Acid and Bases. Physical Properties. Chemical Properties. Indicators. Corrosive when concentrated. Corrosive when concentrated.

Transcription

1 Physical Properties Acid and Bases Chemistry 30 Acids Corrosive when concentrated Have a sour taste Bases Corrosive when concentrated Have a bitter taste Often have a sharp odour Chemical Properties Indicators Acids React with bases to produce a salt React with some metals to produce hydrogen gas and a salt React with carbonates to produce water, carbon dioxide and a salt Bases React with acids to form water and a salt React with fats and oil (may feel slippery due to reaction with body oils) Less reactive than acids, but can more dangerous An indicator is a substance that can be used to determine if something is an acid or a base Some indicators will just tell if the substance is acidic or basic; others will tell how acid or basic the substance is 1

2 Acidity of Solutions Recall: aqueous solutions can be made from dissolving a substance in water When an aqueous solution contains equal amounts of hydroxide and hydrogen ions, it is considered neutral More hydroxide means the solution is basic, and more hydrogen means the solution is acidic Hydronium Ions Hydrogen ions (H + ) are often written as being bonded with a water molecule, making hydronium ions (H 3 O + ). Although these notations are different, they can be used interchangeably. Water is Neutral Water is considered neutral, since it produces equal numbers of hydrogen and hydroxide ions through a reversible process called self ionization: H 2 O (l) H + (aq) + OH (aq) For one litre of water at room temperature, the components of the solution can be broken down approximately as: [H 2 O] = 55 mol/l [H + ] = [OH ] = 10 7 mol/l Arrhenius Model The Arrhenius (ar ay nee us) model of acids and bases states that: Acids are substances that contain hydrogen and ionize to produce hydrogen ions in aqueous solutions. Bases are substances that contain hydroxide and ionize to produce hydroxide ions in aqueous solutions 2

3 Arrhenius Model Examples When hydrogen chloride gas is dissolves in water, it produces hydrogen ions (and is therefore an acid): HCl (g) H + (aq) + Cl (aq) Potassium hydroxide is a base, since it produces hydroxide ions: KOH (s) K + (aq) + OH (aq) Limits of the Arrhenius Model Although the Arrhenius model is suitable for describing most acids and bases, it does not apply to all bases. For example, ammonia (NH 3 ) is a common base, but it does not contain hydroxide. Brønsted Lowry Model Acids are hydrogen ion (proton) donors, and bases are hydrogen ion (proton) acceptors. This is more useful, since in these reactions, all that is needed is to determine how the hydrogen ion moves within the reaction. The general form to examine this model is: HX (aq) + H 2 O (l) H 3 O + (aq) + X (aq) Conjugate Acids and Bases The resulting compound after an acid has donated its proton is called a conjugate base, and the compound after the base has accepted the proton is called a conjugate acid. acid + base conjugate base + conjugate acid HA + B A + HB + This is useful because it indicates what the acid and base would be in the reverse reaction, if it were to occur. 3

4 Example 1: Conjugate Acids and Bases Identify the conjugate acid base pairs. H 2 O (l) + NH 3 (aq) NH 4+ (aq) + OH (aq) Example 2: Conjugate Acids and Bases Identify the conjugate acid base pairs. HF (aq) + H 2 O (l) F (aq) + H 3 O + (aq) Amphoteric Substances Amphoteric is a term used to describe substances that can act as either an acid or a base. Water is an example of an amphoteric substance. Identifying Available Protons Hydrogen ions can only be donated from a molecule if they are part of a polar bond. For example, in acetic acid: (The old term was amphiprotic, but this was too specific and did not accurately describe all substances in the category.) There are three hydrogen atoms that are part of nonpolar bonds, so they cannot be donated. 4

5 Monoprotic and Polyprotic Acids Monoprotic acids have one available hydrogen, e.g. HF, HClO 4, HCH 3 COO Polyprotic acids have more than one (usually two or three) available hydrogen ions, e.g. H 2 SO 4, H 3 PO 4 They go through steps to donate all of their hydrogens it is more difficult for the ionization to occur with each step Example: Ionization of Phosphoric Acid Step 1 H 3 PO 4 (aq) + H 2 O (l) H 2 PO 4 (aq) + H 3 O + (aq) K a = Step 2 H 2 PO 4 (aq) + H 2 O (l) HPO 4 2 (aq) + H 3 O + (aq) K a = Step 3 HPO 4 2 (aq) + H 2 O (l) PO 4 3 (aq) + H 3 O + (aq) K a = Strength of Acids and Bases The strength of an acid or base refers to the extent to which it dissolves in solution. Basically, strength is an indicator of how soluble the compound is. Strong Acids and Bases A strong acid or base will dissolve 100% in solution. This means that in the solution, all of the non water particles are ions. There are six strong acids: Hydrochloric acid Nitric acid Sulfuric acid Hydrobromic acid Hydroiodic acid Perchloric acid 5

6 Strong Acids and Bases There are eight common strong bases: LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH) 2, Sr(OH) 2, Ba(OH) 2 Strong acids and bases have highly polar bonds, which allows them to be easily broken apart by the water molecules. Stronger acids and bases have weak conjugates, but strong acids and bases are considered to not have conjugate. Weak Acids and Bases Acids and bases are considered weak when they ionize less than 50% in solution. Weak acids and bases are solutions in which there is an equilibrium between the ionic compound (as a neutral molecule) and its ions. HA (aq) H + (aq) + A (aq) Weak Acids and Bases Strong Versus Weak Weak acids and bases have almost non polar bonds, and do not dissolve as easily. Note that acids and bases have a varying degree of strength, and not all acids and bases that are not strong are considered to be weak. Temperature and concentration can also affect the level of dissociate of any substance being dissolved in water. 6

7 Concentration Versus Strength A dilute solution has a small amount of acid or base particles (either ions or molecules) per unit volume of solution. A concentrated solution has a large amount of acid or base particles per unit volume. Concentration Versus Strength In the next four pictures, consider any soldiers to be particles. Soldiers with their swords up are ions. Soldiers with their swords holstered are molecules. The strength of an acid or base does not affect its ability to be concentrated or dilute. 7

8 Ionization Equations Strong acids ionize completely: HA H + + A Example: Ionization Equations Write a balanced equation for the ionization of hydrochloric acid: Weak acids ionize partially, so they are in equilibrium: HA H + + A Write the balance equation for the ionization of carbonic acid: 8

9 Dissociation Equations Strong bases dissociate completely: BOH B + + OH Example: Dissociation Equations Write a balanced equation for the dissociation of sodium hydroxide: Weak bases dissociate partially, so they are in equilibrium. A weak base need to be combined with water, to produce hydroxide ions: B + H 2 O OH + HB Write a balanced equation for the dissociation of methylamine: Dissociation Constants K a and K b are acid and base dissociation constants. Similarly to K sp, K a and K b are numbers that indicate how strong an acid or base is (how much it will ionize in solution). Higher values mean the acid or base is stronger. Example 1: K a Which acid is the strongest, given the following K a values? Formic acid Hydrocyanic acid Citric acid Hypochlorous acid Benzoic acid

10 Dissociation Constants Just like K eq, K a and K b have equilibrium expressions. For example, for acetic acid: Example 1: K a for Strong Acid Calculate [H + ] in a 2.00 L solution of hydrogen chloride in which 3.65 g of HCl is dissolved. K a for HCl is very large. For polyprotic acids, there is a dissociation constant for each ionization. Example 2: K a for Weak Acids An acetic acid (HC 2 H 3 O 2 ) solution is 0.25 M. Given that K a for acetic acid is , find [H + ]. Example 3: K b for Weak Bases Calculate the hydroxide ion concentration in a M solution of analine, C 6 H 5 NH 2, a weak base with K b =

11 Self Ionization of Water Remember that water is neutral because it produces equal amounts of H + and OH ions. The ion product constant of water, K w is given by the following equation: Recall that at room temperature (25 C), the concentrations of both ions are 10 7 M. This means that K w is Self Ionization of Water At this temperature, the product of [OH ] and [H + ] in any aqueous solution will equal K w. This means that if the concentration of one increases, the other will decrease proportionally, like in Le Chatelier s Principle. Added H + will bond with OH to make water molecules, decreasing the concentration of OH. Example: Calculating Concentration with K w In an aqueous solution, [H + ] = M. What is [OH ] in the solution? Is the solution basic or acidic? ph and poh To simplify concentration values (which are often very small numbers), the ph scale is used. The ph scale is a log scale that indicates the concentration of hydrogen ions in a solution. ph = log [H + ] 11

12 ph and poh Acidic solutions have ph values below 7. Basic solutions have ph values above 7. Neutral solutions (like water) have ph of 7. Most ph values will fall between 0 and 14, but it is possible for them to be outside of that range. Since it is a log scale, a solution with ph of 3 is ten times more acidic than a solution with ph 4, and a hundred times more than ph 5. ph and poh poh is the same idea as ph, except it expresses the hydroxide concentration in the solution. It is calculated using: poh = log [OH ] All of the values are reversed from ph (low values of poh are basic). To convert between them: ph + poh = ph to Concentration To calculate the concentration from ph or poh: Example: ph and poh A solution has [H + ] = a. What is the ph of the solution? b. What is the poh of the solution? c. What is [OH ]? 12

13 ph of Acids and Bases Given the concentration of a strong acid or base, the concentration of the ions will be equal the concentration times the number of that ion in the formula. For example: For 0.1 M HCl, [H + ] = 0.1 M For 0.1 M Ca(OH) 2, [OH ] = 0.2 M Neutralization Reactions A neutralization reaction is a specific type of double displacement reaction that occurs when a base reacts with an acid. The products of a neutralization reaction is water and an ionic compound (a salt). Stoichiometry of Neutralization Important formulas to remember: n = CV m = nm You should also remember how to use mole ratios! Assumptions You only need to know how to do stoichiometric calculations for reactions with at least one strong acid or base. Assume that the reaction goes to completion. 13

14 Solving Neutralization Problems 1. Write and balance the equation 2. Identify and list all known and unknown variables 3. Determine the relationship of moles of acid to moles of base (mole ratio) 4. Use stoichiometry to solve Example 1: Neutralization How many moles of hydrochloric acid are needed to neutralize 0.50 mol of calcium hydroxide? Example 2: Neutralization What volume of M NaOH is needed to neutralize 21.4 ml of M HCl? Example 3: Neutralization What is the ph of a solution of Ca(OH) 2 if ml is needed to completely neutralize ml of 0.350M H 3 PO 4? 14

15 Titration Titration is a lab method used to determine the unknown concentration of a solution by reacting it with a known volume and concentration of another solution. For example, in acid base titration, the concentration of an acid could be determined by titrating it with a base of known concentration. Titration Titrations are very common lab procedures. For example, they are used in many manufacturing processes, in environmental science and in the health field. For a titration, the end point is either a specific concentration/ph (measured with a ph meter) or an indicator colour change. Titration 15