GCSE Chemistry Notes: Neutralisation, indicators and pH curves
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7. pH changes in acid–alkali neutralisations and choice of indicator for a specified titration
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pH titration curves for a neutralisation reaction: How does
the pH change during a neutralisation reaction? What indicator do you use for a
particular acid–alkali titration? Litmus, phenolphthalein, methyl orange, methyl
red colour changes are given for determining the end–point of a titration. These
revision notes and pH curve graphs for acid alkali titration neutralisations and
their pH changes, should prove useful for the new AQA chemistry, Edexcel
chemistry & OCR chemistry GCSE (9–1, 9-5 & 5-1) science courses.
chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE chemistry, O level
& ~US grades 8, 9, 10 school science courses for ~14-16 year old science
students for national examinations in chemistry topics including acids
bases alkalis salts preparations reactions
Doc Brown's chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE chemistry, O level & ~US grades 8, 9, 10 school science courses for ~14-16 year old science students for national examinations in chemistry topics including acids bases alkalis salts preparations reactions
7. What pH changes go on in a neutralisation reaction?
Typical neutralisation reactions involving mixing a soluble acid with a soluble base (alkali) include
These two graphs, on the same set of axis, 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.). These simple curves represent what happens when eg hydrochloric acid and sodium hydroxide are mixed or nitric acid and potassium hydroxide (1 : 1 molar equations) BUT the curves are complicated for acids like sulfuric acid where the molar ratio is NOT a 1 : 1 molar ratio with the alkali. You need to consult more advanced notes via links at the end of the page.
The pH neutralisation curves
Strictly speaking, they only apply to a strong acid and strong soluble base (alkali), but this pattern of pH change illustrated by the graph is what is happening in the salt preparation method (a) or in acid and alkali titrations.
Note: You can prepare a salt, or analyse an acid or alkaline solution by doing an 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 a strong acid to a strong alkali (univ. ind. = blue), the pH starts at about 13-14 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 ==> red
In terms of H+ and OH– ions: Initially a high concentration of OH–, so solution very alkaline, but as the H+ is steadily added, the OH– ions are neutralised to water. Therefore the OH– concentration steadily falls as does the pH because the solution becomes less alkaline. At pH 7, neutral there are very tiny equal concentrations of H+ and OH–. If excess acid is added, the pH steadily falls to around 1 as the concentration of H+ from the acid rises.
Blue graph line: If you add a strong alkali to a strong acid (univ. ind. = red), the pH starts at about 0-1 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.
In terms of H+ and OH– ions: Initially a high concentration of H+, very acid, but as the OH– of the alkali is steadily added, the H+ ions are neutralised to water. Therefore the H+ concentration steadily falls and the pH rises as the solution becomes less acid. At pH 7, neutral there are very tiny equal concentrations of H+ and OH–. If excess alkali is added the pH steadily rises from 7 to around 13 as the concentration of OH– from the alkali rises ie becoming a much more alkaline solution.
GCSE/IGCSE Acids & Alkalis revision notes sub–index: Index of all pH, Acids, Alkalis, Salts Notes 1. Examples of everyday acids, alkalis, salts, pH of solution, hazard warning signs : 2. pH scale, indicators, ionic theory of acids–alkali neutralisation : 4. Reactions of acids with metals/oxides/hydroxides/carbonates, neutralisation reactions : 5. Reactions of bases–alkalis like ammonia & sodium hydroxide : 6. Four methods of making salts : 7. Changes in pH in a neutralisation, choice and use of indicators : 8. Important formulae of compounds, salt solubility and water of crystallisation : 10. More on Acid–Base Theory and Weak and Strong Acids
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