Chemistry Topic 4- Chemical Changes
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What is the pH scale and how does it work?
The pH scale is a measure of how acidic or alkaline a solution is. The lower the pH, the more acidic (red), the higher the pH, the more alkaline (purple).
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How do you measure the pH of a solution using an indicator?
An indicator is a dye that changed colour depending on the pH. Wide range indicators gradually change colour over a broad range of pH and are useful for estimating pH.
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How else can you measure the pH of a solution?
Use a pH probe attached to a pH meter to measure pH electronically. This will be given on a digital display as a numerical value, making it more accurate.
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What is an acid?
A substance that forms aqueous solutions with a pH of less than 7. Acids form H+ ions in water.
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What is a base?
A substance with a pH greater than 7.
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What is an alkali?
An alkali is a base that dissolves in water to form a solution with a pH greater than 7. They form OH- ions in water.
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Show the neutralisation reaction in word and symbol form.
Acid + base ---> salt + water H+ + OH- ---> H2O
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What kind of indicator is used in a titration?
You use a single indicator, as it changes colour suddenly at the end point. Examples are litmus paper, phenolphthalein and methyl orange.
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What do acids do in water?
They ionise in aqueous solution- they produce hydrogen ions (eg. HCl ---> H+ + Cl-
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What is the difference between a strong and weak acid?
Strong acids ionise completely in water- all the acid particles dissociate to release H+ ions. Weak acids do not fully ionise in solution-only a small proportion of acid particles dissociate.
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What is special about the ionisation of a weak acid?
It is a reversible reaction, setting up an equilibrium between the undissociated and dissociated acid. Only a few of the acid particles release H+ ions, so the position of equilibrium lies well to the left.
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How does the pH of an acid or alkali show the concentration of H+ ions in the solution?
For every decrease of 1 on the pH scale, the concentration of H+ ions increase by a factor of 10. An acid of pH 4 has 10 times the concentration of an acid that has pH 5.
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What is concentration of acid?
It tells you how much acid there is in a certain volume of water (how watered down the acid is)
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How does increasing acid concentration affect pH?
PH will decrease with increasing acid concentration, regardless of whether its a strong or a weak acid.
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What is formed when a metal oxide or metal hydroxide reacts with an acid and why?
They form a salt and water, because all metal oxides and metal hydroxides are bases, or alkalis.
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What happens when a metal carbonate reacts with an acid?
Salt, water and carbon dioxide are produced. Metal carbonated are also bases.
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What is the reactivity series?
It lists metals in order of their reactivity towards other substances- for metals this s determined by how easily they lose electrons (form positive ions).
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What happens when a metal reacts with an acid?
Salt and hydrogen gas is produced.
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How is the speed of reaction indicated for this reaction?
The rate at which the bubbles of hydrogen are given off. You can also measure the temperature change of the reaction (greater temp change = more reactive metal).
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What happens when a metal reacts with water?
Metal hydroxide and hyrdrogen is produced
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Which metals react with water and which do not?
Potassium, sodium, lithium and calcium react with water. Zinc, iron and copper do not.
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What happens when metals react with oxygen and how is the product separated?
They react with oxygen to form oxides (oxidation). These oxides are often the ores that the metals need to be extracted from, and to separate a metal from its oxide a reduction reaction needs to occur.
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What happens during oxidation (the formation of a metal ore), and give an example.
Oxidation = gain of oxygen. Magnesium is oxidised to make magnesium oxide (2Mg + O2 --> 2MgO2)
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What happens during reduction (the extraction of metal), and give an example.
Reduction = loss of oxygen. Copper oxide is reduced to copper. 2CuO + C ---> CO2
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Which metals can be extracted by reduction with carbon and which have to be extracted using electrolysis?
Those above carbon in the reactivity series (e.g. potassium) have to be extracted using electrolysis. Metals below carbon in the reactivity series can be reduced with carbon.
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How are the most unreactive metals obtained?
Some metals are so unreactive (e.g. gold) that they are in the earth as the metal itself. Therefore, it just has to be mined.
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What do oxidation and reduction mean in terms of electrons?
Oxidation is loss of electrons, reduction is gain of electrons.
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What is a REDOX reaction?
When oxidation and reduction happen at the same time?
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What is a displacement reaction?
It is a REDOX reaction that involves a more reactive metal displacing a less reactive metal from its compound.
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Give an example of a displacement reaction.
If you put iron in a solution of copper sulphate, iron will 'kick out' coper from the solution, so you ed up with iron sulphate solution and copper metal.
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What do ionic equations show?
They only show the particles that react and the products they form (e.g. Mg + Zn2+ ---> Mg2+ + Zn)
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How does electrolysis work?
An electric current is passed through an electrolyte (a molten or dissolved ionic compound). The ions move towards the electrodes, where they react, and the compound decomposes.
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What happens at each electrode?
Positive ions move towards the cathode (negative electrode) and gain electrons (reduced). Negative ions move towards the anode (positive electrode) and lose electrons (oxidised).
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What happens after this?
As the ions lose/gain electrons, they form the uncharged element and are discharged from the electrolyte.
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How can you lower the melting point of a substance so it can be melted for electrolysis?
Mix it with cryolite.
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What happens when aqueous solutions are electrolysed?
There will also be hydrogen ions (H+) and hydroxide ions (OH-) from the water.
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How does this change what happens at the cathode?
Hydrogen gas will be produced if the metal ions form an elemental metal that is more reactive than hydrogen. If the elemental metal is less reactive than hydrogen, a solid layer of the pure metal is produced instead.
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How does this change what happens at the anode?
If OH- and halide ions are present, molecules of the halogen will be formed. If no halide ions are present, the OH- ions are discharged and oxygen will be formed.
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How do you test which gaseous products are produced?
Chlorine bleaches damp litmus paper, hydrogen makes a squeaky pop with a lighted splint, oxygen relights a glowing splint.
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What do half equations show?
They show the reactions at the electrodes. The number of electrons must be the same for each half equation.
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What is the half equation for the anode when a halide isn't present?
4OH- ---> O2 + 2H2O + 4e-
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Other cards in this set
Card 2
Front
How do you measure the pH of a solution using an indicator?
Back
An indicator is a dye that changed colour depending on the pH. Wide range indicators gradually change colour over a broad range of pH and are useful for estimating pH.
Card 3
Front
How else can you measure the pH of a solution?
Back
Card 4
Front
What is an acid?
Back
Card 5
Front
What is a base?
Back
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