Acids and Bases Unknown Information
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- Created by: Lotto65
- Created on: 06-03-18 09:35
What is an acid?
A substance that produces hydrogen ions in solution
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What is a base?
A substance that produces hydroxide ions in a solution
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What is a Bronsted-Lowry acid?
A proton donor
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What is a Bronsted-Lowry base?
A proton acceptor
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How could you tell which was the acid in a conjugate acid-base pair?
The one with more hydrogens
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Why is it called a conjugate base?
When the conjugate acid donates a proton to form the conjugate base, there is then availability to accept a proton
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What does amphiprotic mean?
A substance that can donate or accept a proton
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Which substances are amphoteric but not amphiprotic?
Metal hydroxides e.g. aluminium and zinc hydroxide
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Are all amphiprotic substances amphoteric?
Yes
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Name two amphiprotic species
Hydrocarbonate ion and water
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What is formed when aluminium hydroxide, Al(OH)3 reacts with OH- ions?
Al(OH)4-(aq)
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Who discovered that hydrochloric acid did not contain oxygen, paving the way for a new theory on acids and bases?
Humphry Davy
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What is the definition of a Lewis acid?
An electron pair acceptor
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What is a Lewis base?
An electron pair donor
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Which acid and base definition is more general?
Lewis acid
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What is the Lewis acid definition more useful for?
Acid-base reactions involving solvents other than water
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Are all Lewis acids and bases Bronsted-Lowry acids and bases? Why?/Why not?
Yes because the donation of a proton involves the donation of an electron pair
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What does protonation mean?
Adding a proton
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What bond is formed in a Lewis acid-base reaction?
Coordinate covalent bond
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Name a Lewis acid-base reaction that does not involve the transfer of a proton?
Ammonia and boron trifluoride
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What type of reaction is the formation of a complex ion?
Lewis acid-base reaction
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What is the requirement for a substance to be a Lewis acid?
Space for a lone pair of electrons
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What sort of compounds are often Lewis acids?
Boron trihalide
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How do you form salts?
Acid and metal (or ammonium) ion
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Which metals do not react with acids?
Those below hydrogen in the reactivity series
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What is the molecular formula for sodium hydrogen carbonate?
NaHCO3
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What is the charge on a hydrogen carbonate ion?
1-
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What is the charge on a carbonate ion?
2-
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What sort of compounds are bases?
Metal oxides and hydroxides
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What is an alkali?
The solution obtained when a metal hydroxide dissolves in water or when a base reacts with water
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What is produced when sodium oxide reacts with water?
Sodium hydroxide
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What state is sodium oxide?
Solid
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Why are there so few alkalis?
There are very few soluble metal hydroxides
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Give some examples of alkalis
Alkali metal hydroxides, barium hydroxide and ammonia solution
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Why is it called a neutralisation reaction?
H+ ions from acid react with OH- ions from alkali to produce neutral water
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Are neutralisation reactions exo or endothermic? By how much?
Exothermic giving out 57kJ per mole of water produced
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Ammonia solution is equal to...?
Ammonium hydroxide
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What is the reaction between ammonia solution and hydrochloric acid?
NH4OH + HCl --> NH4Cl + H2O
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When making salts, which part comes from the acid and which part comes from the alkali/ base?
Metal comes from alkali/base, non-metal comes from acid
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How can you make a soluble salt?
Titrate an acid against an alkali. Add indicator for the first time and then repeat the titration with the same volumes of acid and alkali but without the indicator
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Why can universal indicator not be used for acid-base titrations?
Composed of many indicators and changes gradually from one colour to the next
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Are the colours of universal indicator temperature dependant?
Yes - 25 degrees celcius
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How can you determine the pH of a solution?
Universal indicator, pH meter, universal indicator paper
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What is the definition of pH?
Negative logarithm to the base 10 of the hydrogen ion concentration in an aqueous solution
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A change of one unit on the pH scale is actually a change of...
Ten times the hydrogen ion concentration (logarithmic scale)
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How do you calculate H+ concentration from pH?
H+ = 10 to the power of -pH
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The formation of the ammonium ion from water is what sort of reaction?
Reversible
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What is Kw?
Ionic product constant [H+(aq)][OH-(aq)]
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What sort of solutions does Kw work for?
All aqueous solutions!
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What is Kw at 298K?
1.0 x 10 to the power of -14
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Why does a solution of acid contain OH- ions?
Ionisation of water means all solutions have H+ and OH- ions
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Why can we always measure pH?
Always H+ ions in an aqueous solution from the ionisation of water
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In a strong acid or alkali, do the H+ and OH- ions dissociate fully? What does this mean about the concentration of the solution?
Yes - it is the same as the concentration of H+ ions (acid) or OH- ions (alkali)
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If you calculate the pH of a solution, does the volume matter?
Not until you dilute it
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What is the general equation when water is added to an acid?
HA + H2O --> H3O+ + A-
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What has the acid donated to the water in its dissociation reaction?
An H+ ion
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What does an acid dissociate into?
H+ and A- ions
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Is the dissociation of an acid reversible or irreversible?
Reversible
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What is H+ equivalent to especially in the dissociation of an acid equation?
H3O+
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Give three examples of strong acids
Hydrochloric acid, sulfuric acid and nitric acid
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What in an equation shows dissociation is complete?
Non-reversible arrow
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What is a diprotic and monoprotic acid?
Dissociates to form two/ one proton per molecule
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Sulfuric acid is a strong acid for which dissociation only?
First
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What is the equation for the first dissociation of sulfuric acid?
H2SO4 +H2O --> HSO4- + H3O+
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What is the equation for the second dissociation equation for sulfuric acid?
HSO4- + H2O --> (reversible arrow) SO4(2-) + H3O+
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Give examples of weak acids and their molecular formula
Carbonic acid (H2CO3) and carboxylic acids
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What is the equation for the dissociation of any acid - weak or strong?
HA --> (reversible or irreversible arrow depending on strength) H+ + A-
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When is carbonic acid formed?
When carbon dioxide dissolves in water
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How do you work out what a carboxylic acid forms when it dissociates?
Take off the H in the COOH group and add a - sign to the last oxygen
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What are coral reefs made of?
Calcium carbonate/ limestone
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What is making the oceans more acidic?
Carbon dioxide dissolving in the oceans from human activities like industrialisation (burning fossil fuels)
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What is the general equation for the dissociation of a base?
B + H2O --> (reversible arrow) BH+ + OH-
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What happens to the base when it comes into contact with water?
It is protonated (definition of a base is that it is a proton acceptor)
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How do we classify strong and weak acids and bases?
How much they ionise in an aqueous solution
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Which bases are strong?
Alkali metal hydroxides and barium hydroxide
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Give examples of weak bases
Ammonia and amines
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What is the equation for the dissociation of ammonia?
NH3 + H2O --> (reversible arrow) NH4+ + OH-
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Which is stronger; ethylamine or ammonia?
Ethylamine
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The stronger the acid...
The weaker its conjugate base
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The stronger the base...
The weaker its conjugate acid
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Why do strong acids dissociate competely?
They have a weak conjugate base which has virtually no tendency to react with water or H+ to reform the strong acid
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Is HCN a strong or weak acid?
Weak
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The stronger the base, where does the position of equilibrium lie more towards?
The right
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When comparing strong and weak acids, why is it important to use equal concentrations of the two solutions?
Otherwise, a weak acid could have a greater concentration of H+ ions than the stronger acid
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Does a stronger or weaker acid conduct electricity better? Why?
Stronger because a stronger acid dissociates fully so there is a higher concentration of ions
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If the solution of a weak acid was connected to a bulb, would it glow and if so, how much?
It would glow because it contains ions but dimly
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When connecting an acid to a bulb, what could you measure to determine conductivity?
Use a conductivity meter to measure conductivity or see the different in brightness of the bulbs
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What sort of electrolytes are strong and weak acids?
Strong acids are strong electrolytes. Weak acids are weak electrolytes
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Why do stronger acids produce more H+ ions?
They dissociate more
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How can you tell the difference between a strong and weak acid in terms of their reactions?
Strong acids react more violently with metals or carbonates (effervescence and a faster rate but hard to measure)
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Why do strong acids react more violently with metals or carbonates?
They have a higher concentration of free H+ ions to react more rapidly with a metal to produce hydrogen
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Stronger acids would have a faster rate of reaction with...
A metal oxide or hydroxide
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What can be observed as the difference between a reaction with a strong acid and weak acid with a metal?
Larger bubbles in strong acid keep metal at surface of fluid. Smaller bubbles in weaker acid reaction spread through solution to make it seem milky
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What does strength refer to?
How much a substance dissociates in aqueous solution
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Can an indicator in a titration be used to distinguish between a weak and strong acid? Why?/ Why not?
No because at the same concentration, both acids will have the same end point (react with the same number of moles of alkali)
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What can be used to compare acid strength if equal concentrations of acids are used?
pH
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How can you get a stronger acid with a higher pH than a weak acid?
Different concentrations means the weak acid partially dissociates to produce higher concentration of H+ than the complete dissociation of a strong acid
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What is the formula for carbonic acid?
H2CO3
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When is rain considered to be acid rain?
pH below 5.6
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What is acid deposition?
Any process where acidic substances (particles, gases and precipitation) leave the atmosphere to be deposited on the surface of the Earth
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Give examples of wet deposition
Snow, fog and acid rain
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Give examples of dry deposition
Acidic gases and particles
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What does the conversion of sulfur dioxide to sulfur trioxide and hydrochloric acid involve?
Hydroxyl radicals, ozone or hydrogen peroxide
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What condition causes oxidation of nitrogen and the formation of nitrogen monoxide?
Very high temperatures
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How can acid rain affect vegetation?
The H+ ions displace the metal ions from the soil that get washed away. Mg2+ ions needed for chlorophyll production and affects photosynthesis. Aluminium ions dissolve from rocks, damaging roots, limits water uptake, stunts growth and yellows leave
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How can acid rain affect lakes and rivers?
Aquatic life sensitive if pH is below 6. Below 5.2 they cannot survive. pH 4 means no life. Dissolves harmful minerals that accumulates and damages life. Al3+ damages fish gills
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Which materials in buildings are dissolved by acid rain?
Marble and limestone
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How does acid rain damage human health?
Irritates mucous membranes leading to asthma and bronchitis. Acid rain dissolves heavy metal ions to release poisonous substances (Cu2+, Pb2+ and Al3++ thought to cause Alzheimer's)
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Give some examples of ways of dealing with acid deposition?
Use catalytic converters, use more public transport, remove sulfur before burning fuels, use renewable power supplies, 'lime' lakes with calcium hydroxide or oxide to neutralise acidity. Design better cars. More efficient power stations
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What is the name of the process to remove sulfur from fuel?
Desulfurisation
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How does pre-combustion desulfurisation occur?
Heat crude oil fractions with hydrogen in the presence of a catalyst. This produces hydrogen sulfide which is removed from reaction mixture by bubbling through alkaline solution
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What is done with the hydrogen sulfide after pre-combustion desulfurisation?
Converted back to sulfur to be sold to companies to make sulfuric acid
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How and when is post-combustion desulfurisation done?
In coal-fired power stations and involves passing exhaust gases from a furnace through a vessel where sulfur dioxide can react with alkalis/ bases to form calcium oxide, hydroxide or carbonate
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What is the acid dissociation constant?
Kc (equilibrium constant) x [H2O]
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What is the general expression for the dissociation of an acid?
HA (aq) --> (reversible) H+ (aq) + A-(aq)
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What does the value of Ka depend on? Why?
It is an equilibrium constant so depends on temperature
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What does the acid dissociation constant measure?
The extent that an acid dissociates
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The higher the value of Ka...
The more the acid dissociates and the stronger the acid
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When a strong acid dissociates, which part is larger in the acid dissociation constant expression?
Products (numerator)
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The lower the pKa...
The stronger the acid
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A low value of pKa gives...
A larger value of Ka and vice versa
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Why are Ka and pKa better measures of acid strength than pH?
Do not depend on the concentration of acid, only temperature
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What is one method of calculating Ka?
ICE diagram!
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What is the general equation for the ionisation of a base?
B(aq) + H2O --> (reversible) BH+ (aq) + OH- (aq)
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Why is water omitted from the base ionisation constant expression?
The concentration of water is much higher than the other concentrations of substances and is essentially constant
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The higher the value of Kb...
The more the base ionises and the stronger the base
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The lower the value of pKb...
The stronger the base
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What is the first thing you should do when you get a calculation question?
Write the chemical equation and expression for Ka/ Kb (if relevant)
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At what temperature is neutral pH 7?
25 degrees celcius
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The pH mark for neutral, acidic and alkaline varies with what...
Temperature
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What is a solution if it has a pOH lower than 7?
Alkaline
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What is a solution if it has a pOH higher than 7?
Acidic
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If pH is less than pOH...
The solution is acidic
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If pOH is less than pH...
The solution is alkaline
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If pH = pOH...
The solution is neutral
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As temperature increases, what happens to Kw?
It increases and the change of increase increases but it is not exponential
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In the expression for the dissociation of water, pH = ...
pOH
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The equation Kw = Ka x Kb only works for what?
Conjugate acid- base pairs
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At higher temperatures, what happens to Ka and pKa?
pH threshold increases so greater concentration of hydrogen ions meaning a higher Ka and lower pKa
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What happens to Kb and pKb at higher temperatures?
pH threshold increases so H+ concentration increases and OH- concentrations decreases so low Kb and high pKb
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What does pKw = pKa + pKb work only for?
Conjugate acid-base pairs
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Is HCN a weak or strong acid?
Weak
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Why do you need different indicators for acid-base titrations of different strengths?
Even with the same concentration, the equivalence point may not be at 7 so the range of pH varies like with indicators themselves
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How can you measure the pH during a titration?
pH meter or pH probe attached to a data logger
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The final pH in a strong acid and strong base titration heads towards but never reaches...
pH 13
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What is the equivalence point of a strong acid strong base titration?
pH 7
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At what volume of NaOH is the equivalence point reached in a titration with HCl? (equal concentrations of 0.1moldm3)
25cm3
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Why does it take a while for the pH to increase in a strong acid strong base titration?
There is a high H+ concentration initially so adding OH- ions does not affect overall pH a lot
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Where the graph is vertical on a pH curve, why does the pH increase so rapidly?
H+ concentration is low so pH is much greater affected by adding OH- ions
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What is the equivalence point?
The point when there are equal numbers of moles of acid and base added
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What does a log scale do on a pH curve?
Magnifies the effect of adding H+ or OH- ions
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Why does the equivalence point occur when you have equal volumes of acid and base added?
Equal concentrations added or acid and base
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What would change if you were adding alkali in an acid-base titration and you doubled the concentration of it?
The end pH would increase and the volume where the equivalence point would occur would be half the original (12.5cm3 instead of 25cm3)
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How would the pH curve change if the concentrations of acid and base were increased in a strong acid strong base titration?
Start closer to pH0, longer steep section and finish closer to pH14, No change to equivalence point
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What is on the x axis of the pH curve of an acid-base titration?
The volumes of whichever substance you put in the burette and did not know the volumes required
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Why can you not calculate the pH at 25cm3 NaOH in titrations?
There will be equimolar solutions of acid and base meaning H+ concentration would be zero which does not fit the logarithm
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What percentage of H+ concentration of a solution depends on the dissociation of water?
8.1 x 10(to the power of -9)%
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How much of the H+ concentration in a solution is entirely due to the dissociation of water when 25cm3 NaOH is added to the titration? Therefore, what does this make the pH?
All of it because water is formed in the neutralisation reaction. So the pH is 7
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With weak acids, what do we assume is negligible?
The dissociation and therefore the change in concentration
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What is formed when ethanoic acid reacts with sodium hydroxide?
CH3COONa + H2O
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Why is the equivalance point higher when you titrate a weak acid with a strong base?
There is a higher concentration of OH- ions
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What is the difference between the original buffer region in a strong acid strong base pH curve and a weak acid strong base pH curve?
The strong acid strong base has a flatter buffer region while the weak acid strong base pH curve is steeper
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Does the strength of acid affect the volume required to reach equivalence point?
No because that does not affect how much it dissociates
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What does the volume of base required to reach equivalence point depend on?
Initial volume of acid used, concentration of acid and concentration of base
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Why does the weak acid strong base pH curve have a higher initial pH?
The weak acid only partially dissociates so there is less H+ ions and thus a higher pH
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Why is the buffer region steeper when a weak acid and strong base titrates?
The weak acid partially dissociates so there is a lower H+ concentration, meaning adding NaOH has a greater relative effect on pH
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At what volume are there equal concentrations of reactant and product?
12.5cm3
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At the half equivalence point, pKa = ...
pH
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Why is the concentration of CH3COONa equal to the concentration CH3COO- at the half equivalence point?
The ionic salt ionises completely in solution
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If you add acid to the titration, what is the initial pH on a pH curve related to?
The initial pH of the base
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At the half equivalence point in the titration of a strong acid and weak base, pKb = ...
pOH
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Is there a buffer region in a strong acid strong base pH curve? Why? / Why not?
No because there is no conjugate acid-base pair present and no acid/ base with its salt
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Where is the equivalence point on a weak acid weak base pH curve?
Could be higher or lower than pH 7 depending on the relative strengths of the acid and base used
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What do acid-base titrations deduce?
Concentrations of acids and bases; Equivalent amounts of acids and bases that react with each other
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What does an indicator show in an acid-base titration?
When an equivalent amount of acid and base have been mixed
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What types of acids and bases are indicators?
Weak
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Why do indicators have to be weak acids or bases?
Partial dissociation means there is an equilibrium so can switch between colours
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What is the general equation for a weak acid indicator?
HIn(aq) --> (reversible) H+(aq) + I-(aq)
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What must occur in the general equation for a weak acid indicator to work?
The ionised and un-ionised forms must have different colours
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If you add acid to the weak acid indicator, which side does the equilibrium position shift to and why?
Shifts to the left because the equilibrium tries to use up as much of the H+ added as possible to form HIn
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What happens to the weak acid indicator equilibrium when a base is added?
Shifts to the right because the original H+ combines with the base to produce water so it shifts to replace the H+ lost
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What is the general equation for the weak base indicator equilibrium?
In-(aq) + H2O(l) --> (reversible) HIn(aq) + OH-(aq)
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What is the endpoint of a titration?
When the indicator changes colour
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What is the pH range of an indicator?
The pH values between which the indicator has intermediate colours because comparable amounts of ionised and un-ionised forms are present
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How can you tell if a substance is a good indicator?
If one drop of the solution from the burette causes a colour change (not gradual)- sharp end point
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What is required for the titration to have a clear end point?
The pH range of the indicator must be completely within the steep section of the pH curve (equivalence point)
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Can you see intermediate colours when you add indicator?
No
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Going dropwise when adding solution from the burette to the conical flask in a titration increases what?
Precision
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Can you detect an equivalence point using an indicator not in the pH range?
No
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Give two indicators suitable for the titration of a weak acid with a strong base
Phenol red and Phenolphthalein
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Why is an indicator not effective on a weak acid weak base titration?
The colour change is very gradual because there is not a steep part of the pH curve. Cannot judge end point because indicator gradually changes so cannot see equivalence point
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With a weak acid indicator, what does pKa =?
The pH at the midpoint of the pH range
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What is roughly the pH range of an indicator?
pKa +/- 1
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Why might the midpoint of the pH range not correspond exactly to the pKa value of an indicator?
Due to our ability to detect various colours and colour changes
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What is the pH range for a weak base indicator?
14-pKb +/- 1
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Why can an indicator not be used to detect a weak or strong acid in a titration?
If they have the same concentration, they will require the same amount of alkali and have the same equivalence point
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Will the salt of sodium ethanoate be basic or acidic when dissolved? Why?
Basic because it was composed of a strong base (NaOH) and a weak acid (ethanoic acid)
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What can the stronger CH3COO- ion do in solution?
Because it is a strong conjugate base from the ethanoic acid conjugate acid, it can react with water to gain a proton and form ethanoic acid and OH- ions
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What is salt hydrolysis?
When a salt is ionised and dissolves in water and reacts to produce a solution with a certain pH
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What makes the solution acidic when ammonium chloride dissolves in water?
The ammonium ion is a strong conjugate base so dissociates to form NH3 and H+
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Can you determine the pH of a solution of a salt consisting of a weak acid and weak base?
Not without looking at pKa and pKb values
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How can you determine the pH of a solution of a salt with a weak acid and base?
See if the pKa or pKb value is lower and that one is stronger (if they are very close, the pH will be about 7)
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Which ions are acidic when dissolved in solution? Why can Fe3+?
3+ ions because they can form complex ions with water as a ligand and when it dissociates, this can form H+
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Why can 3+ ions produce acidic solutions?
High charge density in smaller atomic radius means water molecules can be sufficiently polarised for H+ to delocalise. The Fe3+ ion pulls electrons away from water so it increases the delta positive charge on hydrogen, making them easier to lose
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The larger the ion and lower the charge...
The smaller the charge density and lower the tendency to polarise water molecules
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Which will have a lower pH, Fe3+ ions in solution or Fe2+ ions in solution? Why?
Fe3+ because it has a higher charge density to polarise water molecules greater and produce more H+ ions
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What is a buffer solution?
A solution that resists change in pH when small amounts of acid or alkali are added
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What is required for a buffer solution?
A mixture of an acid and a base
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Where are buffer solutions used?
Blood (pH 7.4), shampoos, cosmetics and contact lens solutions
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What does an acid buffer solution contain?
A weak acid and a salt of that weak acid e.g. ethanoic acid with sodium ethanoate
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In a buffer solution containing ethanoic acid, what happens when a base is added?
CH3COOH + OH- --> CH3COO- + H2O
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In a buffer solution containing sodium ethanoate, what happens when acid is added?
CH3COO- + H+ --> CH3COOH
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What is required in terms of quantities for a buffer solution to occur?
Large concentrations of acid and (conjugate) base ion so any addition of acid and base cause little change in comparison
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What is the ratio of acid to basic salt, H+ concentration and pH like in a buffer solution?
They all change very little
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In a buffer solution, what must be equal?
The concentrations of acid and basic salt
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What does a base buffer solution consist of?
A weak base and a salt of a weak base e.g. ammonia and ammonium chloride
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If an acid is added to a basic buffer solution of ammonia, what is the reaction?
NH3 + H2O --> NH4+
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If a base is added to a basic buffer solution of ammonia, what reacts?
NH4+ + OH- --> NH3 + H2O
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What is the ratio from with a buffer solution?
Kb or Ka expression
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How can buffer solutions be made?
With partial neutralisation of a weak acid with a strong base
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If you are making an acid buffer solution, which component has a smaller volume/ moles to ensure only partial neutralisation?
The base
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Why does the pH range not change very much in the buffer region of a pH curve?
Lots of acid and base salt present so adding OH- or H+ ions has little effect
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Why does the buffer region sometimes have an increase of 1.5 pH on a pH curve given that it should be a buffer region?
We are not just adding small amounts here. We could be adding NaOH of the same concentration as acid and similar volume
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Partial neutralisation of what strength reactants gives a base buffer solution?
Weak base and strong acid
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In general, when partially neutralising to make a buffer solution, which reactant should have less moles?
The stronger one (e.g. the strong acid in a weak base, strong acid buffer)
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Why does the stronger reactant have to have less moles in the partial neutralisation to make a buffer?
So the one with more moles reacts a little and some remains to give both components of the buffer solution
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Other cards in this set
Card 2
Front
What is a base?
Back
A substance that produces hydroxide ions in a solution
Card 3
Front
What is a Bronsted-Lowry acid?
Back
Card 4
Front
What is a Bronsted-Lowry base?
Back
Card 5
Front
How could you tell which was the acid in a conjugate acid-base pair?
Back
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