Acids, alkalis & titrations (Edexcel IGCSE Chemistry (Modular): Unit 1): Flashcards
Exam code: 4XCH1
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Define litmus.
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Define litmus.
Litmus is a two-colour indicator extracted from lichens that turns red in acid and blue in alkali.
True or False?
Litmus is a suitable indicator for acid-alkali titrations.
False.
Litmus is not suitable for titrations because its colour change is not sharp and it passes through a purple transition colour near neutral, making the endpoint difficult to identify.
Complete the table of indicator colours.
Indicator
Colour in acid
Colour in alkali
litmus
phenolphthalein
methyl orange
Indicator
Colour in acid
Colour in alkali
litmus
red
blue
phenolphthalein
colourless
pink
methyl orange
red
yellow
Why are synthetic indicators used in acid-alkali titrations instead of litmus?
Synthetic indicators are used because they produce a sharp colour change at the endpoint, making it easy to identify exactly when neutralisation has occurred. Litmus changes colour gradually and passes through a purple stage, so the endpoint is hard to pinpoint.
What is the pH scale?
The pH scale is a numerical scale from 0 to 14 used to show how acidic or alkaline a solution is. It measures the amount of hydrogen ions present in solution.
Acids have pH values .......... 7, alkalis have pH values .......... 7, and a neutral solution has a pH of .......... .
Acids have pH values below 7, alkalis have pH values above 7, and a neutral solution has a pH of 7.
True or False?
A solution with a pH of 2 is a strong acid.
True.
A pH of 0–3 indicates a strong acid. The lower the pH, the more acidic the solution. Extremely acidic substances can have pH values below 1.
What is universal indicator?
Universal indicator is a wide-range indicator made from a mixture of different plant indicators. It gives an approximate pH value by changing colour across a broad pH range, and the colour is matched to a colour chart to estimate the pH of an unknown solution.
Why is universal indicator not suitable for acid-alkali titrations?
Universal indicator is not suitable for titrations because it cannot produce a sharp colour change at the endpoint. A titration requires an exact endpoint, so an indicator that changes colour gradually cannot reliably show when neutralisation is complete.
What is an acid?
An acid is a substance that, when added to water, releases positively charged hydrogen ions (H+). The presence of H+ ions is what makes a solution acidic.
When an acid is dissolved in water, it releases .......... ions (H+). When an alkali is dissolved in water, it releases .......... ions (OH–).
When an acid is dissolved in water, it releases hydrogen ions (H+). When an alkali is dissolved in water, it releases hydroxide ions (OH–).
What is an alkali?
An alkali is a substance that, when added to water, releases negatively charged hydroxide ions (OH–). The presence of OH– ions makes the solution alkaline.
What does the pH scale measure?
The pH scale measures how acidic or alkaline a solution is. It is a numerical scale that reflects the concentration of hydrogen ions (H+) in the solution.
True or False?
All reactions of acids count as neutralisation reactions.
False.
Neutralisation specifically requires an acid to react with an alkali, producing water. When a metal reacts with an acid, a salt is formed but no water is produced, so it is not a neutralisation reaction.
Define neutralisation.
Neutralisation is the reaction between an acid and an alkali in which H+ ions and OH– ions combine to produce water (H2O). The reaction results in a neutral solution.
In a neutralisation reaction, H+ ions react with OH– ions to form .......... .
In a neutralisation reaction, H+ ions react with OH– ions to form water (H2O (l)).
How is neutralisation used in soil treatment?
Neutralisation is used to raise the pH of acidic soils that are too acidic for some crops. Bases such as limestone or quicklime are added to the soil to neutralise the acid and increase the pH.
True or False?
The equation H+ (aq) + OH– (aq) → H2O (l) represents all acid-base neutralisations.
True.
This net ionic equation applies to every acid-base neutralisation. The H+ and OH– ions always combine to produce water, regardless of which specific acid and alkali are used.
What is a titration?
A titration is a technique used to determine the concentration of a solution. In an acid-base titration, it measures exactly how much of one solution is needed to neutralise another.
Why is the conical flask placed on a white tile during a titration?
The conical flask is placed on a white tile to make it easier to see the colour change of the indicator at the endpoint. The white background provides contrast against the solution.
In a titration, a .......... is used to deliver the solution in controlled amounts, while a .......... and pipette filler are used to measure an exact volume of solution into the conical flask.
In a titration, a burette is used to deliver the solution in controlled amounts, while a pipette and pipette filler are used to measure an exact volume of solution into the conical flask.
True or False?
Two titration results are concordant if they are within 0.1 cm3 of each other.
True.
Two concordant results are within 0.1 cm3 of each other. At least two concordant results are needed before calculating the mean titre.
Why is a rough titration performed before the accurate titrations?
The rough titration gives an approximate endpoint volume. This allows the accurate titrations to be performed by adding the solution drop by drop as the endpoint is approached, making the result more precise.
What are concordant results?
Concordant results are two or more titration readings that are within 0.1 cm3 of each other. They are used to confirm the reliability of the titration and are averaged to give a mean titre.
Burette readings in a titration must be recorded to the nearest .......... cm3, with the eye level with the .......... of the solution.
Burette readings in a titration must be recorded to the nearest 0.05 cm3, with the eye level with the meniscus of the solution.
True or False?
A funnel used to fill the burette can be left in place during the titration.
False.
The funnel must be removed before starting the titration. If left in place, it can drip liquid into the burette, making the initial reading inaccurate and giving a false result.
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