Chemistry C3 - Acids and Alkalis

Acids and Alkalis for AQA Chemistry C3

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  • Created by: RATM33
  • Created on: 08-01-10 19:58

Acid and Alkali Reactions

  • acid + alkali salt + water
  • acid + metal oxide salt + water
  • acid + metal hydroxide salt + water
  • acid + metal carbonate salt + water+ carbon dioxide
  • acid + metal hydrogen gas + salt

When any acids and alkalis react together, the neutralisation can be summarised as
H+(aq) + OH- (aq) H2O (l)

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Development of Acids and Alkalis

Robert Boyle - acids had particles that squeeze in between spaces and break materials apart.

Lavoisier - gave oxygen its name meaning 'acid maker' because he thought all acids contained oxygen

Laurent - showed that acids contain hydrogen, but didn't explain how it worked

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Svante Arrhenius

Arrhenius suggested that in water, substances could be split into positive and negative ions.

He called this 'electrolytic dissociation'

This gave rise to the Arrhenius definition for acids and bases:

  • an acid is any substance that produces positive hydrogen ions (H+) in water
  • an alkali is anything that produces negative hydroxide ions (OH-) in water.
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Svante Arrhenius

Arrhenius' definition was not widely accepted because -

  • People did not believe that molecules can be split
  • Ahead of its time as subatomic particles have not yet been discovered
  • Not enough infomation/proof
  • His scientific reputatation is not regarded highly
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Lowry and Brønsted

Suggested that:

  • Acids are PROTON DONORS
  • Bases/Alkalis are PROTON ACCEPTORS

Brønsted and Lowry realised that in water, the H+ ions bond to water molecules (hydrated) forming H3O (aq) ions - represented as H

By this point, protons and electrons were well understood and accepted, so scientists had little difficulty accepting Brønsted and Lowry's definition

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Concentration, Moles and pH

Concentration - a measure of how much acid or alkali is dissolved in a known volume of liquid, eg water

Molarity - a measure of concentration: the number of moles dissolved in 1000cm3 of water

pH scale - a measure of the concentration of hydrogen ions

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Strong and Weak Acids

Strong Acids have a large number of H+ ions. They are completly (100%) ionised

Examples -

  • Hydrochloric Acid - HCl
  • Nitric Acid - HNO3
  • Sulphuric Acid - H2SO4

Weak Acids have few H+ ions. They are only partly ionised.

Examples -

  • Vinegar
  • Citric Acid
  • Carbonic Acid
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Strong and Weak Bases/Alkalis

Strong Alkalis have a large number of OH- ions. They are completly (100%) ionised

Examples

  • Sodium Hydroxide - NaOH
  • Postassium Hydroxide - KOH

Weak Alkalis have few OH- ions. They are only partly ionised.

Examples -

  • Ammonium
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Titrations

Titrations are used to find concentrations

Titrations Steps

  • Add Alkali to conical flask using pipette
  • Add Acid to burette
  • Add Indicator to flask
  • Open tap and gently release acid
  • Swirl the conical flask to ensure the acid and alkali are reacting
  • Continue until indicator changes colour
  • Record volume of Acid in burette

PIPETTE - used to measure out a fixed solution

BURETTE used to measure the volume of solution added

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Titration Calcuations

MOLES = CONCENTRATION x VOLUME

MASS = MOLES x RELATIVE FORMULA MASS

For Mol/dm3 - (or mol dm -3)

1) Calculate Moles

2.) Balanced Equation & Ratio

3.) Concentration - mol/dm3

For grams/dm3

4.) RFM - relative formula mass of acid

5.) Convert Moles to grams

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Indicators

STRONG acid + STRONG alkali = ANY INDICATOR

WEAK acid + STRONG alkali = PHENOLPHTHALEIN

Colour change -

In Alkalis - Colourless --> Pink

STRONG acid + WEAK alkali = MEYTHL ORANGE

Colour change -

In Acids - Orange --> Yellow

In Alkalis - Orange --> Pink

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Comments

Gaga

thanks, its very useful

Ashir

notes are upside down?

Liata

thank you so much for the notes on arrhenius, lowry and bronsted i didnt understand that before now :D :D :D :D

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