Topic 8: Acids & Ms. Kiely, IB Chemistry SL Coral Gables Senior High. Bases

Transcription

1 Topic 8: Acids & Ms. Kiely, IB Chemistry SL Coral Gables Senior High Bases

2 8.3: Properties of Acids and Bases In IB Chem, you will focus on H+ as being the ion common to making a substance acidic, and OH, hydroxide, as being the ion common to making a substance basic. Topic 8 Test on MONDAY 2/11/19 Concentration (mol dm³ ) is represented in brackets, [H+] means concentration of H+ The concentration of the hydroxide ion [OH ] in a substance is what determines how basic a substance is; the concentration of the hydrogen ion [H+] determines how acidic a substance is.

3 8.3: Neutralization, Endpoint, and Indicators When performing acid-base titrations, it is common protocol to use either an aqueous solution indicator or a paper-based indicator. Litmus is by far one of the most popularly used indicators. It turns pink when the solution is acidic and blue when it is basic/alkaline; however, it is NOT so useful in distinguishing between different strengths of acids or bases. Many of these indicators are derived from natural substances, such as extracts from flower petals and berries. Another common indicator is the universal indicator, which is formed by mixing together several different acids and bases. Color changes for universal indicators are on the next slide.

4 8.3: Neutralization, Endpoint, and Indicators The ph scale and universal indicator: The tubes in the following figure contain universal indicator added to solutions of ph 0-14 from left to right.

5 8.3: Neutralization, Endpoint, and Indicators Neutralization reactions are done in order to calculate the exact concentration of an acid or a base, given that the concentration of the other is known. Drop by drop, one of the solutions is poured into the other until the equivalence point is reached. The equivalence point is the point in a titration where the amount of titrant added is enough to completely neutralize the analyte solution. The moles of titrant (standard solution) equal the moles of the solution with unknown concentration. This is also known as the stoichiometric point because it is where the moles of acid are equal to the amount needed to neutralize the equivalent moles of base. Note this does not necessarily mean the acid to base ratio is 1:1. The ratio is determined by the balanced acid-base chemical equation.the equivalence point is not the same as the endpoint of a titration. The endpoint refers to the point at which an indicator changes color. More often than not, the color change occurs after the equivalence point has already been reached. Using the endpoint to calculate equivalence naturally introduces error. Some indicators show their color right at a ph of 7; but some show their color at a lower or higher ph. The end-point is therefore an indication that neutralization has been reached- not necessarily that you have a neutral substance.

6 8.3: The ph scale Main ideas: ph 1) ph=-log[h+(aq)] and [H+]=10 ph is a logarithmic expression of the concentration of H+ ions in a substance, [H+]. 2) A change of one ph unit represents a 10-fold change in the hydrogen ion concentration [H+] 3) The ionic product constant, Kw=[H+][OH ]=10 ¹⁴ at 298K 4) Be familiar with the use of a ph meter and universal indicator

7 8.3: The ph scale Most acids and bases fall within the ph values of The ph number of a substance is inversely related to the [H+]. High [H+] = low ph; Low [H+] = high ph -Increasing the ph by one unit represents a decrease in [H+] by 10 times; decreasing by one ph unit increases in [H+] by 10 times. The ph scale is determined at 298K; close to average room temperature; 75 F, 24 C.

8 8.3: The ph scale Increasing the ph by one unit represents a decrease in [H+] by 10 times; decreasing by one ph unit increases in [H+] by 10 times. Video: FjM For every 1 factor of change in ph, the concentration of H+ changes by a factor of 10: For example, a ph of 1 = 10 ¹ = 0.1 mol dm ³ a ph of 2 = 10 ² = 0.01 mol dm ³ a ph of 3 = 10 ³ = mol dm ³ etc.

9 8.3: The ph scale The ph scale is a way of quantifying just how acidic or basic a substance is. It is based on the concentration of H+ ions that are roaming in a substance. [H+], these brackets are the symbol used for concentration, mol dm ³ We can determine the ph of a substance if we know the concentration of H+ in the substance by using the following formula: ph=-log[h+] We can find the concentration of H+ in a substance if we know its ph by using this formula: [H+]=10 ph

10 8.3: The ph scale ph ph=-log[h+(aq)] and [H+]=10 ph is a logarithmic expression of the concentration of H+ ions in a substance, [H+]. -The use of log or a logarithmic scale for ph enables us to express ph as a smaller range of simpler numbers; i.e For instance, the majority of acids are weak. This means they have very low [H+], such as 4.6 x 10 ⁸ mol dm ³. Such a small number is not very user-friendly when trying to compare the strength of acids.** -The scale that is constructed so that these numbers that are an equal distance apart, represent values that are in an equal ratio. Therefore, a change of one ph unit represents a 10-fold change in the hydrogen ion concentration [H+]. **Note that logarithms in this topic are to base 10. Don t confuse these with natural logarithms to base e, (ln).

11 Worked Ex pg.250: If the ph of a solution is changed from 3 to 5, deduce how the hydrogen ion concentration changes.

12 Worked Ex pg.250: If the ph of a solution is changed from 3 to 5, deduce how the hydrogen ion concentration changes. ANSWER: -Remember, a change of one ph unit represents a 10-fold change in the hydrogen ion concentration [H+]; meaning if you increase the ph of a substance by one unit, there will be a decrease in [H+] by 10 times; and vice versa decreasing by one ph unit increases [H+] by 10 times. Therefore: in this problem, the ph went from 3 to 5. This means the substance became more basic, less acidic, meaning the [H+] decreased by a factor of 2: 10 x 10 = 100. The [H+] has decreased by 100, by a factor of 2, or by 10 ².

13 Practice 10.0 cm³ of a solution of a strong acid with a ph of 3 is added to a volumetric flask and the total volume is made up to 1.00 dm³ by adding distilled water. The resulting solution is then thoroughly mixed. What is the ph of the diluted solution? A. 1 B. 2 C. 4 D. 5

14 Answer: 10.0 cm³ of a solution of a strong acid with a ph of 3 is added to a volumetric flask and the total volume is made up to 1.00 dm³ by adding distilled water. The resulting solution is then thoroughly mixed. What is the ph of the diluted solution? A. 1 B. 2 C. 4 D. 5 Answer: D cm³ is equivalent to dm³ Compare dm³ to 1.00 dm³. There is a 10 ² difference between them, meaning, that the concentration of hydrogen ions has decreased by a factor of 2, meaning the ph has gone from 3 to 5.

15 Worked Ex pg.250: A sample of lake water was analyzed at 298 K and found to have [H+] = 3.2 x 10 ⁵ mol dm ³. Calculate the ph of this water and comment on its value.

16 Worked Ex pg.250: A sample of lake water was analyzed at 298 K and found to have [H+] = 3.2 x 10 ⁵ mol dm ³. Calculate the ph of this water and comment on its value. ANSWER: Use the following formula: ph=-log[h+] 4.49, which is less than 7, indicating the lake water is acidic.

17 Worked Ex pg.251: Human blood has a ph of Calculate the concentration of hydrogen ions present.

18 Worked Ex pg.251: Human blood has a ph of Calculate the concentration of hydrogen ions present. ANSWER: Use the following formula: [H+]=10 ph [H+]=10 ⁷ ⁴⁰ = [H+] = 4.00 x 10 ⁸ mol dm ³

19 8.3: Introduction to [OH ] -We have been discussing how H+ is the ion that determines just how acidic a substance is. For example, the higher the concentration of H+, the lower the ph, the more acidic the substance is. The lower the concentration of H+, the higher ph, and the more basic/alkaline the substance is. -The hydroxide ion, OH, is an ion that is often used to determine how basic a substance is, given it is available in the substance. -In aqueous solutions where both ions are present, the relationship between [H+] and [OH ] is inverse: a low [H+] (high ph) means a higher [OH ] and vice versa. -The substance is neutral if [H+] and [OH ] are equal.

20 8.3: Kw -The substance is neutral if [H+] and [OH ] are equal. For instance, distilled water is neutral, having a ph of 7. -Since most acid-base reactions involve the ionization of H+ and OH in aqueous solution (a solution of distilled water), we can use the ionic product constant of water, Kw, to help us calculate the concentrations of H+ and OH in a substance. -Kw is basically another way of solving for the concentrations of H+ or OH. At 298K, the ionic product constant of water, Kw, is: Kw = [H+][OH ] = 1.00 x 10 ¹⁴ Distilled water is neutral because it has an equal concentration of [H+] and [OH ], meaning they each respectively equal 1.00 x 10 ⁷

21 Summary of steps in calculations of H+, OH, and ph

22 At 298K, Kw = [H+][OH ] = 1.00 x 10 ¹⁴ We can manipulate this formula for the ionic constant of water to determine the concentration of either [H+] or [OH ], since we know that together their product must equal 1.00 x 10 ¹⁴. Worked ex pg.253/ A sample of blood at 298K has [H+] = 4.60 x 10 ⁸ mol dm ³. Calculate the concentration of OH and state whether the blood is acidic, neutral, or basic.

23 ANSWER Worked ex pg.253/ A sample of blood at 298K has [H+] = 4.60 x 10 ⁸ mol dm ³. Calculate the concentration of OH and state whether the blood is acidic, neutral, or basic. Kw = [H+][OH ] = 1.00 x 10 ¹⁴ [4.60 x 10 ⁸][OH ] = 1.00 x 10 ¹⁴ [OH ] = 1.00 x 10 ¹⁴ = 2.17 x 10 ⁷ 4.60 x 10 ⁸ The [OH ] is greater than the [H+], therefore the solution is basic.

24 Other useful formulas to help solve for ph when you only have [OH ]: poh = -log[oh ] ph = 14 - poh

25 Topic 8 Test on MONDAY 2/11/19

26 ANSWERS