Chemistry B2A Chapter 16 Acids and bases

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1 Chemistry B2A Chapter 16 Acids and bases Arrhenius definitions: an acid is a substance that produces H 3 O + ions in aqueous solution. A base is a substance that produces OH - ions in aqueous solution. CH 3 COOH(aq) + H 2 O(l) CH 3 COO - (aq) + H 3 O + (aq) CH 3 COOH is an acid. NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH - (aq) NH 3 is a base. H 3 O + (hydronium ion): an H+ ion in water immediately combines with an H 2 O molecule (H + cannot exist in water. Because an H + ion is a bare proton, and a charge of +1 is too concentrated to exist on such a tiny particle). H + (aq) + H 2 O(l) H 3 O + (aq) Bronsted and Lowry definitions: an acid donates H + (proton). A base accepts H + (proton). When an acid transfers a proton to a base, the acid is converted to its conjugate base. When a base accepts a proton, it is converted to its conjugate acid. Any pair of molecules or ions that can be interconverted by transfer of a proton is called a conjugate acid-base pair. Note: If water is not involved in a chemical equation and H 3 O + and/or OH - ions are not produced, we should use the Bronsted and Lowry definitions. CH 3 COOH + NH 3 CH 3 COO NH 4 acid base Conjugate base Conjugate acid Conjugate acid-base pair Conjugate acid-base Weak acid and base: an acid or a base that is only partially ionized in aqueous solution. CH 3 COOH(aq) + H 2 O(l) NH 3 (aq) + H 2 O(l) CH 3 COO - (aq) + H 3 O + (aq) NH 4 + (aq) + OH - (aq) Strong acid and base: an acid or a base that ionizes completely in aqueous solution. HCl(aq) + H 2 O(l) NaOH(aq) + H 2 O(l) Cl - (aq) + H 3 O + (aq) Na + (aq) + OH - (aq) Note: In general, the strength of an acid (a base) is defined by the position of its ionization (dissociation) reaction. A strong acid is one for which the forward reaction predominates (only one direction for the arrow). This means that almost all the original acid is dissociated

2 (ionized). For a weak acid (a weak base), almost all of the molecules remain undissociated and the reverse reaction predominates (thus the arrow pointing left is longer). Note: A strong acid (base) is a good electrolyte and it can conduct the electricity current very well. On the other hand, a weak acid (base) is a weak electrolyte, which means that only a few ions are present. Note: It is important to understand that the strength of an acid or a base is not related to its concentration. HCl is a strong acid, whether it is concentrated or dilute, because it dissociates completely in water to chloride ions and hydronium ions. Note: A strong acid contains a relatively weak conjugate base. Note: Acids are classified as Monoprotic (HCl), Diprotic (H 2 SO 4 and H 2 CO 3 ), or Triprotic (H 3 PO 4 ) depending on the number of protons (H + ) each may give up. But not all hydrogen atoms can be given up. For example, acetic acid, CH 3 COOH, has four hydrogens but is

3 monoprotic; it gives up only one of them. This is because a hydrogen atom must be bonded to a strongly electronegative atom, such as oxygen or a halogen, to be acidic. Amphiprotic: a substance that can act as either an acid or a base (such as water). HCl(aq) + H 2 O(l) Cl - (aq) + H 3 O + (aq) H 2 O is a base. NaOH(aq) + H 2 O(l) Na + (aq) + OH - (aq) H 2 O is an acid. Oxyacids: most acids are oxyacids, in which the acidic hydrogen is attached to an oxygen atom. Organic acids: those with a carbon atom backbone, commonly contain the carboxyl group (-COOH). Acids of this type are usually weak. An example is acetic acid, CH 3 COOH. Naming binary acids: we use the name of the anion that they produce when they dissociate. We replace the suffix -ic acid instead of -ide ion. Then, we add it after the prefix hydro-. HF anion: fluoride ion acid name: hydrofluoric acid HCl anion: chloride ion acid name: hydrochloric acid Naming ternary acids: we use the name of the polyatomic anion that they produce when they dissociate. For the smaller charge, we replace the suffix -ous acid instead of -ite ion and for the larger charge, we replace the suffix -ic acid instead of -ate ion. NO - 2 : nitrite ion NO - 3 : nitrate ion HNO 2 : nitrous acid HNO 3 : nitric acid Acid ionization constant (K a ): it shows us how strong any weak acid is. Because the ionisation of weak acids in water are equilibria, we can only use equilibrium constants and acid ionisation constants for these acids. We do not have the K a for a strong acid. HA + H 2 O A - + H 3 O + [A-][H3 O+ ] K = [HA][H2O] [A-][H3O+] Ka = K[H2O] = [HA] - log K a = pk a Note: The p of anything is just the negative logarithm of that thing. the weaker the acid, the smaller its K a, but larger its pk a. Note: K a is always smaller than 1 and the pk a is a positive number.

4 ph and poh: ph is a scale to measure the strength of an acidic and a basic solution. This scale is between 0 and 14. A solution is acidic if its ph is less than 7.0. A solution is basic if its ph is greater than 7.0. A solution is neutral if its ph is equal to 7.0. Note: there is a relationship between ph and hydronium ion concentration [H 3 O + ] (also, between poh and hydroxide ion concentration [OH - ]). ph = - log [H 3 O + ] or [H 3 O + ] = 10 -ph poh = - log [OH - ] or [OH - ] = 10 -poh Note: we can use the following formulas as well: ph + poh = 14 [H 3 O + ] [OH - ] = ph meter: an instrument that can rapidly measure the ph of an aqueous solution precisely. We dip the electrode of the ph meter into the solution whose ph is to be measured, and then read the ph on a display. The accuracy of a ph meter depends on correct calibration. ph paper: is made by soaking plain paper with a mixture of ph indicators. We place a drop of solution on this paper, the paper turns a certain color. We compare the color of the paper with the colors on a chart supplied with the paper. ph indicator: is a substance that changes color at a certain ph. We use a ph indicator to measure the ph of an aqueous solution (for example: Methyl orange and Litmus). ph of strong acids: a strong acid dissociates (ionizes) completely in aqueous solution. Each molecule of an acid dissociates into ions. Therefore, the concentration of ions including H + is the same as the concentration of acid. Reactions of acids and bases: [H + ] = [HA] concentration of a strong acid 1. Reaction with metals: strong acids react with certain metals (called active metals) to produce hydrogen gas and a salt. Mg(s) + 2HCl(aq) MgCl 2 (aq) + H 2 (g) 2. Reaction with metal hydroxides: acids react with metal hydroxides to produce a salt and water. HCl(aq) + KOH(aq) KCl(aq) + H 2 O(l) 3. Neutralization: acids and bases react with each other to produce a salt and water. HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(l)

5 Note: when a strong acid reacts with a strong base, the product has neither acidic nor basic properties. We call such a solution neutral. Titration: an analytical procedure to determine the unknown concentration of an acidic or a basic solution. In a titration, we react a known volume of a solution of known concentration with a know volume of a solution of unknown concentration. B A M A V A = M B V B Equivalence point: the point at which there is an equal amount of acid and base in a neutralization reaction. In this situation, the solution becomes neutral ([H 3 O + ] = [OH - ] ph = 7.0). End Point: the point at which an indicator changes color during a titration. It is convenient if the end point and the equivalence point are the same. Normality (N): is another unit of concentration (when dealing with acids and bases). Normality is defined as the number of equivalents of solute per liter of solution. Normality (N) = Number of equivalents Volume of solution (L) N V = Number of equivalents Equivalent of an acid: is the amount of that acid that can furnish 1 mole of H +. Equivalent of a base: is the amount of that base that can furnish 1 mole of OH -. Equivalent weight: the equivalent weight of an acid or a base is the mass in grams of 1 equivalent of that acid or base.

6 Note: during a neutralization reaction, the number of equivalents of an acid becomes equal to the number of equivalents of a base ([H + ] = [OH - ]). We use this formula if the coefficients of an acid and a base are different in a neutralization reaction. Buffer: a solution that resists change in ph when limited amounts of an acid or a base are added to it. The most common buffers consist of approximately equal molar amounts of a weak acid and a salt of the weak acid (or its conjugate base). Note: The body manages to keep the ph of blood remarkably constant (when we eat the limited amounts of an acidic or a basic food). Our body uses three buffer systems: carbonate (HCO 3 - /H 2 CO 3 ), phosphate (HPO 4 2- /H 2 PO 4 - ), and proteins. How do buffers work in our body? We use the carbonate system (HCO 3 - /H 2 CO 3 ) as an example: HCO H 3 O + H 2 CO 3 + H 2 O If we eat an acidic food. H 2 CO 3 + OH - H 2 O + HCO 3 - If we eat a basic food. Henderson-Hasselbalch equation: it gives us a way to calculate the ph of a buffer when the concentration of the weak acid and its conjugate are not equal [HA] [A - ]: HA + H 2 O A - + H 3 O + ph = pk a + [A-] log [HA]