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March 30, 2024, 02:50:48 am

Author Topic: Exothermic and endothemric reactions  (Read 1947 times)

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Bri MT

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Exothermic and endothemric reactions
« on: July 28, 2019, 06:04:28 pm »
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“exo” means ‘out’ so in an exothermic reaction, there is an overall transfer of energy from the system out to the surrounds. The enthalpy of the system is decreased (negative change in enthalpy).
“endo” means ‘in’ so in an endothermic reaction, there is an overall transfer of energy from the surrounds into the system. The enthalpy of the system is increased (positive change in enthalpy).

Remember that in an energy profile diagram we see that the energy is transferred into the system until the transition stage is reached (the top of the curve) then energy is transferred out of the system. This means that the word “overall” in the definition of exothermic and endothermic reactions is important – without it, reactions would be both exothermic and endothermic.

How does this relate to chemical bonds?
Breaking chemical bonds requires energy – the average amount of energy required to do this is called the bond enthalpy. The difference between the bond enthalpy of the reactants and the bond enthalpy of the products tells us about the enthalpy of that reaction. Caution! Things like changing states involve energy transfers so you can’t always rely on bond enthalpies to tell you the full story.

So what does this mean for temperature?
Temperature is a measure of the average kinetic energy of the particles. Particles move faster when they have higher kinetic energy and if they are stationary they have no kinetic energy.
Let’s say you are holding an instant ice pack and you feel the solution inside the pack becoming colder after activating it.
> the solution making up most of the ice pack (water) is decreasing in temperature
> therefore, energy is being transferred from the surroundings (water) into somewhere else (the system)
> this tells us that the reactions that occur when the ice pack is activated are endothermic.

By contrast, let’s say that you have lit something on fire.
> the surroundings increase in temperature
> thus energy is being transferred from somewhere (the system) to the surroundings
> combustion reactions are exothermic

What if we wanted to know about the size of the energy change rather than just its direction?
One way we can do this is by having a known amount of reactant, having something that changes by a reliable and known amount per unit of energy it absorbs, using that “something” as the surrounds and measuring the temperature change in the surrounds.
For example, we could ignite a 100g of glucose (type of sugar) below a beaker with 500 mL water and measure the temperature change of the water. This only provides a rough estimate since some the of energy will go into the beaker holding the water rather than the water, some will go into the air, and some will be lost in other ways as well.

It’s better to have the reaction occur inside an insulated container designed for experiments like this (i.e. a calorimeter) to reduce this loss, but even then there will still be some uncertainty.



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