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docbacidsbasessalts updated Feb 17th 2008 |
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KS4 Science GCSE-IGCSE Chemistry revision-information notes on The pH scale of acidity and alkalinity, acids, alkalis, salts & neutralisation Sub-index: (1) Examples of using acid-alkali chemistry (2) pH scale, indicators, ionic theory of acids-alkali neutralisation (3) Examples of acid, neutral or alkaline solutions (4) Acid reactions with metals/oxides/hydroxides/carbonates and neutralisation reactions (5) Reactions of bases-alkalis like sodium hydroxide (6) Two Methods of making water soluble salts (7) Changes in pH in a neutralisation (8) Summary of important formulae (9) Further examples of word/symbol equations Multiple choice revision quizzes: easy start KS3 m/c quiz then Foundation GCSE Quiz and finally Higher GCSE Quiz (if appropriate) * also some easy start word-fills on some basic ideas ex KS3 and word fills on reactions ex KS3 and an easy start matching pair quiz * Equation worksheet + answers on reactions of acids There is more on acid-base theory and its history, pH measurement, strong or weak, acids or alkalis, making insoluble salts etc. etc. on the Extra Aqueous Chemistry * EMAIL query?comment Spelling note: sulphuric acid = sulfuric acid, sulphate = sulfate, aluminium = aluminum (US) (1) Introducing a few examples of using this sort of chemistry In the HOME: Alkaline lime (CaO, calcium oxide), is put on soil that is too acid for healthy plant growth. Powdered limestone (CaCO3, calcium carbonate) is slower and less effective. Both react and neutralise acids and you can pre-test the soil with pH paper. They can be used on a larger scale in farming and rivers and lakes.
Bicarbonate or powder* or baking powder can be used with sour milk (acidic) for raising action in baking. The acidic milk reacts with the *sodium hydrogen carbonate (NaHCO3) to form carbon dioxide gas to give the rising action.
In the chemical INDUSTRY: Sodium hydroxide, one of the most commonly used alkalis,is used to neutralise aspirin making 'soluble aspirin'. Aspirin is an organic acid and not very soluble in water, its sodium salt is much more soluble and is absorbed faster by the body.
Neutralising harmful sulphur dioxide gas (acidic, irritating and toxic SO2) in power station smoke from burning fossil fuels, by absorbing it in alkaline calcium hydroxide solution (limewater) to absorb it. Eventually harmless calcium sulphate solution is formed.
So all of this is still pretty important chemistry even for the 21st century, with strong links to agriculture, the environment and leading a stressful life! Of course there are 'downsides' to some of this 'acidic' chemistry: Acid rain increases the rate of corrosion of stonework (particularly limestone) and metal structures. Acid rain makes water too acid for some aquatic organisms to live and this in turn affects food chains e.g. salmon do not like water with a pH below 4.5! Living on Venus could be hard going, its atmosphere is 98% sulphuric acid, mind you, you should be ok in a plastic suit because plastics don't usually react with acids, which is why, as well as being cheaper, plastics are replacing water pipes, drain pipes and gutters etc.
(2) The pH Scale - Acids and Alkali - ion theory
The colours of solutions with universal indicator
An indicator is a substance or mixture of substances that when added to the solution gives a different colour depending on the pH of the solution. Universal indicator solution or paper, is prepared from mixing several indicators to give a variety of colours to match the pH. It is a very handy indicator for showing whether the solution is very weakly/strongly acidic (pH <7) or alkaline (pH > 7) or neutral (pH = 7) and gives the pH to the nearest pH unit. Theoretically there is no limit to the pH scale, but most solutions are between pH 0 and pH 14. For example, looking at the 'extremes', 1M hydrochloric acid (HCl) has a pH of 0 and 10M HCl has a pH of -1, and 1M sodium hydroxide (NaOH) has a pH of 14, but 10M potassium hydroxide (KOH) has a pH of 15! However the solubility limits of substances in water ensures that its almost impossible to get below -1 or above 15! Note 1: M is the old shorthand for solubility in mol/litre or mol dm-3. Note 2: The pH scale is known as a logarithmic scale of base 10. At GCSE level, to put it more simply, a change of one pH unit means a 10x change in the acidity or alkalinity of the solution e.g. from pH 5 to pH 2 means an increase in acidity of 1000x, or to change from pH 13 to pH 11 means to become 100x less alkaline.). Other common indicators used in the laboratory (* often used in titrations)
BASES e.g. oxides, hydroxides and carbonates, are substances that react and neutralise acids to form salts and water.
After a neutralisation, the salt solutions consist of a mixture of positive and negative ions (and their names are in the salt name!) e.g. sodium chloride (NaCl) is a mixture of Na+ and Cl- ions, calcium chloride (CaCl2) is a mix of Ca2+ and Cl- ions; magnesium nitrate (Mg(NO3)2) is a mix of Mg2+ and NO3- ions, aluminium sulphate (Al2(SO4)3) consists of Al3+ and SO42- ions etc. See other GCSE Extra Aqueous Chemistry notes for the advanced proton/hydrogen ion theory of acids and bases (Bronsted-Lowry theory)
Acids are neutralised by reaction with metals, oxides, hydroxides or carbonates to form salts and other products. Apart from metals (which is an electron loss/gain redox reaction), the other reactants listed above are considered as bases (meaning they react by accepting a proton from an acid). Water soluble bases are known as alkalis. The reaction between acids and bases like oxides, hydroxides and carbonates are called neutralisation reactions.
(A2) magnesium + sulphuric acid ==> magnesium sulphate + hydrogen
(B) alkali (soluble base )
+ acid
(B1) e.g. sodium hydroxide + hydrochloric acid ==> sodium chloride + water (method a)
(B2) metal hydroxide + acid ==> a salt* + water
(B3)
insoluble base + acid
==> salt + water
(note:
oxides that react with acids to form salts are known as 'basic
oxides') e.g. metal oxide + acid ==> salt* + water e.g. copper(II) oxide + sulphuric acid ==> copper(II) sulphate + water CuO(s) + H2SO4(aq) ==> CuSO4(aq) + H2O(l) (B4) calcium hydroxide + hydrochloric acid ==> calcium chloride + water
See also extra examples
Note: Using sulphuric acid and calcium carbonate you don't get much of a fizz! because the calcium sulphate salt formed, is not very soluble, and coats the remaining calcium carbonate inhibiting the reaction! This will happen with any reaction between an acid and a water insoluble reactant which forms an insoluble solid product! (C2) magnesium carbonate + sulphuric acid ==> magnesium sulphate + water + carbon dioxide
or sodium hydrogencarbonate + nitric acid ==> sodium nitrate + water + carbon dioxide NaHCO3(s) + HNO3(aq) ==> NaNO3(aq) + H2O(l) + CO2(g) See also extra examples at the end of the page.
NOTE (a)*: The name of the particular salt formed depends on (i) the metal name, which becomes the first part of salt name, and (ii) the acid e.g. H2SO4 sulphuric acid on neutralisation makes a ... sulphate; HCl hydrochloric acid makes a ... chloride; HNO3 nitric acid makes a nitrate etc. NOTE (b)**: There is a list of compound formulae and their solubility at the bottom of the page. The first part of the salt name is ammonium derived from ammonia (with metals or their compounds the metal retains its original name), but the second part of the salt name is always derived from the acid as in NOTE (a) above. NOTE (c): Ammonia is an alkaline gas that is very soluble in water. It is a weak alkali or soluble base and is readily neutralised by acids in solution to form ammonium salts which can be crystallised on evaporating the resulting solution. Sometimes the equations are written with the 'fictitious' 'ammonium hydroxide'
NOTE (d): There are more equations at the end and an extensive structured question on acid reaction equations.
(5) Some important reactions of Bases (alkali = soluble base)
Aqueous solutions of alkalis like sodium hydroxide ('caustic soda') and calcium hydroxide ('limewater') react with the acidic gas carbon dioxide to form carbonate compounds if the gas is bubbled into their solutions.
(6) Two Methods of making Salts which are water soluble
(1) A known volume of acid is pipetted into a conical flask and universal indicator added. The acid is titrated with the alkali from the burette. (2) The acid is added until the indicator turns green, pH 7 neutral. This means all the acid has been neutralised to form the salt (3) The volume of alkali needed for neutralisation is then noted, this is called the endpoint volume. (1)-(3) are repeated with both known volumes mixed together BUT without the contaminating universal indicator. (4) The solution is transferred to an evaporating dish and heated to partially evaporate the water causing crystallisation or can be left to slowly evaporate - which tends to give bigger and better crystals. (5) The residual liquid can be decanted away and the crystals can be carefully collected and dried by 'dabbing' with a filter paper OR the crystals can be collected by filtration (below) and dried (as above). Note (i) You can put the acid in the burette and the alkali in the flask. (ii) Parts (1) to (3) are known specifically as an acid-base (alkali) titration, and the general method is known as a volumetric titration by which it possible to find out exactly what volume ratios are needed for neutralisation. So knowing one concentration, you can calculate the other. (iii) Concentration calculations are on other pages sections 11. and 12. (iv) Apparatus used: (1) pipette and conical flask; (2)-(3) burette and conical flask; (4) evaporating (crystallising) dish, bunsen burner, tripod and gauze; (5) filter paper. (v) Other indicators e.g. phenolphthalein can be used instead (pink alkaline, colourless acid). (vi) The burette and pipette are both used for the accurate measurement of volume. (vii) The pH changes in this preparation are described in section (7).
(1) The required volume of acid is measured out into the beaker with a measuring cylinder. The metal, oxide, hydroxide or carbonate is weighed out and the solid added in small portions to the acid in the beaker with stirring. (2) The mixture may be heated to speed up the reaction. When no more of the solid dissolves it means ALL the acid is neutralised and there should be a little excess solid. (3) The hot solution (with care!) is filtered to remove the excess solid metal/oxide/carbonate, into an evaporating dish. (4) The hot solution is left to cool and crystallise. Then collect and dry the crystals with a filter paper. Note (i) Apparatus used: (1) balance, measuring cylinder, beaker and glass stirring rod. (2) beaker/rod, bunsen burner, tripod and gauze; (3)-(4) filter funnel and filter paper, evaporating (crystallising) dish. (ii) A measuring cylinder is adequate for measuring the acid volume, you do not need the accuracy of a pipette or burette required in method (a). (iii) How to calculate amounts required and % yield is dealt with in Chemical Calculations Part 14. More on other methods of making salts on the "Extra Aqueous Chemistry" page.
(7) What pH changes go on in a neutralisation reaction?
The graphs show how the pH changes when an alkali (soluble base) and an acid neutralise each other and what you see visually using universal indicator (univ. ind.). This what is happening in the salt preparation method (a) above. Note: you can prepare a salt by doing the acid-alkali addition either way round but in either case the volume of acid or alkali needed for neutralisation = the volume reading X at pH 7 (univ. ind. green). Red graph line: If you add acid to an alkali (univ. ind. = blue), the pH starts at about 13 and only falls little at first as the colour changes from purple ==> blue. Then the pH falls much more steeply as the indicator colour changes from 'bluey' green ==> dark green ==> pale green. The solution is then neutralised at pH 7. This is the point where the salt is 100% formed. With further addition of excess acid, the pH falls and then levels out to about pH 1 as the colour changes further from green ==> yellow ==> orange. Blue graph line: If you add alkali to an acid (univ. ind. = red), the pH starts at about 1 and only rises a little at first with the colour still quite red. Then on further addition of alkali the pH rises more sharply as the colour changes from red ==> orange ==> yellow and eventually at the neutralisation point at pH 7 the univ. ind. is green. This is the point where the salt is 100% formed. With excess alkali the pH continues to rise and then levels out to about 13 as the indicator colour changes through dark green ==> blue ==> purple. Universal indicator, and most other acid-base indicators, work for strong acid and alkali titrations, but universal indicator is a somewhat crude indicator for other acid-alkali titrations because it gives such a range of colours for different pH's. Examples of more accurate and 'specialised' indicators are:
(8) A Summary of important formulae The original acids are hydrochloric acid HCl, sulphuric acid H2SO4, nitric acid, HNO3 which give the salts when reacted with a metal, oxide, hydroxide or carbonate.
(9) FURTHER EXAMPLES of WORD & SYMBOL EQUATIONS for Salt Preparations i.e. reactions of oxides, hydroxides or carbonates with acids where it says similar, you can just change the metal name/symbol These reactions apply to the two salt making methods described above word/symbol equation worksheet questions with answers * more examples and details above 1a. copper(II) carbonate + sulphuric acid ==> copper(II) sulphate + water + carbon dioxide (method b) 1b. CuCO3(s) + H2SO4(aq) ==> CuSO4(aq) + H2O(l) + CO2(g) 1c. Similar for many other Group 2 and Transition metal carbonates e.g. Mg, Ca and Fe, Co, Ni, Zn 2a. magnesium hydroxide + hydrochloric acid ==> magnesium chloride + water (method b) 2b. Mg(OH)2(s) + 2HCl(aq) ==> MgCl2(aq) + 2H2O(l) 2c. Similar for many other Group 2 and Transition metal hydroxides e.g. Ca, Sr, Ba and Fe, Co, Ni, Cu, Zn 3a. zinc + sulphuric acid ==> zinc sulphate + hydrogen (method b) 3b. Zn(s) + H2SO4(aq) ==> ZnSO4(aq) + H2(g) 3c. Similar for many other Group 2 and Transition metals e.g. Mg, Ca and Fe, Co, Ni 4a. ammonia + nitric acid ==> ammonium nitrate (method a) 4b. NH3(aq) + HNO3(aq) ==> NH4NO3(aq) 5a. zinc oxide + hydrochloric acid ==> zinc chloride + water (method b) 5b. ZnO(s) + 2HCl(aq) ==> ZnCl2(aq) + H2O(l) 5c. Similar for many other Group 2 and Transition metal oxides e.g. Mg, Ca, Ba and Co, Ni, Cu 6a. calcium carbonate + hydrochloric acid ==> calcium chloride + water + carbon dioxide (method b) 6b. CaCO3(s) + 2HCl(aq) ==> CaCl2(aq)+ H2O(l) + CO2(g) 6c. Similar for many other Group 2 and Transition metal carbonates e.g. Mg, Sr, Ba and Ni, Co, Zn 7a. sodium carbonate + hydrochloric acid ==> sodium chloride + water + carbon dioxide (method a) 7b. Na2CO3(s) + 2HCl(aq) ==> 2NaCl(aq) + H2O(l) + CO2(g) 8a. potassium hydroxide + hydrobromic acid ==> potassium bromide + water (method a) 8b. KOH(aq) + HBr(aq) ==> KBr(aq) + H2O(l) 8c. Its the same for any Group 1 Alkali Metal hydroxide e.g. LiOH, NaOH, RbOH etc. and other Group 7 halogen acids e.g. HCl hydrochloric acid. 9a. sodium hydrogencarbonate + hydrochloric acid ==> sodium chloride + water + carbon dioxide 9b. NaHCO3(s) + HCl(aq) ==> NaCl(aq) + H2O(l) + CO2(g) 9c Note that hydrogencarbonate is commonly known as 'bicarbonate'.9d. Its the same for any Group 1 Alkali Metal hydrogencarbonate e.g. for Li, K etc.
ks4 science examinations gcse-igcse chemistry revision * ks4 science examinations-gcse-igcse chemistry revision * ks4 science examinations-gcse-igcse chemistry revision * ks4 science examinations-gcse-igcse chemistry revision * ks4 science examinations-gcse-igcse chemistry revision * ks4 science examinations-gcse-igcse chemistry revision * SITE PURPOSE EDUCATION - online learning or 'self-private-tuition' using revision notes, quizzes, practice tests involving GCSE Science CHEMISTRY in the areas of REVISING only the CHEMISTRY-Earth Science-Radioactivity at Doc Brown's Chemistry Clinic via HOMEPAGE in secondary school/schools, 6th form college/colleges, academy/academies or home self-study. Hopefully it will encourage interest and understanding of Chemistry, Earth Science and Radioactivity in any country of the world, though the site is written entirely in English. The website is designed to help and unofficially support students/teachers revise-learn/teach the chemistry for modular or co-ordinated examination science courses from UK QCA based AQA, OCR (Oxford and Cambridge) Twenty First (21st) Century and Gateway Science, Edexcel 360Science , Nuffield, Salters, Cambridge International (CIE), London International, WJEC, CCEA exams etc. Also, national award assessments-examinations for GCSE-IGCSE-KS4-O level-BTEC-NVQ applied, additional and chemistry national science courses. Also covers, mainly via quizzes the UK National KS3 SATs Science-biology/chemistry/physics (SAT revision levels 3-5 or 5-7) and covers much of the revising, learning and teaching chemistry examinations for the national curriculum for secondary schools and colleges. The site does not support the content of England, Wales or Northern Ireland primary science KS1 or KS2. The notes should also provide some background theory for a coursework assignment or project. BUT please note that my on-line revision notes and quizzes are no substitute for good classroom teaching-lecturing and thorough studying of your own notes and textbooks, practicing past papers and a copy of the syllabus which are readily downloaded from the examination board sites, but I hope here and there they will lend a tutoring hand on some topic, unit, module etc. For final revision you have to be intellectually honest about what you don't know or follow, YOU have to take the stuff to pieces, analyse what you do/do not understand and reconstruct it so it all makes sense in the end. There is no other way, there are no magic secrets on how to revise and learn, its mainly down to hard work and just good old fashioned study and employing teach-yourself strategies without the need for extra tutors and tutoring lessons. I also think there is too much hit and miss revision using past papers (which I do NOT supply) and not enough systematic revision. I also hope it will help teachers in planning lessons and developing schemes of work for science-chemistry. There are no lesson plans on the site but there are plenty of quizzes to incorporate into classroom activities whether photocopied or on electronic whiteboard projector for use as self-tuition-assessment purposes and a variety of teaching and learning styles and the images may be used in Microsoft Word documents and powerpoint projections. The site seems to be used by a large number of home study tutors, particularly the revision notes. An individual tutor may print out the notes for science-chemistry learning teaching-tuition purposes and for background material for assignments and projects. I have no interest or time in producing WORD.doc or xxxx.pdf files of the notes at the moment. Neither have I time to write up many practical laboratory experiments ('lab'-'labs') at the moment, but the notes contain lots of background information of chemical reactions in terms of observations-balanced equations-reactants-products-theory etc. I also find it difficult to recommend specific exam websites or syllabus textbooks, it depends exactly on what you need, what you have time for, and there are so many of them to choose from and I do not supply past examination papers for classes. The sites resources include revision notes, quizzes and worksheets which provide support for home study or tuition for homework and coursework help e.g. science investigations for any of the key stage courses indicated, but I do not supply lesson plans. Dr W P Brown gcse 10-11-2007 * ks4 science examinations gcse-igcse chemistry revision * ks4 science examinations-gcse-igcse chemistry revision * ks4 science examinations-gcse-igcse chemistry revision * ks4 science examinations-gcse-igcse chemistry revision * ks4 science examinations-gcse-igcse chemistry revision * ks4 science examinations-gcse-igcse chemistry revision
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Antacid indigestion tablets are mild
alkalis that react by neutralising excess stomach acid
Acidic bee stings
(pH 5.0-5.5)
can be soothed,
i.e. neutralised by calomine lotion, which is a mild alkali and
antiseptic and anti-itching agent based 
on
zinc oxide. You can also use baking soda ('bicarb of soda' or
sodium hydrogen carbonate), another mild alkali.
Wasp stings are supposed to be alkaline, but apparently not so! they are almost
neutral at pH 6.8-6.9 but are 'traditionally' treated with vinegar which is a weak
acid (and then perhaps you need the calomine too!).
I've come across references on the web to say that wasp stings are
not alkaline so 'English folklore' and mild-weak acid treatment has no real
scientific basis. It should be pointed out that sting venom is a complex mixture, including many
protein-enzymes, which, with other 'foreign' substances, might well trigger a response from the bodies
immune system, so, in all honesty, I'm not quite sure what the truth
is! However, what is known is that (i) bees and wasps have
glands that can secrete either acids or alkalis with other substances and (ii) ants sting
venom often contains methanoic acid ('formic acid') which can
have a pH
of 3 and is presumably 'soothed' by mild alkalis and just to confuse matters
more, (iii) many people claim the 'folklore' remedies
work! and maybe they do!
Ammonia gas is
a weak alkali and neutralised by
sulphuric acid or nitric acid to form ammonium sulphate or ammonium
nitrate salts. These are important agri-chemical fertilisers
supplying nitrogen
to the soil for better plant growth. Of course
some people prefer organic growing using good old muck and compost,
but it doesn't involve neutralisation, but it does involve my wife,
who is a member of the Soil Association!
Acids can be used to clean corroded
metal surfaces because of their reactivity to metals and metal
oxides to form soluble salts which can be washed away to leave a
cleaner metal surface.
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docbacidsbasessalts updated Feb 17thd 2008 |