Flashcards in module 4 Deck (19):
what is an indicator?
is a dye that changes colour depending on whether it's above or below a certain pH.
How do some indicators work?
Contain mixture of dyes, that means thy gradually change colour over a broad range of pH. Called wide range indicators and they're useful for estimating the pH of a solution. E.g. universal indicator gives the colours.
What is a pH probe and how does it work?
Attached to pH meter can also be used to measure pH electronically. Probe is placed in solution you are measuring and pH is given on digital display as numerical value, meaning it's more accurate than indicator.
What is an acid?
Substance that forms aqueous solutions with a pH of less than 7 Acids form H+ ions in water.
What is a base? And what is an alkali?
A base is a substance with a pH greater than 7. An alkali is a base that dissolves in water to form a solution with a pH greater than 7. Alkalis form OH- (revision guide) ions in water.
What is the neutralisation equation?
acid+base=salt+water. Or H+(aq)+OH-(aq)=H2O(l) (revision guide).
What happens when an acid neutralises the base or vice versa?
Products are neutral i.e. they have a pH of 7. An indicator can be used to show that a neutralisation reaction is over.
What can a neutralisation reaction of strong acids and alkalis be useful?
Can be used to calculate the concentration of an acid or alkali by titration.
What are titrations?
Allow you to find out exactly how much acid is needed to neutralise a quantity of alkali-or vice versa.
What do you do to find the concentration of an alkali?
Use a pipette and pipette filler, add a set volume of alkali to a conical flask. Add 2 or 3 drops of indicator too.
What do you use a funnel to do?
To fill burette with some acid of known concentration. Make sure do this bellow eye level-don't want to be looking up if some acid spills over. (wear safety glaces). Record initial volume of acid in burette.
What do you use to burette to do?
Add acid to alkali to bit at time-giving conical flask a regular swirl. Go especially slowly when think end-point (colour change) is about to be reached.
What is the final step of titration when the indicator changes colour and is neutralised?
Record final volume of acid in burette, and use it, along with initial reading to calculate volume of acid used to neutralise alkali.
What do you do to increase accuracy of titration and to spot any anomalous readings?
need several consistent readings.
What does the first titration need to be?
Rough titration to get approximate idea of where solution changes colour (end-point).
Why do you need to repeat the experiment?
To make sure get (pretty much) same answer each time (within 0.10cm3). Calculate mean of results ignoring any anomalous results.
What is universal indicator used for?
To estimate pH of solution because can turn variety of colours. Each colour indicates narrow range of pH values.
What is universal indicator made from and why is it used in titration?
made from mixture of different indicators. Colour gradually changes from red in acidic solutions to violet of alkaline solutions. During titrations between alkali and acid, want to see sudden colour change at end-point.