Chemistry Exo/Endothermic Reactions Help

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ashjaytsr
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#1
Report Thread starter 4 years ago
#1
got this in my spec
Recall that changes in heat energy accompany the following
changes:
a salts dissolving in water
b neutralisation reactions
c displacement reactions
d precipitation reactions
and that, when these reactions take place in solution,
temperature changes can be measured to reflect the heat
changes

Can someone please explain this to be properly, with specific detail on each type of reaction.
Much appreciated.
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CheeseIsVeg
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#2
Report 4 years ago
#2
(Original post by ashjaytsr)
got this in my spec
Recall that changes in heat energy accompany the following
changes:
a salts dissolving in water
b neutralisation reactions
c displacement reactions
d precipitation reactions
and that, when these reactions take place in solution,
temperature changes can be measured to reflect the heat
changes

Can someone please explain this to be properly, with specific detail on each type of reaction.
Much appreciated.
a - The chemical change when a salt is dissolved into water can be endothermic or exothermic. It explains it rather nicely here. A delicate balance determines whether the dissolving of a salt crystal in water will absorb or give off heat. Energy is required to pull apart the oppositely charged ions in the crystal, and this is called the lattice energy, DHlat.
In contrast, energy is liberated when the separated ions attract polar water molecules around themselves, and this is the heat of hydration, DHhyd. The observed heat effect on solution depends on which factor predominates.
b - A neutralisation reaction transfers energy to its surroundings. Energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter. The temperature increase can be detected using a thermometer. This includes neutralisation reactions between acids and alkalis.
c - Displacement reactions, such as that of halogens and halide ions or different metals based on the reactivity scale, are exothermic. In the reaction A + B-C → B + A-C, we must break the B-C bond and form an A-C bond.
If formation of the A-C bond releases more energy than it took to break the B-Cbond, there will be a net release of energy.
The reaction will be exothermic.
It the process is endothermic, the reaction will not occur spontaneously, because the system is already in its more stable state.
Hence, single displacement reactions are exothermic.
d - Precipitation has to be an exothermic process. We know this because the overall entropy change having to be positive (second law of thermodynamics) - and because the entropy change of the system is negative (two liquids forming one solid and one liquid) the entropy change of the surroundings must be positive (and also large enough to make up for the decrease in entropy of the system) for the overall entropy to be positive and the reaction to be spontaneous. Positive entropy of the surroundings equates to a negative value for delta H (and therefore an exothermic process).
You can do calorimetry experiments with chemicals in solution and measure the change in temperature when adding reagents to further confirm these above points.I am aware some of these explanations may be beyond your level so if you have any further queries do not hesitate to ask.
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