Topic 4 - Chemical Changes.

Cards (157)

  • What is the pH scale?
    A measure of how acidic or alkaline a solution is. It goes from 0-14
  • What pH is an acid?
    less than 7
  • what pH is an alkali
    pH is greater than 7
  • What is the pH of a neutral substance?
    exactly 7
  • What is indicator?
    dye that changes colour depending on if its above or below a certain pH
  • What is a wide range indicator?
    An indicator that contains multiple dyes so it gradually changes colour over a wide range of pH
  • What is an example of a wide range indicator?
    universal indicator
  • Give one way other than indicator that can be used to determine the pH of a solution?

    a pH probe can be placed in the solution with a pH meter attached to it
  • What is released when acids are dissolved in water?
    Acids form hydrogen ions (H+)
  • What is released when alkalis are dissolved in water?
    alkalis form hydroxide ions (OH-)
  • what is a neutralisation reaction and how can it be shown that one has finished?
    a reaction in which an acid will react with a base to form salt and water. When an acid neutralises an alkali (or vice-versa) the solution that is formed is neutral and has a pH of 7. An indicator can be used to show that a neutralisation reaction is over
  • Why is water formed in neutralisation reactions?
    During the reaction, hydrogen ions from the acid react with the hydroxide ions from the base to produce water
  • How can we tell when a neutralisation reaction is over?

    Use indicator on the solution. It should have a pH of 7
  • What is the neutralisation reaction equation?
  • What is the equation for the formation of water in a neutralisation reaction?
  • What colour does universal indicator turn at the end of a neutralisation reaction?
    green (pH 7)
  • what is an alkali?
    A soluble base
  • What is a titration?
    A titration is and experimental technique used to find an unknown concentration of an acid or an alkali
  • What is an example of something you might use a titration to find out?
    exactly how much acid is needed to neutralise a certain qauntity of alkali (or vice-versa)
  • Explain in detail how to carry out a titration to work out how much acid is needed to neutralise an alkali

    Use a pipette with a filler to to measure out a volume of alkaliput the alkali into a flask with a few drops of indicator and swirl the flaskUsing a funnel, fill a burette with some acid of a known concentration then place the flask containing hte alkali under the buretteTake a reading of the volume of acid in the burette by measuring at the bottom of the meniscuscomplete a rough titration by adding the acid to the alkali a bit at a time, giving the flask a regular swirlContinue until the indicator changes colour. this happens when the alkali has been fully neutralisedrecord the volume of acid left in the bueretteCalculate the amount of acid needed to neutralise the alkali by subtracting the initial reading of acid in the burette from the final reading of acid in the burette Now do an accurate titrationUse the qauntity of acid you found doing the rough titration and add all but 2cm of itadd the rest of the acid drop by drop very slowly and spot exactly where the indicator changes colourcalculate the volume of acid used to neutralise the acidrepeat the accurate titration until you get at least 3 concordant resultscalculate the mean volume of acid that was needed to neutralise the acid. Only use concordant results
  • Why do we do a rough titration first?
    It helps make your final results more accurate by giving you an approximate idea of how much acid is needed to neutralise the alkali
  • what is a rough titre
    the final reading of acid in the burette after a rough titration
  • Why can universal indicator NOT be used in a titration experiment?
    The colour change is too gradual. An indicator must be used that gives a definite colour change so its easy to see when the alkali is neutralised
  • What is the end point in a titration reaction?

    the point where the alkali is neutralised by the acid
  • What is a single indicator?
    An indicator that contains only one colour changing compound so only have one distinct colour change at a particular pH
  • What is the best type of indicator for a titration reaction?
    single indicator (phenophtalin, methyl orange and litmus)
  • What are some examples of single indicators?
    phenophtaleinmethyl orangelitmus
  • What colours does phenophtaelin change?
    phenophtaelin is pink in alkalis and colourless in acids
  • How does methyl orange react to different pH?
    methyl orange is yellow in alkalis but red in acids
  • How does litmus respond to different pH?
    litmus is blue in alkalis and red in acids
  • In titrations what is used to identify the end point?
    Indicator
  • Suggest 2 safety methods that should be used when carrying out titration using acid and alkali
    wear safety gogglesdilute the acid and alka
  • What is a strong acid?
    an acid that ionises completely in water. All the particles release H+ ions
  • Give three examples of strong acids
    sulfuric acid (H₂SO₄)hydrochloric acid (HCL)nitric acid (HNO₃)
  • reaction equation for hydrochloric acid in water
    HClH+ + Cl
  • reaction equation for nitric acid in water
    HNO3 = H + NO3
  • Equation for sulfuric acid dissolved in water
    H₂SO₄ → H+ + SO₄2-
  • What is a weak acid?
    An acid that only oartially ionises in water. Not every molecule will release H+ ions
  • Give three examples of weak acids
    ethanoic acid (CH₃COOH)citric acid (C₆H₈O₇)carbonic acid (H₂CO₃)
  • reaction for ethanoic acid in water
    CH₃COOH- ⇌ + H+ + CH₃COOH-