Page 39 - ACE YR IGCSE A TOP APPROACH TO CHEM

OH will lose electrons instead of Cl since the Energy needed to break the bonds:
–
–
–
concentration of Cl is low. C–H 435 × 4 = +1740 kJ/mol
–
4OH ➞ O + 2H O + 4e O=O 498 × 2 = +996 kJ/mol
2
2
The cathode has no change. H gains electrons, Total = +2736 kJ/mol
+
leaving Na in the solution. Energy needed to form the bonds:
+
H and OH are attracted to the electrodes and turn C=O 803 × 2 = –1606 kJ/mol
+
–
into molecules. So, water molecules (H O) in the O–H 464 × 4 = –1856 kJ/mol
2
solution are being removed. Total = –3462 kJ/mol
Enthalpy change = 2736 – 3462
5 Chemical Energetics = –726 kJ/mol
10 D
Part 1: Multiple-choice Questions This statement is not related to enthalpy change.
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1 D 11 D
The reaction between calcium carbonate and sulfuric Uranium-235 is a radioisotope. It is used to generate
acid is neutralisation. energy. But energy is released through the decay of
Neutralisation is mostly exothermic. Thermal atoms. It does not need to react with oxygen.
decomposition and cracking are both endothermic as
the reactions take in heat to break down the molecules. Part 2: Structured Questions
Rusting is an exothermic reaction as it releases heat. 1 (a) Ag (aq) + Cl (aq) ➞ AgCl(s)
–
+
2 B Correct formula [1]
During photosynthesis, energy from sunlight is absorbed. Correct state symbols [1]
3 B The ionic equation shows the reacting ions in a
The bottom of the flask freezes and this shows that part chemical reaction. The precipitate formed is a solid.
of the flask is cold.
Therefore, this reaction is endothermic. Heat is taken (b) Exothermic [1]
in from the surroundings to break the bonds. In an The temperature increases at the end of the
endothermic reaction, the energy released is lower than reaction. Heat was given out.
the energy absorbed. Exothermic reactions give out heat.
4 C (c)
An exothermic reaction releases heat to the surroundings. AgNO 3 + KCI
N + 3H ➞ 2NH is a reaction in the Haber process.
2
3
2
Making ammonia is an exothermic reaction.
The rest are all endothermic reactions.
5 C AgCI + KNO 3
The energy level diagram shows an exothermic reaction
as the energy level in the molecule reduces. The energy
in the molecules is released to the surroundings. Correct energy level change [1]
The condensation of water vapour releases heat. Energy Correct label [1]
is released when the particles from far apart move
closer. Therefore, it is an exothermic reaction. Arrow pointing down [1]
The energy level decreases since the energy in the
6 B molecules is given out to the surroundings.
Energy is absorbed to break the bonds in order to form
new compounds – glucose and oxygen. (d) The precipitate will form faster. [1]
More particles per unit volume. [1]
7 A Particles collide more frequently. [1]
1 When the enthalpy change is negative, it is an
exothermic reaction. 2 (a)
2 In an exothermic reaction, the reactants have a lower H H
energy than the products. O O O C O H O H
3 In an exothermic reaction, the energy in the molecules H C C O H + O O O C O + H O H
O
H O H
O
is released to the surroundings. Therefore, the reading H H
on the thermometer will increase. All the molecules are displayed, [1]
8 A showing all bonds. [1]
This is a neutralisation reaction. Neutralisation reaction Oxygen and carbon dioxide have double bonds.
is an exothermic reaction. In an exothermic reaction, [1]
energy is released. Therefore, the energy level reduces
after the reaction. (b) Energy taken in to break the bonds
9 B = 4586 kJ/mol [1]
Enthalpy change = energy taken in to break the bonds – Energy given out to form the bonds
energy given out to form the bonds = 5410 kJ/mol [1]

Answers 151

Answers.indd 151 3/4/22 3:54 PM