Chemical changes
Acids and bases
Indicator- dye that changes colour depending on pH.
Wider range indicators- contain mixture of dyes that gradually change colour over broad range of pH- useful for estimating pH of solution.
pH probe attached to pH meter- used to measure pH electronically. Probe placed in solution; gives numerical value- more accurate.
Acid- substance that forms aqueous solutions less than pH 7; form H+ ions in water.
Base- substance with pH greater than 7.
Alkali- base that dissolves in water to form solution greater than pH 7. Form OH- ions in water.
Reaction between acids and bases= neutralisation; products are neutral (pH7).
Strong and weak acids
Acids ionise in aqueous solutions- produce H+ ions.
Strong acids- ionise completely in water; all acid particles dissociate to release H+ ions (sulfuric, HCL, nitric acids).
Weak acids- don't fully ionise in solution (ethanoic, citric, carbonic acids). Only small proportion of acid particles dissociate to release H+ ions.
Ionisation of weak acid is reversible reaction; sets up equilibrium between undissociated and dissociated acid. As only a few acid particles release H+ ions, position of equilibrium lies to the left.
Reaction of acids involve H+ ions reacting with other substances. If concentration of H+ is higher, rate of reaction will be faster, so strong acids will be more reactive than weak acids of same concentration.
pH- measure of concentration of H+ ions in the solution. For every decrease of 1 on pH scale, concentration of of H+ ions increased by 10. For decrease of 2 on pH scale, concentration increases by 100.
pH of strong acid is always less than pH of weak acid in same concentration.
Acid strength- proportion of acid molecules that ionise in water.
Concentration- how much acid in certain volume of water- the more acid in certain volume of liquid, the more concentrated it is.
pH will decrease with increasing acid concentration regardless of strong or weak acid.
Reactions of acids
Some metal oxides and meal hydroxides dissolve in water- these soluble compounds are alkalis. Even bases that won't dissolve in water, take part in neutralisation reaction with acids.
Some metal oxides and meal hydroxides dissolve in water- these soluble compounds are alkalis. Even bases that won't dissolve in water, take part in neutralisation reaction with acids.
Acid + metal oxide ➡ salt + water
Acid + metal hydroxide ➡ salt + water
Metal carbonates also bases: Acid + metal carbonate ➡ salt + water + carbon dioxide
Reactivity series
Reactivity determined by how easily metals lose electrons to form positive ions. The higher up the reactivity series a metal is, the easier to form a positive ion.
When metals react with H2O or acid, lose electrons to form positive ions- the higher it is, the easier it reacts with H2O or acid.
Acid + metal ➡ salt + H2
Speed of reaction indicated by rate at which bubbles of H2 are given off. The more reactive, the faster the reaction. Very reactive metals (K, Na, Li and Ca) react explosively. Less reactive metals (Mg, Zn, Fe) react less.
In general. Cu won't react wit cold, dilute acids.
Also investigate reactivity of metals by measuring temperature. Use same mass and surface area- more reactive metal will have greater temperature change.
Metal + H20 ➡ metal hudroxide + H2
Only more reactive metals will react with H2O.
Separating metals from metal oxides
Lots of common metals (Fe, Al) react with O2 to form oxides- Oxidation. These oxides are often ores that metals need to be extracted from. Reduction- reaction which separates metals from oxide; loses 02.
Some metals can be extracted from ores chemically by reduction using carbon- the ore is reduced as O2 removed from it and carbon gains O2 so oxidised; only metals below carbon can do this.
Metals higher than carbon in reactivity series have to be extracted through electrolysis- expensive.
Some metals so unreactive that they are in Earth as metal itself- gold.
REDOX reactions
This is in terms of electrons- OIL RIG
Reduction and oxidation happen at same time- REDOX reaction.
Fe atoms oxidised to Fe2+ ion when react with dilute acid: Fe + 2H+ ➡ Fe2+ + H2
Fe atoms lose electrons; oxidised by H2 ions: Fe - 2e- ➡ Fe2+
Hydrogen ions gain electrons; reduced by iron atoms: 2H+ + 2e- ➡ H2.
Displacement reactions- more reactive metal will displace a less reactive metal from its compound- has to be a solution so particles able to move around.
In displacement reactions, metal ions gain electrons; reduced and metal atoms lose electrons; oxidised.
Ionic equations- only show particles that react and products formed. Concentrates on substances which are oxidised and reduced. Spectator ions- ions that don't change in the reaction- don't include this in equation.
Electrolysis
During electrolysis, electric current passed through an electrolyte (molten, dissolved in ionic compound). Ions move towards electrodes, where react and compound decomposes.
- ions in electrolyte to cathode (-ve electrode) and gain electrons; reduced.
- ions in electrolyte move to anode (+ve electrode) and lose electrons; oxidised.
This creates a flow of charge through electrolyte as ions travel to electrode.
As ions gain or lose electrons, form uncharged element and are discharged from electrolyte.
Ionic solid can't be electrolysed because ions are in fixed position and can't move. Molten ionic compounds can be electrolysed as ions can move freely and conduct electricity. Molten ionic liquids e.g. lead bromide are always broken up into their elements.
Positive metal ions are reduced to the element at cathode. Negative non-metal ions are oxidised to the element to anode.
If a metal is too reactive to be reduced with carbon or reacts with carbon; electrolysis used.- extracting metals like this can be expensive; lots of energy required to melt the ore and produce required current.
Sometimes, other element added to lower the melting point and make decomposing easier.
Electrolysis of aqueous solutions
In aqueous solutions, there will be ions from ionic compound, H+ ions and OH- ions from water.
At cathode, if H+ ions and metal ions present, H2 gas be produced if metal ions form elemental metal that is more reactive than H2.
If metal ions form elemental metal that is less reactive than H2, then a solid layer of the pure metal will be produced instead.
At anode, if OH- ions and hallide ions are present, molecules of Cl, Br and I will be formed. If no hallide ions present, the OH- ions are discharged and O2 formed.
You can set up electrolysis experiment in lab. Once experiment over, test gaseous products to see what's produced:
- CL bleaches damp litmus paper- turns white.
- H2- squeaky pop with lighted splint.
- O2- relight glowing splint.