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AQA A2 acids

QuestionAnswer
Write the full equation between Calcium carbonate and hydrochloric acid 2 HCl + CaCO3  CaCl2 + CO2 + H2O (remember to check the formula)
Write the ionic equation between the solutions of sodium carbonate and hydrochloric acid 2H+ + CO3-2  CO2 + H2O (remember to check the formula)
Write the ionic equation between the solutions of potassium hydroxide and hydrochloric acid H+ + OH-  H2O
Write the ionic equation between the solutions of magnesium and hydrochloric acid Mg + 2H+  Mg+2 + H2
Write the ionic equation between the solutions of magnesium oxide and hydrochloric acid MgO + 2H+  Mg+2 + H2O
Name the two conjuguate pairs in this reaction: HCl + H2O  H3O+ + Cl-; for each pair, indicate which species is the acid, which is the base HCl (acid) and Cl- (base); H2O (base) and H3O+ (acid)
Name the two conjuguate pairs in this reaction: NH3 + H2O  NH4+ + OH-; for each pair, indicate which species is the acid, which is the base H2O (acid) and OH- (base); NH3 (base) and NH4+ (acid)
Identify the acid-base pairs in the reaction: HIO3 + H2O  H3O+ + IO3- H2O (base) and H3O+ (acid); HIO3 (acid) and IO3- (base)
Ka for Propanoic acid is 1.35x10^-5 and for Methanoic acid 1.78x10^-4; complete the equation CH3CH2COOH + HCOOH  Methanoic acid has the highest Ka so will dissociate more easily so it will be the acid; whereas Propanoic acid will be the base; CH3CH2COOH + HCOOH  CH3CH2COOH2+ + HCOO-
Kw=1.0x10^-14 at 25C; what is the pH of water at 25C? Kw=[H+][OH-]; here [H+]=[OH-] so pH=-log[H+]=-log(square root Kw); remember to type brackets on your calculator
H2O  H+ + OH-; delta-H=+57kJ.mol^-1 and Kw=1.0x10^-14 at 25C; would the pH of water at 50C be lower or higher than at 25C? as temp increases, the equilibrium will shift to the right as forward is endo; so CONCENTRATION of H+ increases; Kw will increase so pH=-log[H+] will decrease
Why is water neutral at 37C, even though the pH is 6.87? there are equal numbers of moles of H+ and OH-
How do you work out the pH of a strong acid given [HX]? strong acid= fully dissociates so [H+]=[HX]; pH=-log[H+]; BEWARE of H2SO4 and other DI-protic acids for which [H+]=2x[H2X]!!!
How do you work out the pH of a strong (monoprotic) base given [base]? strong base= fully dissociates so [OH-]=[base]; using Kw, [H+]=Kw/base; pH=-log[H+]
How do you work out the pH of a strong (diprotic) base given [base]? strong base= fully dissociates BUT for each mole of the base, 2 OH- will be produced so [OH-]=2x[base]; using Kw, [H+]=Kw/base; pH=-log[H+]
Calculate the pH of a 0.1mol.dm-3 solution of NaOH [OH-]=[NaOH]=0.1mol.dm^-3; [H+]=Kw/[OH-]=1x10^-14/0.1=1x10^-13; pH=-log[H+]=13
What is the expression of Ka and units for Ethanoic acid? Ka=[CH3COO-]x[H+]/[CH3COOH]; water is in excess so does not appear in Ka; units are mol.dm^-3
How do you work out the pH of a weak acid given [HX] and Ka? we assume that 1) [H+]=[X-] and 2) [HX] hardly changes so after rearranging expression of Ka, [H+]=square root(Ka x [AH]); pH=-log[H+]
Calculate the pH of a 0.1 mole.dm-3 Ethanoic acid with Ka=1.7x10^-5 mol.dm^-3 2.48
Calculate the pH of a 0.65 mole.dm-3 Ethanoic acid with Ka=1.7x10^-5 mol.dm^-3 4.39
Calculate the pH of a 0.044 mole.dm-3 HClO with Ka=3.7x10^-8 mol.dm^-3 5.32
Calculate the pKa of a weak acid when a 0.13mol.dm^-3 has a pH of 3.52 Ka=7.02x10^-7 and pKa=6.15
Calculate the pKa of a weak acid when a 0.078mol.dm^-3 has a pH of 5.19 Ka=5.34x10^-10 and pKa=9.27
Chloroethanoic acid has a pKa of 2.88 and ethanoic acid of 4.77. Which is the strongest acid? Chloroethanoic acid; the smaller the pKa, the stronger the acid
Ka of Benzoic acid is 6.3x10^-5 and of Hydrocyanic of 4.9x10^-10. Which is the strongest acid? Benzoic acid; the larger Ka, the more dissociated, the stronger the acid
What is the equivalence point? the point where volume of one solution has reacted exactly with the volume of the second solution (moles of H+=moles OH-); the point in the middle of the vertical section of the titration curve
What is the end point? the point for which the indicator changes colour; the point for which there is equal amounts of the weak acid and its conjuguate base form of the indicator
What are the conditions to choose an indicator? the end point must be the same as the equivalence point so the indicator must change colour within the vertical section of the pH curve
How can you determine pKa from a pH curve? at the half-equivalence point pH=pKa, because half of the AH has been converted to A- and H+; so find the equivalence point, divide by two
What is a buffer? A solution for which pH remains almost constant/changes only slightly for small additions of H+ or OH-
How would you make a buffer? By mixing a weak acid with its conjuguate base (for example CH3COOH with CH3COO-) OR by mixing a weak acid with a base (for example CH3COOH with NaOH)
How do you work out the pH of a buffer given volumes and concentrations for [acid], [salt] and Ka? work out moles of acid(AH) and salt(A-); calculate the concentrations using TOTAL volume; rearrange expression of Ka: [H+]=( Ka x [acid] )/ [salt]
How do you work out the pH of a buffer given volumes and concentrations for [acid], [NaOH] and Ka? work out moles of acid(AH) and NaOH; the difference between molesAH and molesNaOH is the moles of AH unreacted; moles NaOH=moles salt;calculate the concentrations using TOTAL volume; rearrange expression of Ka: [H+]=( Ka x [acid] )/ [salt]
Calculate the pH of the buffer made when 0.2mole.dm^-3 of CH3COOH is mixed with 0.1mole.dm^-3 CH3COONa? (pKa=4.77)? 4.50
Calculate the pH of the solution made when 500cm3 of 1mole.dm^-3 of CH3COOH is mixed with 500cm3 of 0.4mole.dm^-3 NaOH? (Ka=6.25x10^-5)? 4.03
Explain how a buffer is able to resist a change in pH when an acid is added; HA H+ + A- 1) the conjuguate base A- will react with the added H+ to form AH; 2) as [H+] is increased, system minimise change by decreasing [H+]/removing the added H+ and equilibrium shifts to the left
Explain how a buffer is able to resist a change in pH when an alkali is added; HA H+ + A- 1) the H+ will react with the added OH-; 2) as the H+ is reacting, the system minimise change by increasing [H+] and equilibrium shifts to the right
Explain how buffers in the blood prevent pH from falling when extra acids are produced H+ is removed by HCO3- to form H2CO3/carbonic acid; the carbonic acid is then converted into CO2 by an enzyme; the CO2 is exhaled
Calculate the pH of 0.02 mol dm-3 sulphuric acid pH= -log (0.04) = 1.398
State the two forms of the equation that link Ka and pKa pKa = -logKa and Ka = 10^-pKa
Calculate the pKa of CH3COOH (Ka=1.74x10-5) pKa=4.76
Calculate the Ka of Phenol (pKa=9.89) Ka=1.28x10-10
Describe what a buffer solution is A solution in which minimises the changes of PH on additions of small amounts of acid or alkali.
Give an example of a weak acid and a strong acid A weak acid is ethanoic acid and a strong acid is hydrochloric acid
Write out the expression for Ka. Ka = [H+] x [conj base] / [weak acid]
Calculate the pH when there is 0.35 moldm-3 of sodium hydroxide and 0.025 moldm-3 of propanoic acid when Ka is 1.45 x 10-5.
Define a Bronsted-Lowry acid. A proton donor
Explain what is meant by the strength of an acid. How much an acid dissociates in aqueous solution – strong acids 100% dissociate.
Identify the conjugate acid-base pairs in this equilibrium:HNO3 + HNO2  NO3- + H2NO2+ HNO3 = acid 1. HNO2 = base 2. NO3- = base 1. H2NO2+ = acid 2.
Write an expression for the acid dissociation constant, Ka, of phenol, given the full equation: C6H5OH + H2O <=> H30+ + C6H5O- Ka = [H3O+][C6H5O-] / [C6H5OH]
How can you prove an acid is a weak acid by taking a single pH measurement? If the single pH measurement is greater than 1, the acid is weak.
CHALLENGE: A weak organic acid AH has the following percentage composition by mass: C 40.0%, H 6.7%, O 53.3% and the RMM is 60.0. Calculate the empirical and molecular formula of AH. (3) 40/12=3.33, 6.7/1=6.7, 53.3/16=3.33; /by smallest 3.33= 1, 2, 1;Empirical formula= CH2O; Mass CH2O= 12 +2 +16=30;Emp mass x2= Molecular mass; Molecular formula= C2H4O2
Define pH (1) pH=-log[H+]
CHALLENGE: pH of stomach is 1.3. Patient should produces 2dm^3 of gastric juice in a day. Patient is prescribed tablets containing Al(OH)3. Calculate mass of aluminium hydroxide required to raise the pH from 1.3 to 2.0. 3HCl + Al(OH)3 --> AlCl3 + 3H2O pH2:[H+]=10^-2=0.01;pH1.3:[H+]=10^-1.3=0.05;To raise pH from 1.3 to 2.0 mean [H+] changes from 0.05 to 0.01=-0.04mole.dm-3;As 2dm^3 is made, the total amount is 2x0.04=0.08;using ratio 0.08/3mol of Al(OH)3 needed= 0.0266mol;RMM=78;mass=0.0266 x78=2.08g
The control of blood pH is important. This is achieved by HCO3- ions in blood plasma. Using appropriate equations, explain how these ions can act as a buffer solution. (3) when adding extra H+= HC03- + H+ <=> CO2 + H2O, equilibrium shifts riht AND HCO3- reacts with extra H+.; when adding extra OHequilibrium shifts left (as [H+] decreases) and OH- reacts with CO2 to form HCO3-
What is the approximate pH at equivalence point for a weak acid-strong base titration above 7
What is the approximate pH at equivalence point for a weak base-strong acid titration below 7
What is the colour change when you add HCl onto 25cm3 of a weak base? yellow to red
What is the colour change when you add NaOH onto 25cm3 of a weak acid? colourless to pink
Define a Bronsted acid/base A chemical that can donate/accept a proton
Created by: UrsulineChem