Acids, Alkalis and Titrations
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What colour is litmus indicator in acid?
Red
What colour is litmus indicator in alkali?
Blue Important: Litmus is not suitable for titrations because it changes colour gradually, but two colour indicators like methyl orange or phenolphthalein are
What colour is phenolphthalein indicator in acid?
Colourless
What colour is phenolphthalein indicator in alkali?
Pink
What colour is methyl orange in acid?
Red
What colour is methyl orange in alkali?
Yellow
What is the formula for ethanoic acid?
CH₃COOH
What is the formula for nitric acid?
HNO₃
What three types of substances are considered to be bases/alkalis
Hydroxides, oxides and carbonates
What pH is necessary for a substance to be considered a strong acid?
0-3
What pH is necessary for a substance to be considered a weak acid?
4-6
What pH is necessary for a substance to be considered a neutral?
7
What pH is necessary for a substance to be considered a weak alkali?
8-10
What pH is necessary for a substance to be considered a strong alkali?
11-14
What colour is universal indicator with a strong acid?
Red
What colour is universal indicator with a weak acid?
Orange/Yellow
What colour is universal indicator with with a neutral solution?
Green
What colour is universal indicator with a weak alkali?
Blue
What colour is universal indicator with a strong alkali?
Purple
What ions are acid a source of when dissolved in an aqueous solution?
H+ (Hydrogen)
What ions are alkalis a source of when dissolved in an aqueous solution?
OH- (Hydroxide)
What happens in a neutralisation?
• An acid reacts with an alkali to form a salt and water • The water is formed due to the hydrogen ion from the acid and the hydroxide ion from the alkali reacting • For example: HCl + NaOH –> NaCl + H₂O (Except for ammonia)
What are acid-alkali titrations used for?
• Finding the concentration of a solution • Determining exactly how much acid is needed to neutralise a quantity of alkali and vice versa
What are the steps of a titration? Using the example of hydrochloric acid and sodium hydroxide
• Use a pipette to place exactly 25cm³ of sodium hydroxide which you know the concentration of into a conical flask, and add a suitable indicator to it • Rinse and fill a burette with hydrochloric acid and measure how much is in the burette, making there there is liquid in the space between the tap and tip • Add the hydrochloric acid to the conical flask from the burette drop by drop until it changes colour, indicating it has been neutralised • Swirl the conical flask continuously while doing so • Measure the new value of how much is in the burette, and calculate the difference to find exactly how much acid has been added • Repeat twice more or until concordant results have been reached • For the repeated times, you only need to add it dropwise towards the end You can do a rough titration first if you want to find out roughly when you need to start going drop by drop