What is ph? Power of Hydrogen

Transcription

1 What is it?

2 What is? Power of Hydrogen

3 What is? A measure/scale that allows us to determine if a solution is acidic (H + ), neutral or basic (OH - ). Acidic Neutral Basic (alkaline) stronger weaker stronger

4 of common substances Should know: of rain is 5

5 Scale used to measure the concentration of hydrogen ions,, in a solution. H +

6 Scale used to measure the concentration of hydrogen ions,, in a solution. H + Scale generally runs from 0-14

7 Scale generally runs from 0-14

8 In pure water a few of the water, H 2 O, molecules will + split up into hydrogen, _ H, and hydroxide, OH, ions. H 2 O H + + OH - H + OH - OH - H +

9 In pure water the number of + - H and OH ions are equal. + - H and OH are balanced. This corresponds to a of 7. 7 = Neutral H + OH - OH - H +

10 In pure water the number of + - H and OH ions are equal. Neutral + - H and OH are balanced. This corresponds to a of 7. 7 = Neutral H + OH - OH - H +

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12 If an acid is added to the water, the quantity of H + will increase. Acids release H + more H + value drops

13 Acids If an acid is added to the water, the quantity of H + will increase. Acids release H + more H + value drops

14 Acids The stronger the acid, the lower the.

15 If a base is added to the water, the quantity of OH - will increase. Bases release OH - less H + value rises

16 Base s If a base is added to the water, the quantity of OH - will increase. Bases release OH - The stronger the base, the higher the. less H + value rises

17 Dissolving a salt in the water (normally) does not affect the balance between H + and OH ions. Saline (salt) solutions are usually neutral: = 7

18 Calculating the Strength of For every unit on the scale there is a 10x difference between strengths. x10 x10 x How much weaker is an acid of 4 vs 1? How much stronger is a base of 13 vs 8? How much stronger is a base of 9 vs an acid of 5? 10x10x10 = 1000 or or or 10 4

19 Strength to Neutralize Each specific unit has its opposite on the scale To neutralize you must have the same amount of solution of the opposite strength

20 Strength to Neutralize What would you add to neutralize 30 ml of a of 6? 30 ml of 8 2- What would you add to neutralize 60 ml of a of 10? 60 ml of 4 3- You want to neutralize 50 ml of a of 3. You only have 8 available. What do you do? Add more than 50 ml of 8 We ll figure out the calculations next class!

21 How antacids work

22 Identifying unknowns using indicators Buffer solutions and buffers Clear liquids (chemicals) which have the strengths of specific levels. ex: buffer 8 = 8 buffer 4 = 4 Indicators Liquids (chemicals) which will produce various colours depending on the of the solution it is mixed with.

23 Indicators indicators can be in the form of a paper or a solution.

24 Indicators Sometimes the colour change gives a lot of info, sometimes very little info That s why it s often best to use them in combination

25 Many indicators have 2 main colours.

26 Some can be colourless for certain values.

27 Some indicators change colour several times.

28 Some natural substances can act as indicators.

29 Get a sample of each of the different values (these are called buffer solutions)

30 Add a drop of the indicator (for example litmus) to each of the buffer solutions

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32 Now we can make a reference chart of the colour for each of the values Litmus: Red Purple Blue

33 Now we can use the indicator, with the chart, to narrow down the of an unknown solution. range: Litmus: Red Purple Blue

34 If we use a few different indicators, we can narrow down the value even more. Methyl orange indicator Phenolphthalein indicator

35 Methyl orange indicator Phenolphthalein indicator

36 For example: If one indicator gives a range of 4-9 And different indicator shows a range of 1-5 Then we know the solution being tested has a of

37 Red Cabbage Indicator

38 Titration What is it?

39 Acid-Base Titration A technique whereby a certain reactant (whose nature and concentration are known) is reacted with another reactant (whose concentration is unknown) in order to determine the unknown s concentration An indicator is used in order to determine the end point (neutralization)

40 Acid-Base Titration Last year, all you had to do was determine the volume needed to neutralize your unknown acid or base This year we will go one step further and you will need to determine the unknown s concentration

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42 Example (Volume) You are given an unknown solution, and after testing it with litmus paper these are your results: What is the nature of your unknown solution? Base!

43 Titration procedure 3.7ml 1) Obtain a 25ml beaker 2) Obtain a 25ml burette 3) Using a graduated cylinder, pour 10.0 ml of unknown base into the beaker 4) Add 5 drops of Bromothymol blue 5) Fill burette with 1.0 M solution of HCl 6) Record starting volume

44 Example (Volume) 3.7ml 7.3ml 7) Slowly add acid from burette into beaker, swirling after each addition 8) Stop adding when solution in beaker turns green 9) Record end volume on burette 10) Calculate volume of acid needed to neutralize unknown base. 11) Repeat steps 1-10 two more times 12) Determine average volume needed to neutralize. 7.3ml 3.7ml = 3.6 ml

45 TITRATION CALCULATIONS

46 Titration Calculations In order to be able to do the calculations necessary we have to be sure we fully understand neutralization. Review: in order to have full neutralization, there needs to be the same number of hydrogen ions (H + ) as hydroxide ions (OH - )

47 Titration Calculations Review: Acids release H+ ions in solution Bases release OH- ions in solution When there are more H+ ions than OH- ions, the solution is acidic When there are more OH- ions than H+ ions, the solution is alkaline (basic) Therefore: in order to have full neutralization, there needs to be the same number of hydrogen ions (H + ) as hydroxide ions (OH - )

48 Titration Calculations Review: Molarity is: C = n V C : concentration in mol/l n: number of moles V: volume of solution in L

49 Titration Calculations If there has to be the same number of H+ and OH- ions then that means they have to have the same number of moles (n) C A = n A V A C B = n B V B n A = C A V A n B = C B V B

50 Titration Calculations If Note: there has this to equation be the same only number works of H+ for and acids OH- ions and then bases that that means are they in have a 1:1 to mole have the same number of moles (n) ratio n A = n B n A = C A V A n B = C B V B C A V A = C B V B

51 Example Let s revisit the example from last class What was the volume of 1.0M HCl needed to neutralize 10.0ml of the unknown base? 3.6 ml ml C A V A = C B V B (1.0mol/L)(0.0036L) = C B (0.010L) mol = C B (0.010L) 0.36 mol/l = C B