AQA Topic 4 - Chemical Changes

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  • Created by: Grace
  • Created on: 05-05-18 09:01
How do you know if a substance is an alkali or acid via the pH scale?
1-6 = Acid, 8-14 = Alkali
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What sort of indicator is universal indicator?
A 'wide range' indicator
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Wha is an indicator?
An indicator is a dye that changes colour depending on whether its above or below a certain pH, some indicators contain a mixture of dyes so they gradually change colour over a BROAD range of pH, useful for estimating the pH
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Describe how to measure the pH of a solution
1. Use a few drops of an indicator via a pipette 2. Attach a pH probe to a pH meter to measure pH electronically the probe is placed in the solution and the pH is given as a numerical value on the screen = more accurate than an indicator
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Define an acid
A substance that forms aqueous solutions with a pH of less than 7 and forms H+ ions in water
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Define a base
A substance with a pH greater than 7
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Define an alkali
A base that dissolves in water to form a solution with a pH greater than 7 and forms OH- ions in water
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Give the neutralisation equation
Acid + base = salt + water
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What is the equation for neutralisation in terms of ions?
H+ + OH- = H2O
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What are titrations used for?
To find concentrations as they allow you to find out how much acid is needed to neutralise a quantity of alkali or vice versa. This data is then used to find the concentration of the acid/alkali
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Describe the experiment for titration for an alkali (1)
Use a pipette and pipette filter to add a set volume of the alkali to a conical flask, add two or three drops of indicator too. Use a funnel to fill a burette with some acid of a KNOWN concentration and record its initial volume. Using the burette
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Describe the experiment for titration for an alkali (2)
add the acid to the alkali a drop at a time. When you think the end-point (colour change) is about to be reached go slowly, the indicator will change colour when all the alkali has been neutralised. Repeat the titration to exclude anomalous results
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How do you calculate chemical quantities in titrations?
Make sure the volume is in dm3 then determine the number of moles in the acid/alkali. Work out the moles in the substance you're trying to find by a balanced equation. Then using the moles find the concentration by conc = moles ÷ volume
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Why must you use single indicator for titrations?
You need to see sudden colour change not a gradual to be accurate
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Describe three different single indicators and what colour they go in acid/alkalis
LITMUS - Red in acid and blue in alkalis, METHYL ORANGE - Red in acid and yellow in alkalis, PHENOLPHTHALEIN - Colourless in acid and pink in alkalis
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What do acids do in water?
Acids produce protons in water, they ionise in aqueous solutions and produce H+ ions, 1 H+ ion is a proton
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Define a strong acid
Strong acids ionise completely in water, all acid particles dissociate to release H+ ions
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Define a weak acid
Weak acids do not ionise completely in water and only a small portion of acid particles dissociate to release H+ ions
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What sort of reaction is the ionisation of a weak acid? Why?
A reversible reaction because not all the particles dissociate to produce H+ ions
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Where does the equilibrium lie for a weak acid reversible reaction? Why?
The equilibrium lies to the left because there
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What happens during the reactions of acids?
The H+ ions react with other substances
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What happens if the concentration of H+ ions is high?
The higher the concentration of the H+ ions the faster the rate of reaction so therefore strong acids are more reactive than weak acids of the same concentration
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What is pH a measure of?
The concentration of Hydrogen Ions in the solution
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Complete the sentence: For every decrease of ___ on the pH scale, the concentration of H+ ions increases by a factor of ___
1. One 2. Ten
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If a acid decreases by 2 on the pH scale, how much does its concentration of H+ ions increase by?
A factor of 100 = 10 squared
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Give the general rule for the factor of H+ ions change
Factor H+ ion concentration changes by = 10 the the power of -x
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Explain the rule for factor of H+ ions
The x is the difference in pH, so if pH falls from 7 to 4 the factor difference is -3 and the factor the H+ ion concentration has increased by is 10- to the power of (-3) = 10 cubed
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What is the difference between strength and concentration of acids?
Acid strength tells you what proportion of the acid molecules ionise in water whereas concentration measures how much acid there is in a certain volume of water
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How do you increase concentration?
The larger the amount of acid there is in a certain volume of liquid, the more concentrated the acid is
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Complete the sentences: pH will ____ with increasing acid concentration regardless of whether it's a ____ or ___ acid
1. Decrease 2. Strong 3. Weak
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True or false: Even bases that won't dissolve in water will still take part in neutralisation reactions with acids
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What do metal oxides produce with acids?
Acid + Metal Oxide = Salt +Water
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What do metal hydroxides produce with acids?
Acid + Metal Hydroxide = Salt + Water
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Wha do acids and metal carbonates produce?
Acid + Metal Carbonate = Salt + Water + Carbon Dioxie
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Describe an experiment to get soluble salts using an insoluble base (1)
You need to pick the right acid and insoluble base i.e an insoluble metal oxide, hydroxide or carbonate. Warm the dilute acid using a bunsen burner and then add the insoluble base to the acid a bit at a time until no more reacts (the base is in
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Describe an experiment to get soluble salts using an insoluble base (2)
excess) The acid has been neutralised when the excess solid sinks to the bottom of the flask. Filter out the excess solid to get the solution and then use the process of crystallisation using a water bath
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Define the reactivity series
The reactivity series is the order of metals in terms of their reactivity towards other substances. Whilst hydrogen and carbon are non-metals they are included as well
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Give the reactivity series in descending order
Potassium, Sodium, Lithium, Calcium, Magnesium, Carbon, Zinc, Iron, Hydrogen and Copper
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How is metal reactivity determined?
How easily they lose electrons so the higher a metal is on the reactivity series the easier it is for said metal to form positive ions and how easily it reacts with water/acid
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What does acid + metal make?
Acid + metal = Salt + Hydrogen
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How is the speed of reaction indicated?
Indicated by the rate at which the bubbles of hydrogen are given off
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What is one of the ways in which you can investigate reactivity of metals?
By measuring the temperature change of the reaction with an acid or water over a set time period. If you use the same mass and surface area of metal each time, then the more reactive metal the greater the temperature change should be
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Give the reaction for metal and water
Metal + Water = Metal Hydroxide + Hydrogen
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True or false: Less reactive metals only react with water
False, less reactive metals won't react with water but more reactive ones like potassium will react with water
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Define oxidation for metal ores
Gain of oxygen so magnesium is oxidised to make magnesium oxide
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Define reduction for metal ores
A loss of oxygen so copper oxide is reduced to copper
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How do metals get extracted from their ores?
Some metals can be extracted from their ores chemically by reduction using carbon
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Why can some metals be reduced by carbon and others can't?
Because they're below carbon in the reactivity series so carbon displaces it
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Why are some metals in the earth as the metal itself?
Because they are so unreactive like gold
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Define oxidation in terms of electrons
A loss of electrons
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Define reduction in terms of electrons
A gain of electrons
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Give the mnemonic for oxidation and reduction
Oxidation Is Loss, Reduction Is Gain - OILRIG
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What is a redox reaction?
When electrons are transferred i.e when reduction and oxidation happen at the time hence REDOX
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Refer to page 57 for an example of a Redox Reaction
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True or false: Displacement reactions are Redox reactions
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Why are displacement reactions redox reactions?
The metal ion gains electrons is reduced (gain) but the metal atoms always loses electrons so is oxidised
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What do ionic equations show?
Only the particles that react and the products they form are shown i.e the useful bits
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Give the ionic equation for the displacement of Zinc from Zinc Chloride by magnesium
Mg + Zn(2+) = Mg(2+) + Zn
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What are spectator ions
Ions that don't change in the reaction
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What do ionic equation for displacement equations focus on?
The substances that are being oxidised or reduced
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Define an electrolyte
A molten or dissolved ionic compound
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What is electrolysis?
The process of which an electric current is passed through an electrolyte and the ions move towards the electrodes where they react, causing the compound to decompose
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Define a cathode
The negative electrode
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Define an anode
The positive electrode
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Describe what happens at the cathode
The positive ions in the electrolyte will move towards the cathode and gain electrons, they are reduced
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Describe what happens at the anode
The negative ions in the electrolyte will move towards the anode and lose electrons, they are oxidised
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What happens as a results of the ions moving to the different anodes?
The movement of ions creates a flow of charge through the electrolyte and as the ions gain/lose electrons they form the uncharged element and are discharged from the electrolyte
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Why can't an ionic solid be electrolysed? How can it be?
The ions are in a fixed position so cannot move therefore the solid needs to be molten so the ions can move freely and conduct electricity
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Why is electrolysis used for extraction?
Some metals are too react so cannot be reduced by carbon
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What is the disadvantage of electrolysis?
It's incredibly expensive due to the large amount of energy required to melt the ore and produce the required current
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Describe the electrolysis for aluminium from its ore (1)
Aluminium is extracted from the ore bauxite which contains aluminium oxide, due to aluminium's high melting point it is mixed with cryolite to lower its melting point. The molten mixture now contains free ions to conduct electricity, the positive
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Describe the electrolysis for aluminium from its ore (2)
Al(3+) ions are attracted to the cathode where they each pick up 3 electrons and turn into neutral aluminium atoms which then sink to the bottom of the electrolysis tank. The negative O(2-) ions are attracted to the anode where they each lose 2
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Describe the electrolysis for aluminium from its ore (3)
electrons, the neutral oxygen atoms will then combine covalently to form O2 molecules
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What is the difference between a half equation and a ionic equation?
Ionic equations leave out spectator ions but half equations show either reduction or oxidation i.e Fe2+ + 2e- = Fe is a half equation
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What sort of indicator is universal indicator?


A 'wide range' indicator

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Card 4


Describe how to measure the pH of a solution


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Card 5


Define an acid


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