are acids

Svante Arrhenius Acid-Base Theory

• Acids dissociate in water forming $H^{+}$ ions as one product
• Bases dissociate in water forming $O{H}^{-}$ ions as one product
• Neutralisation involves the reaction of $H^{+}$ and $O{H}^{-}$ (an acid and a base) forming a salt in water
• Flaws
  • Theories only apply to aqueous solutions
  • Some substances such as $N{H}_3$ are bases and do not contain $O{H}^{-}$
  • Relative strengths are not addressed
  • Amphoteric substances are not addressed

Strong and Weak Acids

• Acids ionise in aqueous solutions, releasing protons
• Strong acids (hydrochloric, sulfuric and nitric) completely ionise in an aqueous solution $HCl+H_{2}O\to H_3{O}^{+}+C{l}^{-}$ $H_{2}S{O}_4+2H_{2}O\to 2H_3{O}^{+}+S{O}_4^{2-}$ $HN{O}_3+H_{2}O\to H_3{O}^{+}+N{O}_3^{-}$
• A dilute solution of hydrochloric acid is a strong acid
• Weak acids (e.g. carboxylic acids) only partially ionise in aqueous solutions $C{H}_{3}COOH+H_{2}O\rightleftharpoons C{H}_{3}CO{O}^{-}+H_3{O}^{+}$
• About 4 in every 1000 ethanoic acid molecules are ionised in a dilute solution
• A concentrated ethanoic acid is a weak acid

Strong	Weak
$HCl$	$H_{3}P{O}_4$
$HN{O}_3$	$H_{2}C{O}_3$
$H_{2}S{O}_4$	$HF$
$HBr$	$HC{l}_3$
$HI$	$H_{2}C{O}_4$
$HCl{O}_4$	$HS{O}_4^{-}$

Strong and Weak Bases

• Bases dissociate in aqueous solutions
• Strong bases fully dissociate in aqueous solutions $NaO{H}_{(aq)}\to N{a}_{(aq)}^{+}+O{H}_{(aq)}^{-}$
• A dilute solution of sodium hydroxide is a strong base

Strong	Weak
$NaOH$	$N{H}_{4}OH$
$KOH$	$C{O}_3^{2-}$
$Ba(OH{)}_2$	$N{H}_3$
$Sr(OH{)}_2$	$F^{-}$
$Ca(OH{)}_2$	$C{H}_{3}CO{O}^{-}$

Electrolytes

• Electrolytes are substances that dissolve in water and form ions
• Strong electrolytes are good conductors of electricity in an aqueous solution because water causes them to dissociate (ionic compounds) or completely ionise (covalent molecules)

Reactions of Acids with Metals

$Metal+Aci{d}_{(aq)}\to salt+H_2$ $Metaloxide+aci{d}_{(aq)}\to sal{t}_{(maybe)}+H_2{O}_{(l)}$ $Metalhydroxide+aci{d}_{(aq)}\to salt+H_2{O}_{(l)}$ $Metalcarbonate+aci{d}_{(aq)}\to salt+H_2{O}_{(l)}+C{O}_2(g)$ $Metalhydrogencarbonate+aci{d}_{(aq)}\to salt+$

• Metal oxides are always solid, unless specified otherwise

Acids

• Substances that:
  • Have a pH below 7 and turn universal indicator yellow, orange, or red
  • Turn litmus red
  • Form solutions containing hydrogen ions ($H^{+}$)
• Hydrochloric acid ($HCl$) is a strong acid
• Ethanoic acid/vinegar ($C{H}_{3}COOH$) is a weak acid
• Acids can be defined as substances that ionise to release $H^{+}$ ions when they dissolve in water
• Since acids release $H^{+}$ ions (protons), we call them $H^{+}$/proton donors

What are $H^{+}$ ions?

• Hydrogen atoms contain one proton and one electron
• A $H^{+}$ ion is a hydrogen atom that has lost its single electron. This leaves a lone, positively charged proton
• Because acids donate protons, they are sometimes called proton donors.
• When dissolved in water, the $H^{+}$ ion is hydrated.
• This is represented in formulae by adding a state symbol, $H_{(aq)}^{+}$.
• This is shorthand for the hydronium ion, $H_3{O}_{(aq)}^{+}$

Alkalis/Bases

• Substances that:
  • Have a pH above 7 and turn universal indicator blue or purple
  • Turn litmus blue
  • Can neutralise acids
  • Form solutions containing hydroxide ions ($O{H}^{-}$)
  • Sodium hydroxide is a strong alkali.
  • Ammonia is a weak alkali
    • When ammonia is dissolved in water, it forms ammonium hydroxide, ($N{H}_{4}OH$), a fertiliser.
• Bases are substances that react with acids.
• They absorb hydrogen ions ($H^{+}$). Bases are therefore $H^{+}$ acceptors, or proton acceptors.
• The oxides, hydroxides and carbonates of metals, such as sodium carbonate, are bases.
• Some bases are soluble in water: these bases are called alkalis.
• Alkalis release hydroxide ions ($O{H}^{–}$) in solution.
• All alkalis are bases, but not all bases are alkalis.

What is pH?

• The acidity of an aqueous solutoin depends on the number of $H^{+}(H_3{O}^{+})$ ions in solution
• The pH is defined as: $-lo{g}_{10}[H^{+}]$
  • Where $[H^{+}]$ is the concentration of $H^{+}$ in $mol{L}^{-}$
• The pH scale is a logarithmic scale with base 10
• This means that each value is 10 times the value below it
  • E.g., pH 5 is 10 times more than pH 6
• pH values are usually given to 2 decimal places
• The pH of an acid depends on both its strength and concentration
• A strong acid has a low pH (usually 0 or 1)
  • This means that the concentration of $H^{+}$ is high, as the acid is fully dissociated into its ions
• A weak acid has a higher pH (but still less than 7)
  • This means that the concentration of $H^{+}$ is lower than for a strong acid, as the acid is not fully dissociated into its ions

Concentrated vs Diluted

• Strong and concentrated do not mean the same thing.
• Strong – high proportion of substance forms ions in solution.
• Weak – low proportion of substance forms ions in solution.
• Concentrated – lots of solute per volume of solution.
• Dilute – less solute per volume of solution.