acids & bases (c2.6) - secondary science 4 all · 1. acid + alkali neutralisation titrate exact...
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Acids & Bases (C2.6) Section 1: Practicals (making salts & neutralisation)
Section 2: Defining terms
Section 3: Neutralisation reactions
Section 4: Making soluble and insoluble salts
Section 5: Extension Questions
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1. ACID + ALKALI NEUTRALISATION Titrate exact quantity of acid to alkali (or vice-versa) using indicator, then repeat the titration without indicator using the exact volume determined before. Crystallise the salt solution formed.
2. ACID + INSOLUBLE BASE (oxide or carbonate) NEUTRALISATION Add solid base to acid until there is excess (unreacted) base (seen visually). Filter off the solid excess base to leave a filtrate of salt solution. Crystallise the salt solution.
3. REACT TWO SOLUBLE IONIC COMPOUND SOLUTIONS TO GIVE AN INSOLUBLE IONIC COMPOUND Add exact quantities of aqueous ions (according to chemical equation). An insoluble solid is formed (precipitate). Filter off the solid precipitate and keep (this is the salt). Discard the filtrate as this contains soluble spectator ions.
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Clean/fill burette HCl
NaOH + indicator in conical flask
Titrate, drop by drop to indicator end-point
Titrate without indicator
Crystallise product
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Defining terms
An acid is a substance that produces hydrogen ions H+(aq) in aqueous solution HCl(aq) H+(aq) + Cl-(aq) A base is a substance that can neutralise an acid e.g. NaOH
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pH is a scale that indicates the ********** of aqueous hydrogen ions [H+(aq)]in solution. The pH scale extends from ** to **. At room temperature, • pH below ** means the solution is acidic • pH of 7 means the solution is neutral • pH above 7 means the solution is ***** • Pure water is always **** because it has an equal concentration of [H+(aq)] and [OH-(aq)] • If the concentration of [H+(aq)] is more than that present in pure
water, then the solution is ***** • If the concentration of [H+(aq)] is **** than that present in pure
water, then the solution is alkaline .
Alkaline Acidic Less 0 14 7 Neutral Concentration
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pH is a scale that indicates the concentration of aqueous hydrogen ions [H+(aq)]in solution. The pH scale extends from 0 to 14. At room temperature, • pH below 7 means the solution is acidic • pH of 7 means the solution is neutral • pH above 7 means the solution is alkaline • Pure water is always neutral because it has an equal concentration of [H+(aq)] and [OH-(aq)] • If the concentration of [H+(aq)] is more than that present in pure
water, then the solution is acidic • If the concentration of [H+(aq)] is less than that present in pure
water, then the solution is alkaline .
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Strong acid Weak acid Neutral Weak Base Strong Base
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What makes an acid strong or weak?
Why does adding hydroxide ions [OH-(aq)] make a solution less acidic and more alkaline?
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What makes an acid strong or weak?
Why does adding hydroxide ions [OH-(aq)] make a solution less acidic and more alkaline?
A strong acid produces high concentration of H+(aq) in aqueous solution, so the pH is very low (e.g. pH of hydrochloric acid is 2) A weak acid, produces aqueous H+(aq), but at a lower level than a strong acid, so the pH is around 3-6 (e.g. pH of ethanoic acid is 5)
OH-(aq) react with the H+ (aq) present in the solution to make H2O. Hence, there are fewer H+(aq) in solution because they have been neutralised by the OH-, so the pH will be less acidic and become more alkaline.
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Neutralisation reactions
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
H+ (aq) + OH-(aq) H2O(l)
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The salt produced in an acid + base (or alkali or metal) neutralisation reaction depends on: • the acid used (e.g. HCl produces chlorides, HNO3 produces nitrates,
H2SO4 produces sulfates) • the metal in the base or alkali.
Acid + Base → Salt + Water Acid + Alkali → Salt + Water Acid + Metal → Metal salt + Hydrogen gas Acid + Metal Oxide → Salt + Water Acid + Metal Carbonate → Metal salt + Water + Carbon Dioxide Acid + Metal Hydrogen Carbonate → Metal salt + Water + Carbon Dioxide
Acid + Ammonia solution → Ammonium salt + Water
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The salt produced in an acid + base (or alkali or metal) neutralisation reaction depends on: • the acid used (e.g. HCl produces chlorides, HNO3 produces nitrates,
H2SO4 produces sulfates) • the metal in the base or alkali.
Acid + Base → Salt + Water Acid + Alkali → Salt + Water Acid + Metal → Metal salt + Hydrogen gas Acid + Metal Oxide → Salt + Water Acid + Metal Carbonate → Metal salt + Water + Carbon Dioxide Acid + Metal Hydrogen Carbonate → Metal salt + Water + Carbon Dioxide
Acid + Ammonia solution → Ammonium salt + Water
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What is the difference between a BASE and an ALKALI ?
Acid + Base → Salt + Water Acid + Alkali → Salt + Water Acid + Metal → Metal salt + Hydrogen gas Acid + Metal Oxide → Salt + Water Acid + Metal Carbonate → Metal salt + Water + Carbon Dioxide Acid + Metal Hydrogen Carbonate → Metal salt + Water + Carbon Dioxide
Acid + Ammonia solution → Ammonium salt + Water
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What is the difference between a BASE and an ALKALI ?
A base can neutralise an acid
Acid + Base → Salt + Water Acid + Alkali → Salt + Water Acid + Metal → Metal salt + Hydrogen gas Acid + Metal Oxide → Salt + Water Acid + Metal Carbonate → Metal salt + Water + Carbon Dioxide Acid + Metal Hydrogen Carbonate → Metal salt + Water + Carbon Dioxide
Acid + Ammonia solution → Ammonium salt + Water
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What is the difference between a BASE and an ALKALI ?
An alkali is a base that is a soluble metal hydroxide
Acid + Base → Salt + Water Acid + Alkali → Salt + Water Acid + Metal → Metal salt + Hydrogen gas Acid + Metal Oxide → Salt + Water Acid + Metal Carbonate → Metal salt + Water + Carbon Dioxide Acid + Metal Hydrogen Carbonate → Metal salt + Water + Carbon Dioxide
Acid + Ammonia solution → Ammonium salt + Water
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In metal + acid reaction, the metal has to be
more reactive than H2 in order to produce a
metal salt
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Aqueous hydrogen ions (from acid) react with aqueous hydroxide ions (from alkali) to produce water
magnesium chloride
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(neutralisation reaction)
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(neutralisation reaction)
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2NaHCO3 + H2SO4 Na2SO4 + 2H2O + 2CO2
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Making salts
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1. ACID + ALKALI NEUTRALISATION Titrate exact quantity of acid to alkali (or vice-versa) using indicator, then repeat the titration without indicator using the exact volume determined before. Crystallise the salt solution formed.
2. ACID + INSOLUBLE BASE (oxide or carbonate) NEUTRALISATION Add solid base to acid until there is excess (unreacted) base (seen visually). Filter off the solid excess base to leave a filtrate of salt solution. Crystallise the salt solution.
3. REACT TWO SOLUBLE IONIC COMPOUND SOLUTIONS TO GIVE AN INSOLUBLE IONIC COMPOUND Add exact quantities of aqueous ions (according to chemical equation). An insoluble solid is formed (precipitate). Filter off the solid precipitate and keep (this is the salt). Discard the filtrate as this contains soluble spectator ions.
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MAKING SALTS
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The salt produced in an acid + base (or alkali or metal) reaction depends on: • the acid used (e.g. HCl produces chlorides, HNO3 produces nitrates,
H2SO4 produces sulfates) • the metal in the base or alkali.
Acid + Base → Salt + Water Acid + Alkali → Salt + Water Acid + Metal → Metal salt + Hydrogen gas Acid + Metal Oxide → Salt + Water Acid + Metal Carbonate → Metal salt + Water + Carbon Dioxide Acid + Metal Hydrogen Carbonate → Metal salt + Water + Carbon Dioxide
Acid + Ammonia solution → Ammonium salt + Water
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In metal + acid reaction, the metal has to be
more reactive than H2 in order to produce a
metal salt
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Making copper sulfate crystals
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Making copper sulfate crystals
increase rate of reaction H2SO4 H2O
filtration
evaporation, heating, cooling (crystallisation)
incomplete reaction, some sample lost during filtration, weighing error
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Q Answer Extra Information Mark
(a)
O marks
No relevant content.
Level 1 ( 1-2 marks)
There is a simple description
of a laboratory procedure for
obtaining potassium chloride
Level 2 (3-4marks)
There is a clear description of
a laboratory procedure for
obtaining potassium chloride
from potassium hydroxide
solution and hydrochloric acid
that does not necessarily allow
the procedure to be completed
successfully by another
person. The answer must
include the use of an indicator
or a method of obtaining
crystals.
Level 3 (5-6marks)
There is a detailed description
of a laboratory procedure for
obtaining potassium chloride
from potassium hydroxide
solution and hydrochloric acid
that can be followed by
another person. The answer
must include
the use an indicator and a
method of obtaining crystals
•One reagent in the beaker (or similar)
•Add (any named) indicator
•Add other reagent. Swirl or mix
•Add drop by drop near to titration
end- point
•Stop addition at change of indicator
colour. Note volume of reagent added
• Repeat titration without indicator
adding same volume of reagent
• Pour solution into evaporating dish
• Heat using Bunsen burner and
evaporate around half the water
away
• Leave to crystallise
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Section 5: Extension questions
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2 2 6
OH-
H+
dry environment, no water present
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in water, HCl is ionic and dissociates and forms [H+](aq). As a gas, HCl is a covalent molecule and does not produce [H+](aq)
KOH fully dissociates and produces a high concentration of [OH-](aq)
equal concentrations of [H+](aq) and [OH-](aq)
base is a proton acceptor
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• C2 2.6.1
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• C2 2.6.1
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Q Answer Extra Information Mark
(b) Nitric (acid) Allow HNO3
Ignore incorrect formula
1
(c)
(i)
Because it is a fertiliser / helps
plants grow
Allow plant food
Do not accept pesticide /
herbicide / neutralising soil
1
(c)
(ii)
Tick by: Should farmers stop using
ammonium nitrate on their land ?
And two from
•Cannot be done by experiment
•Based on opinion / view
•Ethical or economic issue
Accept difficult to get / not
enough evidence
Allow must be done by survey
If top box tick allow 1 mark for
drinking water varies from
place to place
1
2
Total 11