Acids and Bases

Definitions: 1.Arrhenius- Acid- substance that dissociates in water to produce hydrogen ions - H + Examples: HC l, HNO 3, H 2 SO 4, etc Base- substance that dissociates in water to produce hydroxide ions- OH - Examples: NaOH, KOH, Mg(OH) 2 According to this definition, H must be present in an acid, and OH in a base

HNO 3 + H 2 O H 3 O + + NO 3 - H 3 O + is the Hydronium ion You may see HNO 3 H + + NO 3 - HC l + KOH KC l + H 2 O Neutralization reaction Acid + Base a Salt + water

2. Brønsted-Lowry- Acid- substance that can donate H + ions Base- substance that can accept H + ions Substances do not need to be in water, and the base does not need to have OH. This expands the Arrhenius definition so that other substances can be considered as acids and bases. H + is simply a proton, so the definition of an acid can now be a proton donor and a base is a proton acceptor.

NH 3 + H 2 O NH OH - What is the acid? H 2 O – it donated the H + What is the base? NH 3 – it accepted the H + Conjugate Acid- Base Pairs: Many times a reaction occurs in a forward and reverse direction, so what is an acid in the forward direction becomes the conjugate base in the reverse direction, and the base becomes the conjugate acid in the reverse reaction

HC l + H 2 O H 3 O + + C l - acid base conjugate conjugate acid base Monoprotic acid- an acid that will donate 1 H + HNO 3 Diprotic acid- an acid that donates 2 H + H 2 SO 4 Triprotic acid- an acid that donates 3 H + H 3 PO 4 Amphoteric- a substance that can act as both an acid or a base- Water

3. Lewis Acids and Bases- Acid- substance that can accept a pair of electrons to form a covalent bond Base- substance that can donate a pair of electrons to form a covalent bond This now allows other substances to be considered an acid or a base

Acid – Base Definitions TypeAcidBase ArrheniusH + producerOH - producer Bronsted-LowryH + donorH + acceptor LewisElectron- pair acceptor Electron-pair donor

Properties of Acids and Bases Acids: -sour or tart taste -can burn skin if stronger, may burn if you get it in a cut -react strongly with most metals, usually to produce hydrogen gas -form weak or strong electrolytes -change blue litmus paper to red

Bases: -bitter taste -feels smooth, slippery -not reactive with most metals -weak or strong electrolytes -turns litmus paper from red to blue

Strengths of Acids and Bases Strong acid- easily dissociates H + to water - becomes strong electrolyte Weak acid- does not easily dissociate H + ions - weak electrolyte Common strong acids: HC l, HBr, HI, HNO 3, H 2 SO 4, HC l O 4 Weak acids:HC 2 H 3 O 2, HCN, HNO 2, HF, HC l O, HCO 3 -

Strong Bases: substances with a strong affinity for H +, those with OH - Weak Bases- ions that react only partially in water to form OH - ions Common strong bases: CaO, NaOH, KOH, Ca(OH) 2 Weak bases- NH 3, H 2 NNH 2, CO 3 2-, PO 4 3- Strong bases are strong electrolytes, weak bases are weak electrolytes

Self-Ionization of Water When you have pure water, due to the motion of the water molecules and the high polarity of water, there is a very small amount of H 3 O + and OH - that exist H 2 O + H 2 OH 3 O + + OH -

In pure water, the [H + ] = [OH - ], where [X] is equal to the concentration of X in solution, usually measured in M When the concentrations are equal, the substance is known as a neutral solution [H + ] =[OH - ] = 1.0 x M So, [H + ] x [OH - ] = 1.0 x M 2 = K w = ion product constant of water Since these concentration values are multiplied to give a constant, if [H + ] increases, [OH - ] must decrease

Because K w is a constant, if you know the concentration of H + or OH -, you can calculate the other. Example: If [H + ] = 1.0 x 10 -2, then (1.0 x ) [OH - ] = 1.0 x [OH - ] = 1.0 x x = 1.0 x When [H + ] > [OH - ], acidic solution When [H + ] < [OH - ], basic solution or alkaline solution

Because these concentrations are very small, another scale, the pH scale is used to describe [H + ], and pOH is used to describe [OH - ] pH = -log[H + ] In a neutral solution, [H + ] = 1.0 x 10 -7, so pH = - log (1.0 x 10 -7) = -(0 + -7) = 7 pH < 7 = acidic solution pH = 7 = neutral solution pH > 7 = basic solution

pOH = - log [OH - ] pH + pOH = 14 To calculate pH, use – log [H + ] To calculate pOH, can use 14 – pH or – log [OH - ] To calculate the concentration from a pH value, use the antilog or 10 x button