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GCSE Chemistry Notes: Neutralisation, indicators and pH curves
pH changes in acid–alkali neutralisations & choice of indicator for a specified titration Doc Brown's Chemistry GCSE/IGCSE/O level Science–Chemistry Revision Notes pH scale of acidity and alkalinity, acids, bases–alkalis, salts and neutralisation 7. The pH changes in a neutralisation reaction and choice of indicator for a titration
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. 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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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
The 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).
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.
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. The pH titration curves involving weak acids and weak alkalis are explained in section 10. 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. So, to get accurate titration results you need to use a special indicator for a particular acid–alkali titration. The complications arise because not all acids and soluble bases (alkalis) are as strong as each other. There is more on weak/strong acids in section 10. More on Acid–Base Theory and Weak and Strong Acids
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