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75 Cards in this Set
- Front
- Back
HX + H2O -> |
H3O+ + X- |
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What is a Brønsted-Lowry Acid? |
A hydrogen ion (proton) donor |
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What is a Brønsted-Lowry Base? |
A hydrogen ion (proton) acceptor |
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What is a strong acid? |
An acid 100% dissociated in solution e.g. HCl, H2SO4 and HNO3 |
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What are weak acids? |
Acids that only slightly dissociate in solution e.g. CH3COOH |
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HA + water -> |
H+ + A- |
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What are strong bases? |
Bases that fully (100%) dissociate in solution e.g. NaOH |
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What are weak bases? |
Bases that only slightly dissociate in solution e.g. NH3 |
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H20 <=> |
H+ + OH- |
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KC = |
[H+][OH-] ÷ [H2O] |
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How much does 1 mole of water weigh? |
18 |
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What type of process is dissociation of water? |
Endothermic as bonds are broken |
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pH = |
-log [H+] |
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Why is the pH value used to express concentration of hydride ions? |
As there is such a wide range in values |
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pH = |
-log10 [H+] |
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What is complete dissociation? |
All the acid molecules have dissociated into hydrogen ions |
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HCl + water -> |
H+ + Cl- |
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How does H+ concentration affect pH value? |
As H+ value decreases, pH value increases |
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What does complete dissociation mean we can conclude? |
In a monoprotic acid such as HCl, H+ is equal to the original concentration of the acid |
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What is difference from HCl from sulfuric acid? |
Sulfuric acid is diprotic so each acid molecule produces 2 hydrogen ions in solution |
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H2SO4 + water -> |
2H+ + SO4²- |
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What will all aqueous solutions contain? |
H+ ions and OH- ions which are derived from the partial dissociation of water |
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Acidic solution |
[H+ (aq)] > [OH-(aq)] |
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Alkaline solution |
[OH-(aq)] > [H+ (aq)] |
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Neutral solution |
[H+(aq)] = [OH-(aq)] |
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(using KW) pH = |
-log√kw |
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Why is water still neutral? |
The concentration of hydrogen ions is the same as the concentration of hydroxide ions |
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NaOH + H2O -> |
Na+ + OH- |
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[H+] = |
kW / [OH-(aq)] |
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What are the 5 stages of calculating the pH after mixing strong acids and strong alakalis? |
Mol of H+ Mol of OH- Mol of H+ in excess (H+ - OH-) Calculate [H+] (c = n/v) pH = -log c |
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How do you calculate pH after dilution? |
Calculate moles of acid Divide by total volume in dm³ pH = -log10 (ans) |
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Why can we not use the same method to work out the pH of a weak acid that we used for a strong acid? |
In weak acids, the hydrogen ion concentration is not the same as the concentration of the acid |
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HA(aq) + H2O <=> |
H3O+(aq) + A-(aq) |
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HA(aq) <=> |
H+(aq) + A-(aq) |
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Ka = |
[H+][A-] / [HA] mol dm-³ |
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[H+] = |
√(Ka × [HA(aq)]) |
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How do you find the pH of a weak acid? |
Insert into √(Ka × [HA(aq)] -log10(ans) |
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What is the relationship between the strength of an acid and it's pKa? |
The lower the pKa values, the stronger the acid. |
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What is the purpose of an indicator in an acid base titration? |
To show when the exact amount of acid and alkali are mixed to 'neutralise' each other |
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What is the 'end-point' of a titration? |
When the indicator changes colour |
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What is the equivalence point? |
Equal quantities of acid and base |
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What is the pH curve of strong acid-strong base? |
Equivalence point = pH 7.00 |
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What is the pH curve of weak acid-strong base? |
Equivalence point = pH more than 7 |
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What is the pH curve of strong acid-weak base? |
Equivalence point = pH less than 7 |
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What is the pH curve of weak acid - weak base? |
Difficult to detect accurate equivalence point as pH change is more gradual |
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What is on the X and Y axis of a pH curve? |
Y = pH X = Volume of base added |
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When do indicators change colour? |
Over a narrow pH range approximately centred around the pKa of the indicator |
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What are the features of phenolphthalein? |
Pink in any solution with a pH greater than 10.0 and colourless at a pH of 8.4 or less |
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Colour of methyl orange in acid |
Red |
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Colour of methyl orange in alkali |
Yellow |
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Colour of phenolphthalein in acid |
Colourless |
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Colour of phenolphthalein in alkali |
Pink |
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What must the pH range of the indicator lie between? |
The equivalence range, for it to work as an indicator |
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Why type of indicator is suitable for a strong acid-strong base titration? |
Phenolphthalein and methyl orange |
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Why type of indicator is suitable for a weak acid-strong base titration? |
Phenolphthalein |
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Why type of indicator is suitable for a strong acid-weak base titration? |
Methyl orange |
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Why type of indicator is suitable for a weak acid-weak base titration? |
You would never titrate a weak acid and a weak base in presence of an indicator |
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What is the equivalence range of phenolphthalein? |
8.4-10.0 |
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What is the equivalence range of methyl orange? |
3.0 - 4.4 |
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At half equivalence, [HA (aq)] = |
[A-(aq)] |
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What is a buffer? |
A buffer solution opposes change in pH when small quantities of acid or base are added |
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What does an acidic buffer consist of? |
A weak acid (HA) A salt of the weak acid (A-) |
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What does a basic buffer consist of? |
Weak base (B) Salt of the weak base (BH+) |
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What must an acidic buffer contain to work effectively? |
Large concentration of the weak acid and it's salt |
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What happens to the position of equilibrium if OH- ions are added to acidic buffer? |
Removes H+ (aq) ions so equilibrium shifts right to oppose change, producing H+ so solution doesn't become acidic |
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What happens to the position of equilibrium if H+ ions are added to acidic buffer? |
Equilibrium shifts left to oppose change and CH3COO-(aq) react with H+ (aq) ions, salt remove any H+, so small change to pH |
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What must a basic buffer contain to work effectively? |
Large concentration of the weak base and it's salt |
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What happens to the position of equilibrium if OH- ions are added to basic buffer? |
Removes H+ ions so equilibrium shifts left to oppose change so more H+ produced so solution doesn't become alkaline |
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What happens to the position of equilibrium if H+ ions are added to basic buffer? |
Equilibrium shifts right to oppose change so if there's a large concentration of both the weak base and it's salts then addition of small quantities of an acid/base will have little effect upon the [H+] |
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Why are buffers important in the body? |
The pH of the blood must remain constant for the chemical reactions that occur |
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Blood equilibrium |
HCO3- + H+ <=> CO2 + H2O |
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How does the blood remove excess hydroxide/hydrogen ions to keep pH constant? |
HCO3- + H+ -> CO2 + H2O H+ + OH- -> H2O |
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How can the pH of an acid buffer be made and calculated? |
Add salt to weak acid, add strong base to excess of weak acid. [H+] = Ka × [HA(aq)] / [A-(aq)] |
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How can the pH of an acid buffer after the addition of acid be calculated? |
Mol of H+ Mol of A- / Mol of HA A- - H+ HA + H+ Insert into equation for [H+] PH = -log [H+] |
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How can the pH of an acid buffer after the addition of base be calculated? |
Mol of OH-Mol of A- / Mol of HA A- + H+ HA - H+Insert into equation for [H+]PH = -log [H+] |