What are the properties of acids and bases?

Transcription

1 Text CH. 14 and 15

2 What are the properties of acids and bases?

3 identify acids and bases based on general observable properties explain how an indicator is used to determine whether a solution is acidic, basic, or neutral Indicator: An indicator is a molecular substance that changes color when it comes into contact with an acid or a base.

4 Produce H + (as H 3 O + ) ions in water (the hydronium ion is a hydrogen ion attached to a water molecule) Taste sour Corrode metals (react to form H 2 (g)) Electrolytes React with bases to form a salt and water ph is less than 7 Turns blue litmus paper to red

5 Produce OH - ions in water Taste bitter, chalky Are electrolytes Feel slippery React with acids to form salts and water (neutralize) ph greater than 7 Turns red litmus paper to blue Basic Blue

6 What are the properties of acids and bases? Acids and bases are solutions that are classified according to their observable behavior. Acids and bases change the color of indicators. Substances that are not acids or bases are considered neutral.

7 follow the proton H +

8 How are acids and bases defined?

9 define Arrhenius and Brønsted-Lowry acids and bases explain the behavior of acids and bases on a particulate level explain the difference between strong and weak acids and bases

10 Acids are substances that add/increase H + in solution. Bases are substances that add/increase OH - in solution.

11 Note: H 2 O can function as both an ACID and a BASE! 2H 2 O --> H 2 O + H + + OH - --> H 3 O + + OH -

12 Note: H 2 O can function as both an ACID and a BASE! 2H 2 O --> H 2 O + H + + OH - --> H 3 O + + OH - The H 3 O + is called a hydronium ion and it acts as an acid. For simplification purposes when we talk about acids, we are referring to the formation of hydronium ions but just indicate the H +

13 Because water reacts with itself, the amount of hydronium ions (H 3 O + ) formed = the amount of hydroxide ions (OH - ) formed so the solution overall stays neutral. So then, how does a solution become acidic or alkaline?

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16 The positive ends of the water molecule attract the OH - ions leaving positive and negative ions in solution with an imbalance in OH - ions.

17 The definitions of acids and bases has changed over time.

18 Svante Arrhenius (1887) came up with a way to define acids and bases. He saw that when you put certain molecules in water, sometimes they break down and release H + ions. At other times, you find the release of OH - ions. When a hydrogen ion is released, the solution becomes acidic. When a hydroxide ion is released, the solution becomes basic. Cl H + H O H H H O H + + Cl

19 Arrhenius Acids increase [H + ] in aqueous solutions (H 3 O + ) Bases produce OH - ions in solution OR react with water to remove a H leaving OH -

20 Arrhenius acid is a substance that produces H + (H 3 O + ) in water Arrhenius base is a substance that produces OH - in water

21 Acid a molecule or ion that is a proton donor Bases a molecule or ion that is a proton acceptor After all, a proton is really just a hydrogen atom that has lost it s electron!

22 For example:

23 A Brønsted-Lowry acid is a proton donor A Brønsted-Lowry base is a proton acceptor base acid The Brønsted-Lowry definition also means NH 3 is a BASE in water and water is itself acts as an!

24 G.N. Lewis Lewis acid - a substance that accepts an electron pair Lewis base - a substance that donates an electron pair H +.. H-N-H H H H:N-H H

25 Lewis acid - a substance that accepts an electron pair

26 Formation of hydronium ion is also an excellent example. H + ACID O H H BASE Electron pair of the new O-H bond originates on the Lewis base. H O H H

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28 P+

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30 conjugate acid-base pairs Conjugate acids and bases are found on the products side of the equation.

31 Conjugate base: everything that remains of the acid molecule after a proton is lost. Conjugate acid: formed when the proton is transferred to the base. A conjugate acid-base pair consists of two substances related to each other by the donating and accepting of a single proton.

32 Label the acid, base, conjugate acid, and conjugate base in each reaction: HCl + OH - Cl - + H 2 O acid base conjugate base conjugate acid H 2 O + H 2 SO 4 HSO H 3 O + base acid conjugate base conjugate acid

33 The strength of an acid or base is determined by the amount of.

34 Acids and bases that break apart (dissociate) completely in solution are called strong acids and strong bases. Acids and bases that do not dissociate completely in solution are called weak acids and weak bases.

35 Strong and Weak Acids/Bases The strength of an acid (or base) is determined by the amount of IONIZATION. HNO 3, HCl, H 2 SO 4 and HClO 4 are among the only known strong acids.

36 Weak acids are much less than 100% ionized in water. One of the best known is acetic acid - CH 3 CO 2 H (vinegar)

37 Strong and Weak Acids/Bases Strong Base: 100% dissociated in water. NaOH (aq) ---> Na + (aq) + OH - (aq) Other common strong bases include KOH and Ca(OH) 2. CaO (lime) + H 2 O --> Ca(OH) 2 (slaked lime) CaO

38 Strong and Weak Acids/Bases Weak base: less than 100% ionized in water One of the best known weak bases is ammonia NH 3 (aq) + H 2 O (l) NH 4 + (aq) + OH - (aq)

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40 [H+] and ph

41 How is ph related to the acid or base concentration of a solution?

42 explain the mathematical relationship between the H + and OH concentrations in a solution define ph and explain the relationship between H + concentration and ph determine the H + concentration of a solution given the [OH ], and vice versa

43 * Common log is the power to which 10 must be raised to equal the number another way to indicate [H 3 O + ] from French pouvoir hydrogene meaning hydrogen power Defined as: the negative of the common logarithm* of the hydronium ion concentration

44 [H + ] is critical in many chemical reactions. A quick method of denoting [H + ] is via ph. By definition ph = log [H 3 O + ], [H + ] = 10 -ph The ph scale describes a wide range of values The ph scale condenses possible values of [H + ] to a 14 point scale Also, it is easier to say ph = 7 vs. [H + ] = 1 x 10 7

45 The ph scale is a way of expressing the strength of acids and bases. Instead of using very small numbers, we just use the negative power of 10 on the Molarity of the H + (or OH - ) ion.

46 There are several ways to test ph Blue litmus paper (red = acid) Red litmus paper (blue = basic) ph paper (multi-colored) ph meter (7 is neutral, <7 acid, >7 base) Universal indicator (multi-colored) Indicators like phenolphthalein Natural indicators like red cabbage, radishes

47 Paper tests like litmus paper and ph paper Place a drop of the solution onto a piece of the paper Read and record the color change. Note what the color indicates. You should only use a small portion of the paper. You can use one piece of paper for several tests.

48 Indicators are dyes that can be added that will change color in the presence of an acid or base. Some indicators only work in a specific range of ph Some dyes are natural, like radish skin or red cabbage

49 Tests the voltage of the electrolyte Converts the voltage to ph Very cheap, accurate Must be calibrated with a buffer solution

50 The ph scale is a logarithmic scale that describes the concentration of H + ions in solution. ph is related to [H + ] by the formula ph = -log [H + ]

51 NOTE: this is an inverse relationship: As acids get stronger, [H + ], ph. A ph of 1 is a very strong acid. As bases get stronger, [H + ], ph. A ph of 14 is a very strong base.

52 Let s start with water Water undergoes slight self-ionization: in H 2 25 C: [H 3 O + ] = 1.0 X 10-7 M [OH - ] = 1.0 X 10-7 M Pure water has an [H + ] concentration of 1.0 x 10 7 M and an [OH ] of 1.0 X 10 7 M. What does this mean?

53 The ionization constant of water is the product of the [OH - ] and [H 3 O + ], and is expressed by the equation: K w = [H 3 O + ] [OH - ] Therefore: K w = [1.0x10-7 ][1.0x10-7 ] = 1.0x10-14 because [OH - ] = [H 3 O + ] it is considered neutral

54 because [OH - ] = [H 3 O + ] it is considered neutral, [OH - ] < [H 3 O + ] is an acid and [OH - ] > [H 3 O + ] is a base

55 ph [H + ] [OH - ] poh

56 Remember: for water [OH - ][H 3 O + ] = 1.0 X 10-14

57 Strong acids and bases are considered completely ionized or dissociated in water. It has been determined that 1 mole of acid (or base) = 1 mole of H 3 O + (or OH - ). In other words:.01 M NaOH has 1.0 X 10-2 moles [OH - ] For example 2 X 10-4 M HCl has 2 X 10-4 M [H 3 O + ]

58 So, if 2 X 10-4 M HCl has a 2 X 10-4 M [H 3 O + ], what is the [OH - ]? hint: remember that K w = [OH - ][H 3 O + ] = 1.0 X hint: use your Algebra skills to solve for the unknown [OH - ] = 1.0 X [H 3 O + ] Luke, use the force = 1.0 X = 5.0 X X X 10-4 > 5.0 X acid

59 another example: You have a.01 M solution of HNO 3. Calculate the [H 3 O + ] and the [OH - ] of this solution. Because this is a strong acid, it ionizes completely therefore the [H 3 O + ] is equal to the molarity of the solution and can be written as: [H 3 O + ] = 1 x 10-2 M Now for the [OH - ]. Remember that K w = [OH - ][H 3 O + ] = 1.0 X 10-14

60 Now for the [OH - ]. Remember that K w = [OH - ][H 3 O + ] = 1.0 X Rearranging the formula: [OH - ]= 1.0 X [H 3 O + ] [OH - ]= 1.0 X x 10-2 [OH - ]= 1.0 X 10 (-14 +2) [OH - ]= 1.0 X 10-12

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62 another way to indicate [H 3 O + ] ph is defined as: the negative of the common logarithm* of the hydronium ion concentration ph = - log [H 3 O + ] * Common log is the power to which 10 must be raised to equal the number

63 ph = - log [H 3 O + ] Example: If a neutral solution has a [H + ] = 1 x 10-7 ph = - log (1 x 10-7 ) ph = - (- 7) ph = 7

64 ph = - log [H 3 O + ] Example: If [H + ] = 1 x ph = - log (1 x ) ph = - (- 10) ph = 10 Example **: If [H + ] = 6.2 x 10-9 ph = - log (6.2 X 10-9 ) ph = - (- 8.21) ph = 8.21 **use your calculator

65 Youtube review What is the ph of a M NaOH solution? [OH - ] = (or 1.0 X 10-3 M) poh - = - log (1.0 X 10-3 ) poh - = -(-3) = 3 ph = 14 3 = 11 confirm using K w : 1 x = [H 3 O + ] [OH - ] [H 3 O + ] = 1.0 x [1.0 X 10-3 ] ph = - log (1.0 x ) = 11.00

66 H + concentration OH concentration ph 1.0 X 10 0 M 1.0 X M 0 More acidic 1.0 X 10 1 M 1.0 X M X 10 2 M 1.0 X M X 10 3 M 1.0 X M X 10 4 M 1.0 X M X 10 5 M 1.0 X 10 9 M X 10 6 M 1.0 X 10 8 M X 10 7 M 1.0 X 10 7 M 7 neutral 1.0 X 10 8 M 1.0 X 10 6 M X 10 9 M 1.0 X 10 5 M X M 1.0 X 10 0 M 14 More basic

67 What if you know the ph can you convert that to find the [H 3 O+] and/or [OH]? How to do the math: ph = - log [H + ] -ph = log [H + ] antilog (-ph) = [H + ] 10 -ph = [H + ]

68 What is the [H + ] if ph = 7.4? [H + ] = 10 ph 3.98 x 10 8 M ph = -log[h + ] -ph = log [H + ] Antilog (-ph) = [H + ] 1 x 10 -ph = [H + ]

69 A solution has a ph of 8.5. What is the concentration of hydronium ions in the solution? ph = - log [H + ] 8.5 = - log [H + ] First think: is the solution going to be < or > -8.5 = log [H + ] antilog -8.5 = [H + ] = [H + ] 1x10 7? Why? 3.16 X 10-9 = [H + ]

70 If the ph of Coke is 3.12, [H + ] =??? Because ph = - log [H + ] then 10 -ph = [H + ] [H + ] = = 7.6 x 10-4 M *** to find antilog on your calculator, look for Shift or 2 nd function and then the log button

71 It s very important that the ph levels of water don t increase or decrease too much out of a certain range. This change could affect food chains and the survival of species.

72 Hank on ph Both humans and aquatic organisms depend on water. The ph of water must be within a range of 5 to 9 in order for organisms to survive. Waters with a ph less than about 5 are too acidic for humans to drink and will not allow most aquatic life to survive. Waters that are too basic can also harm humans, plants, and animals. Water with a ph of greater that 9 can dissolve materials such as animal scales and skin.

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75 How can ph levels change? ph can be affected by the chemicals in the water. The ph of water affects organisms living in the water. A changing ph in water can mean that there is an increase of pollution or some other environmental factor. Things such as burning fossil fuels (like with cars), mining, chemical spills, runoff (washing cars and farming), and sewage cause ph levels to change.

76 How is ph related to the acid or base concentration of a solution? The ph scale is a logarithmic scale that describes the concentration of hydrogen ions, H +, in solution: ph = -log [H + ]. The H + concentration is related to the OH concentration: [H + ][OH ] = So, as [H + ] increases, [OH ] decreases, and vice-versa. The ph of water is 7. In water the H + concentration is equal to the OH concentration. Thus, water is neutral.

77 The last few words Adding water to an acid or a base dilutes the solution, making it less acidic or less basic. NOTE! An acid can never be turned into a base by diluting it with water, and a base can never be turned into an acid by diluting it with water.

78 Each time the H+ concentration is diluted tenfold, the ph number goes up 1 unit. ph H+ concentration (decimal number) H+ concentration (scientific notation) mol/l 1.0 X 10 2 mol/l mol/l 1.0 X 10 3 mol/l mol/l 1.0 X 10 4 mol/l mol/l 1.0 X 10 5 mol/l mol/l 1.0 X 10 6 mol/l mol/l 1.0 X 10 7 mol/l

79 You have a stock bottle of hydrochloric acid, which is 12.1 M. You need 400 ml of 0.10 M HCl. How much of the acid and how much water will you need? 12.1(x) = (400)(0.10) 12.1x = 40 x = 3.3 You need 3.3 ml HCl in ml water