SC8 acids and alkalis

SC8 acids and alkalis

1. Acids, alkalis and indicators

   • Indicators and PH
     • A measure of how acidic or alkaline a solution is. A neutral substance has pH7.
       • Acids have a pH lower than 7. Alkalis have a pH higher than 7.
       • Methyl orange turns yellow in alkaline solutions and red in acidic solutions.
       • Litmus turns blue in alkaline and red in acid.
       • Phenolphthalein turns pink in alkaline and colourless in acid.
   • Ions in acids and alkalis
     • An ion is an atom that has become charged by losing or gaining electrons.
       • Losing electrons forms + ions
       • Gaining electrons forms - ions
       • Polyatomic ions are formed when small groups of atoms, held together by covalent bonds lose or gain electrons.
       • Acids produce excess hydrogen ions when aqueous. Alkalis produce excess hydroxide ions in water.
     • The higher the number of H+ ions in a certain volume, the higher their concentration so the more acidic the solution and the lower the pH
     • The higher the concentration of OH- ions, the more alkaline the solution and the higher the pH.
     • Neutral solutions contain equal concentrations of H+ and OH- ions.

2. Bases and salts

   • Bases are substances that neutralise acids to form a salt and water only.
     • All metal oxides are bases. Metal oxide + acid --> salt + water
       • During neutralisation, H+ ions combine with oxide ions to form water. This removes the H+ ions and so the Ph increases and becomes more neutral.
       • The salts are produced by replacing the H+ ions with metal ions.

3. Alkalis and balancing equations

   • A base that is aqueous can dissolve in water. Eg sodium and calcium hydroxide.
     • Group 1 and 2 hydroxides.
   • Reactants and products must be balanced in an equation.
   • Modelling reactions of alkalis with acids
     • During the neutralisation reaction, the mixture becomes warmer.
     • When an alkali is added to an acid, the pH increases and may go higher than 7 if enough alkali is added.

4. Alkalis and neutralisation

   • Ions and neutralisation
     • HCL and other acids are a source of H+ ions. NaOH and other alkalis are a source of OH- ions.
     • In a neutralisation reaction, H+ ions from acid react with OH- ions from alkali. Water, a simple molecular substance containing covalent bonds, is formed.
     • The other ions from the acid and alkali stay in the solution as ions of the dissolved salt. These ions combine to form a solid salt when the water evaporates.
   • Soluble salts from alkalis
     • Can be obtained by crystallisation. Make sure solution is neutral before evaporating water, or the salt will be contaminated with excess of one reactant.
     • To obtain neutral solution, mix acid and alkali in correct proportions so that you end up with a solution that contains only water and the desired salt. This can be done using titration.
       • Acid is added from burette to alkali in conical flask. Flow is controlled using tap at bottom which makes it possible to add one drop at a time. Pipette is used to measure out alkali. A few drops of indicator are added to alkali to see colour change. End point is when indicator changes colour.
         • Universal indicator is not used as it is slightly acidic and may affect titration and also it doesn't have a sharp end point.

5. Reactions of acids with metals and carbonates

   • Acids and metals
     • Less reactive metals don't react with dilute acids. More reactive react explosively. Effervescence is seen as hydrogen gas bubbles are produced. The reaction also produces salt. Metal + acid --> salt + hydrogen.
       • In ionic equation we don't show spectator ions which are the ones that stay the same during the reaction.
   • Acids and carbonates
     • Acids react with metal carbonates to form a salt, H20 and CO2. Bubbles of CO2 are produced and the solid metal carbonate disappears if there is enough acid to react with it. Metal carbonate + acid --> salt + water + carbon dioxide.
       • The H+ ions from the acid reacts with the carbonate ions to form water and CO2 molecules.

6. Looking at acids

   • Concentration is how watered down your solution is.
     • An acid with a large number of acid molecules compared to the volume of water is said to be concentrated.
     • An acid with a small number of acid molecules compared to the volume of water is said to be dilute.
     • A dilute strong acid can have a similar pH and reactivity to a concentrated weak acid as they have similar concentrations of H+ ions.
     • concentration = amount dissolved/volume of solution
   • Strong acid molecules dissociate completely into ions in a solution. Weak acids do not fully dissociate into ions.

7. Solubility

   • A precipitation reaction is one where soluble substance in a solution cause an insoluble solution to form.
   • Preparation of insoluble salts
     • Wear eye protection, mix the two solutions in beaker then filter mixture, rinse beaker with distilled water and pour through funnel, pour distilled water over the precipitate in the funnel, carefully remove the filter paper containing the precipitate and dry it in a warm oven.