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Acids and Bases Unit 12
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Properties of an Acid Sour taste Turns litmus paper red (and responds uniquely to other indicators) Reacts with: Hydroxide bases to produce water and an ionic compound. Pure metals (i.e. Zn & Mg) to produce H 2 (g) Carbonates to produce CO 2 (g).
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Properties of a Base Bitter taste Slippery, soapy feeling Turns litmus paper blue (and responds uniquely to other indicators)
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Arrhenius Definition of an Acid/Base Acids produce hydrogen ions, or protons (H + ) in aqueous solution. HA represents a generic acid. HA ⇄ H + (aq) + A - (aq) Example: HCl (g) ⇄ H + (aq) + Cl - (aq) Bases produce hydroxide ions (OH – ) in aqueous solution. MOH represents a generic base. MOH ⇄ M + (aq) + OH - (aq) Example: NaOH ⇄ Na + (aq) + OH - (aq) The Arrhenius definition is very limited – Not all bases contain OH - Example – NH 3 or ammonia is a common base. (1859 – 1927)
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Neutralization of an Arrhenius Acid and Base An Arrhenius acid and an Arrhenius base will react to form a salt and water. Example: HCl (aq) + NaOH (aq) ⇄ NaCl (aq) + H 2 O (l) The salt is formed from the positive ion of the base (Na + ) and the negative ion of the acid (Cl - ). The water is formed from hydrogen ion of the acid (H + ) and the hydroxide ion of the base (OH - ). If such a reaction results in a solution with no hydrogen or hydroxide ions, the products would be neither an acid nor a base. This type of reaction is called a neutralization reaction.
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Bronsted-Lowry Definition Arrhenius’ model was limited because it only applied to substances with hydrogen and hydroxide ions. Bronsted and Lowry proposed a new definition for acid and bases: Acids are proton donors (give off an H + ) Bases are proton acceptors What do we mean by a proton donor? Hydrogen!What is H + ?
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Bronsted-Lowry Acid/Base Below is the general reaction that occurs when an acid (HA) is dissolved in water. HA (aq) + H 2 O (l) ⇄ H 3 O + (aq) + A - (aq) The acid (HA) is donating a proton to the base (H 2 O). When the H 2 O accepts the proton it become H 3 O + which is known as the hydronium ion. AcidBase
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Conjugate Acid/Base Pairs HA (aq) + H 2 O (l) ⇄ H 3 O + (aq) + A - (aq) This is a reversible reaction. If we look at the reverse reaction, which substance would donate a proton? Because the H 3 O + is a proton donor in the reverse reaction, it is considered the conjugate acid. If we look at the reverse reaction, which substance would accept a proton? Because the A - is a proton acceptor in the reverse reaction, it is considered the conjugate base. Acid donates proton, becomes the conjugate base. Base accepts proton, becomes the conjugate acid. AcidConjugate BaseBaseConjugate Acid
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Is water an acid or a base? Pure water is neutral! Water can act as an acid or a base: H 2 O (l) + H 2 O (l) H 3 O + (aq) + OH - (aq) One water molecule acts as an acid by donating a proton; the other water molecule acts as a base by accepting the proton. The reverse reaction dominates in normal conditions, indicated by the longer arrow.
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