Chemical Changes Flashcards
pH scale
- measure of how acidic or alkaline a solution is
- neutral substances have a pH of 7
What pH does an acid have?
Below 7
What pH would an alkali have?
Above 7
Give two examples of strong acids
- car battery acid
- stomach acid (hydrochloride)
- vinegar
- lemon juice
Give two examples of weak acid
- acid rain
- lemon juice
- normal rain
Give two examples of strong alkalis
- caustic soda (drain cleaner)
- bleach
Give two examples of weak alkalis
- pancreatic juice
- washing-up liquid
Indicator
- a dye that changes colour depending on acidity or alkalinity of substance
- wide range indicators have a mixture of dyes that gradually change colour over a broad range of pH
Neutralisation
- reaction between an acid and a base
- react to form a salt and water
- pH 7
Acid + base =>
Salt + water
Ionic equation for neutralisation (acid + base)
H+ + OH- => H2O
Titration
- used to find the exact volume of acid required to neutralise a measured volume of alkali (or vice versa)
- data used to work out concentration of the alkali
Titration method to find concentration of alkali
- add a set volume of alkali to a conical flask (using pipettes and pipette filler)
- add two or three drops of indicator
- use a funnel to fill a burette with an acid of known concentration - record initial volume of acid
- add the acid slowly to the alkali; go especially slowly when you think the end-point is about to be reached
- indicator changes colour when alkali has been neutralised
- record final volume of acid and use in concentration calculation
Why must you repeat titrations?
- repeat the experiment to increase accuracy and spot any anomalous results
- several consistent readings
- first is a rough titration to get an approximate idea of end-point
- calculate a mean of results (ignoring any anomalous results)
Phenolphthalein colour in acid and alkali
- colourless in acid
- pink in alkali
Litmus colour in acid and alkali
- red in acid
- blue in alkali
Methyl orange colour in acid and alkali
- red in acid
- yellow in alkali
Difference between strong and weak acids
- strong acids ionise completely in water
- all acid particles dissociate to release H+ ions
- weak acids do not fully ionise in solution
- small proportion of acid particles dissociate to release H+ ions
Difference between strength and concentration of acids
- strength tells you proportion of acid molecules that ionise in water
- concentration measures how much acid there is in a certain volume of water; how pure or dilute it is
- pH affected by concentration and strength of acid
Acid + metal oxide =>
Salt + water
Acid + metal hydroxide =>
Salt + water
Acid + metal carbonate =>
Salt + water + carbon dioxide
Soluble salts from insoluble bases
- right acid + insoluble base
- gently warm dilute acid with Bunsen burner, then turn off burner
- add base slowly until no more reacts (it is in excess)
- filter out excess to get just salt solution
- evaporate remaining water to get pure, solid crystals
Reactivity Series
- lists metals in order of reactivity to other substances
- determined by how easily they lose electrons, forming positive ions
- reacts more easily with water and acid if it is higher in the reactivity series
- carbon and hydrogen are often included, as it gives information about extraction
Reactivity series order
Potassium Sodium Lithium Calcium Magnesium Aluminium *Carbon* Zinc Iron Tin Lead *Hydrogen* Copper Silver Gold
Acid + metal =>
Salt + Hydrogen
Metal + water =>
Metal hydroxide + hydrogen
Separating metals from ores
- most metals found in compounds, not as pure form
- common metals (e.g. iron, aluminium) form oxides, which are found in the ground
- reduction reactions separate metals from oxides
Oxidation and Reduction
- oxidation is the gain of oxygen
- reduction is the loss of oxygen
BUT… - oxidation is loss of electrons
- reduction is gain of electrons
- OILRIG
Extracting Metals by reduction with carbon
- chemically extracted with carbon
- ore is reduced (loses oxygen) and carbon is oxidised (gains oxygen)
- only metals lower than carbon in reactivity series; higher than carbon must be extracted using electrolysis
Redox reactions
- loss of electrons is oxidation
- gain of electrons is reduction
- redox reactions are where oxidation and reduction are happening at the same time
- e.g. metals reacting with acids
What is electrolysis?
- ‘splitting up with electricity’
- electric current passed through electrolyte (molten/dissolved ionic compound)
- ions move towards electrodes, where they react and the compound decomposes
- positive ions towards cathode where they are reduced
- negative ions towards anode where they are oxidised
- flow of charge through electrolyte as ions travel towards electrodes
Why might we use electrolysis to extract metals from ores?
- more reactive than carbon
- would react with carbon
Electrolysis of Aluminium ore
- aluminium extracted from bauxite, which contains Al2O3
- Al2O3 had a very high MP, so it is mixed with cryolite to lower MP
- molten mixture contains free ions - it will conduct electricity
- positive Al3+ ions move to negative electrode, where they pick up three electrons and turn into Aluminium atoms (sink to bottom of tank)
- negative O2- ions to positive electrode, where they lose two electrons each and combine to form O2 molecules
What happens at the cathode during electrolysis of aqueous solutions?
- H+ and metal ions are present
- hydrogen gas formed if metal is more reactive than hydrogen
- solid metal layer produced if less reactive than hydrogen
What happens at the anode during electrolysis of aqueous solutions?
- OH- and halide ions present
- molecules of chlorine, bromine or iodine formed
- no halide ions; OH- ions discharged and oxygen gas (and water) will be formed
Test for chlorine
Bleaches litmus paper, turning it white
Test for Hydrogen
Makes a “squeaky pop” with a lighted spliny
Test for Oxygen
Will relight a glowing splint
General word equation for reaction between acid and a base
Acid + base => salt + water
What colour does universal indicator turn in a neutral solution?
Green
Why do we do a rough titration first?
To find the approximate end-point
State three indicators you could use for a titration and the colours they turn in acidic and alkaline conditions
Phenolphthalein (colourless - pink)
Litmus (red - blue)
Methyl orange (red - yellow)
What is a strong acid?
The acid particles in a strong acid fully ionise in water (dissociate into H+ ions)
By what factor does the H+ concentration increase for a decrease of 1 on the pH scale?
10
Write a word equation for the reaction between hydrochloric acid and sodium carbonate
Hydrochloric acid + sodium carbonate => sodium chloride + water + carbon dioxide
Write a word equation for the reaction between sulphuric acid and calcium hydroxide
Sulphuric acid + calcium hydroxide => calcium sulphate + water
Write a word equation for the reaction between nitric acid and magnesium oxide
Nitric acid + magnesium oxide => magnesium nitrate + water
Is zinc more or less reactive than iron?
More reactive
General word equation for a metal and an acid
Acid + metal => salt + hydrogen
Calcium + water =>
Calcium Hydroxide + Hydrogen
Will copper react with water?
No
If a metal is above carbon in the reactivity series, can it be extracted by reduction with carbon?
No
Why is gold found as a pure metal in the earth
Gold is extremely unreactive, so it found in its unreacted form
In terms of electrons, define oxidation
Oxidation is the loss of electrons
In electrolysis, which electrode are the positive ions attracted to?
Cathode (negative electrode)
Why can’t ionic solids undergo electrolysis?
The ions are held in place and aren’t free to move carrying electrical charge
Do ions get oxidised or reduced at the anode in electrolysis?
The ions get oxidised, as they lose electrons
Which electrode is aluminium formed at in electrolysis?
Aluminium is formed at the cathode
Why would hydrogen gas be produced at the anode in electrolysis
If the metal ions present are more reactive than hydrogen
If halide ions are present during electrolysis of an aqueous solution, will oxygen be produced at the anode?
No
