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Flashcards

Acids, bases and buffers

Sparkle Shine Chemistry A level English 54 cards

Cards in this deck (54)

  1. 1Define a weak acid-An acid that only partially dissociates its H+ ions at equillibrium
  2. 2Define an electrolyteA substance that is able to conduct electricity
  3. 3How are strong acids s as electrolytes?-They dissociate all H+ so they are good at being electrolytes
  4. 4HANDY TIP!!Weak acids tend to be organic and weak bases tend to be nitrogen containing! :)
  5. 5How would you measure the strength of an acid?-Calculate the PKa
    -The LOWER the PKa value then the STRONGER the acid
    -Ka> 1 = more PRODUCTS - strong, gave up it's everything
    -Ka<1= more REACTANTS -weak; is holding back
  6. 6How can you tell the strength of an acid based on its PKa?-The smaller the PKa, the stronger the acid
    -This is because PKa is like the inverse of Ka
  7. 7How are pH and pOH linked?pH + pOH = 14
    -So if you know one of the two, you can find the other by minusing it from 14
    -But only @ 25 degrees
  8. 8How do you relate the pH of acidic and diluted compositions?-Find
  9. 9What is the formula to find the pH of a WEAK acid?-pH = -log ( square root of Ka x [initial])
  10. 10TOP TIP-The pH in exams should usually be to 2dp
  11. 11What are the assumptions that need to be made when finding the pH of WEAK acids?1) That the constituent ions and their concentrations are equal to one another
    2) That the concentration of the weak acid at the beginning is the same as the concentration of the acid at equilibrium, because it is so weak
    3) That the reaction is taking place @ 298K
  12. 12What is the most accepted definition of an acid?-A substance that donates protons to a base
  13. 13What are the conjugate acid-base pairs when ammonia reacts with waterPair 1 = NH3 and NH4
    Pair 2= H2O and OH
  14. 14TOP TIP!Species with the EXTRA proton are the conjugate acids
    -Those which have lost protons are ALWAYS conjugate BASES
  15. 15Distinguish between dilute and weak-Dilute refers to the number of moles per unit volume
    -Weak refers to degree of ionisation
  16. 16Name one way in which you can find out the strength of an acidTHE Ka BABYYYYY!!!!
    -The larger the Ka..... the stronger the acid
  17. 17What are the two main assumptions that you make when dealing with strong acids?-That they completely ionise
    -That there are only H+ and A- ions... no HA
    -The [H+] is EQUAL to [A-]
  18. 18Why is the pH scale used?-It is more convenient as it allows a huge range of H+ values to be shown in simple +ve numbers
  19. 19What equation would you use to show that two species are acid-base pairs?H+ + A- <----> HA
    -You just relate them both to an H+
  20. 20What does Kw stand for?-Ka x [H20]
    -This is the Ionic product of water
  21. 21What assumptions do you make with Kw?-The dissociation is so small that the concentration of H2O is said to stay constant
  22. 22What are the units of Kw.........ALWAYSmol^2dm^-6
  23. 23How can you find the [H+} from [OH-] the long winded way?-If you have Kw you can divide it by [OH]
  24. 24Under the umbrella of weak acids, there is another subsection -You have strong weak acids -You have weak weak acids -Unfortunately, you cant use the classic pH equation for weak acids on strong weak acids. How can you tell is an acid is a strong weak acid or not?-The 5% rule
    -Use the equation as normal
    -Use this to find [H+]
    Substitute this into [H+]/ [HA] x100
    -If answer is over 5% you can't use equation
  25. 25At 298K, the Kw is 10^-14 -What is the value of [H+]?10^-7
    -H + OH needs to add up to pH 14
    -As Kw refers to neutral solutions the H must = the OH
    -So 14/2 = 7
  26. 26The Kw of a solution is 8.0 x10^-14 -Find the pH It's crucial to know that pH HAS to be and I'm mean HAS to be to two decimal placesKw = pH + pOH
    Kw= -log [H+] + -log[OH-]
    As pH and pOH are equal Kw = [H+]^2
    aka
    8.0 x10^-14 = [H+]^2
    [H+]= sqr of (8.0 x10^-14)
    [H+] =ANS
    pH = -log(ANS) = 6.35
  27. 27How can you find whether acid dissociation reaction is endo or exothermic?-See how the temperature change correlates to the pH
    -If pH decreases, it means that there is an increase in H+
    Forward reaction is likely to produced H+
    -As systems want to oppose change the forward reaction must be ENDOTHERMIC
  28. 28How would you get from pH to [OH-]?10^-pH = [H+]
    Kw/ ANS
    OR
    Kw - pH= ANS
    10^-ANS
  29. 29Explain the relationship between Kc and KwKc is the equillibrium constant for water
    -As concentration of water is assumed to remain constant due to its huge volume, we can multiply both sides by [H20]
    -This leaves us with [H+][OH-] which is Kw
  30. 30Define a bufferThis is a solution that resists changes in pH
  31. 31How can you identify a buffer solution?If it includes a weak acid and its conjugated base/ a salt made from it
  32. 32How do buffers not change the pH so much?-They make sure that the H+/OH- ions don't build up...because this build up is what is detected or shown in the change of pH
  33. 33TOP TIP!!!When you have x10 more acid (in terms of mol OR concentration....it has to be exactly x10) than base, the pH will decrease by 1!
    -If more base than acid, the pH will increase by 1! Neat huh?!
  34. 34How can you tell which one is an acid and which one is its conjugate base? This is a top tip in disguise?-The conjugate base always has one LESS H than its acid
  35. 35What can you tell me about the pKa of a solution that has more acid than base?-The pKa is bigger than the pH
  36. 36What can you tell me about the make up of a solution which has a pH higher than it's pKa?-This solution contains more OH- than H+
  37. 37TOP TIP!Don't be shy of moles....they are always your plan B!
    You better use 'em or lose 'em!
  38. 38How can you increase buffer capacities?YOU BEST INCREASE THE CONCENTRATION!
    -This means more ions can be absorbed so less will have an impact on pH
    -Think of it having more soldiers/ those bodyguard people in movies to fight off the heros...more people to get through so not as much damage done
  39. 39Name 3 strong acidsHBr
    HI
    HNO3
  40. 40State 3 weak basesNH3
    NaF
    H2CO3
  41. 41How do you get from an acid to its conjugate base?Remove an H+ from molecule
  42. 42Give one example when water acts as an acid and one which acts as a base-When water reacts with a weak base it acts as an acid because it donates an H+ ion
    -When water reacts with an acid it acts as a base because it accepts an H+ ion
  43. 43TOP TIP!!DO NOT INCLUDE SOLIDS AND LIQUIDS IN ANY KA/KB/KP......ANY OF THE K/ EQUILIBRIUM EXPRESSIONS!
  44. 44How many different equations equal to 14?-pKa + pKb
    -pH +pOH
  45. 45Using a made up example, explain why a change in pH by a certain integer means a chage in concentration by factor of ten-Give two examples of acid with different concentrations
    -Use x10^-pH equation
    -Show how that changes [ ]
  46. 46NEAT TRICK!!When checking if -log ANS is right, look at the power to ten first.... e.g if -3 is power then 3 is focus number.... now if the accompanying number is between 1 and 10 (standard form) the answer witll be slightly lower than focus number.....
    Kinda comp but you're welcome!
  47. 47What is this I.C.E and B.C.A. thisng?-Initial Change Equilibrium is used when finding the K(letter) THIS ONE USES CONCENTRATION (at the end of the column)
    -Before Change After is when you want to see how a quantity has changed during a reaction or after adding something.....THIS ONE NEEDS TO BE IN MOLES
  48. 48Define conjugate acids. Now define conjugate bases.Conjugate acids are species formed after an acid donates its proton
    Conjugate bases are species formed after a base accepts its proton
  49. 49What happens during the autoionization of water?This is when, in the correct conditions, the lone pairs water molecule form a dative bond with one of the hydrogens on another molecule.
    -This makes a hydronium ion and a hydroxide ion
  50. 50State an example of a buffer that would produce a pH less than 7CH3COOH and CH3COO-Na+
  51. 51If OH- ions were added to and acid buffer solution, what would happen to the equilibrium?-It would shift to the r
  52. 52TOP TIPTo remember what affects the pH of the equivalence point; The weak part of the regents will react with the other to form a product.
    This product then continues to react with water to produce either more OH- or H+
  53. 53What makes a good indicator/-One that has a narrow colour change range
    -The colour change should occur within the equivalence point
  54. 54—

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A level Chemistry Flashcards on Acids, bases and buffers, created by Sparkle Shine on 22/11/2017.
Created by
Sparkle Shine · Nov 2017
Updated
May 9, 2018
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