Chemistry
Chapter 4
Chapter 4
Chemical reactions
In chemical reactions, new substance are produced.
There is no loss or gain in total mass during a chemical reaction (the law of conservation of mass).
4.1
Energy changes in chemical reactions
During a chemical reaction, there is always an energy change.
Energy is given out or taken in.
So reactions can be divided into two groups: exothermic and endothermic.
Exothermic reactions
Exothermic reactions give out energy. So there is a temperature rise.
Examples of exothermic reactions
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the neutralisation of acids by alkalis.
the combustion of fuels.
respiration in your body cells.
freezing water
condensing water vapour
making a hydrate from an anhydrous salt
combustion of hydrogen
dissolve lithium chloride in water
Endothermic reactions
Endothermic reactions take in energy from their surroundings. So there is a temperature fall.
Examples of endothermic reactions
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Cooking
Photosynthesis
melting ice cubes
evoparating liquid water
making anhydrous salt from a hydrate
Synthesis
The formation of more complex compound from its elements.
Decomposition
Breakdown of a complex compound into simpler substances.
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Displacement
A reaction in which a more reactive element displaces a less reactive element from a solution of a
salt.
Reactivity series
Private (potassium)
Second (Sodium)
Class (Calcium)
M (Magnesium)
a (aluminum)
c (Carbon)
Z (Zinc)
i (iron)
t (Tin)
l (Lead)
He (Hydrogen)
Can (Copper)
Make (Mercury)
Some (Silver)
Gun (Gold)
Powder (Platinum)
Private Second-class MacZitl he can make some gun powder
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Reduction: oxidation number decreases
Oxidation: oxidation number increases
Oxidising agent
O
Reducing agent
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Electrolysis
Decomposition (breakdown) of an ionic compound, when molten or in aqueous
solution, by the passage of an electric current.
Electrolyte
o Aqueous solution of ionic substance or molten ionic salt
o Conducts electricity due to the presence of mobile ions
Electrodes
o Rods which help current enter the electrolyte
o Inert electrodes: do not take part in the reaction
o Reactive electrodes: take part in the reaction
Why solid ionic compound does not conduct electricity?
ions cannot move in solid OR are in fixed positions in solid
[1]
ions can move when in solution
[1]
The rules for the electrolysis of a solution
1.
At the cathode (-), either a metal or hydrogen forms.
The one less reactive will be formed at cathode.
2.
At the anode (+), a non-metal other than hydrogen forms.
If it is a concentrated solution of a halide, then chlorine, bromine, or iodine
form. But if the halide solution is dilute, or there is no halide, oxygen forms.
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The masses of the copper electrodes changed during the electrolysis.
State how and explain why the masses of the two copper electrodes changed. Use
the ionic half-equations to help you.
anode decreases (in mass)
[1]
copper removed (from anode) / solid (copper from anode) becomes aqueous
[1]
cathode increases (in mass)
[1]
Cu2+deposited as Cu (on cathode)
[1]
Explain why, during the electrolysis, the colour of the copper(II) sulfate solution
does not change.
copper is both added and removed (at same rate)
[1]
OR
the concentration (of copper ions) does not change
[1]
Explain why the concentration of copper ions in the electrolyte remains constant.
formation of Cu2+ at the anode happens at the same rate as
[1]
removal of Cu2+ at the cathode
[1]
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Two important uses of electrolysis: for refining(or purifying) copper,
and for electroplating.
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Electroplating
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Coating one metallic object with another metal using electrolysis
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Make things look better
Prevent corrosion
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Question 1
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Question 2
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Question 3
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Question 6
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Question 9
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Question 11
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Question 12 (S24QP41Q3)
S24QP43Q3
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W23QP42Q4
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S23QP41Q5
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S23QP43Q5
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S22QP43Q5
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Answer S24QP41Q3
S24QP43Q3
W23QP42Q4
S23QP41Q5
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S23QP43Q5
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S24QP43Q5
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