endothermic/exothermic processes
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endothermic/exothermic processes
If a reaction gives off heat, does it matter if the change in enthalpy is positive at a certain point? Does this change anything? This is assuming that this particular reaction ends at a lower energy than at the start (negative change in enthalpy).
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Re: endothermic/exothermic processes
According to Hess's law, the net change in enthalpy is dependent on the start and end energy of the reactants and products. Therefore, there can be a point at which the graph is positive, but if the products are lower than the reactants then it will be an exothermic reaction.
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Re: endothermic/exothermic processes
It may matter if the change in enthalpy is positive at some point if you are looking to carry out the reaction in a lab. For example, you must add heat to start the exothermic process of burning wood - you must supply the activation energy. However, I don't think this matters for our calculations.
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Re: endothermic/exothermic processes
The change in enthalpy will always be negative for an exothermic reaction because such reaction gives off heat as it goes from reactants to products. On the other hand, the change in enthalpy will always be positive for an endothermic reaction because such reaction absorbs heat as it goes from reactants to products.
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Re: endothermic/exothermic processes
If the change in H is a negative number, the reaction is giving off heart from reactants to products and the process is exothermic (and vice versa for a positive change in H for an endothermic reaction). Although the change in H can very from positive to negative, we are looking at the overall change and the overall process (which it overall endothermic or exothermic). The details is too specific and I doubt we will need to know to solve problems. Hope this helps!
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Re: endothermic/exothermic processes
How can you tell a reaction is endothermic from the enthalpy number?
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Re: endothermic/exothermic processes
Roni Nissim 1D wrote:How can you tell a reaction is endothermic from the enthalpy number?
You can tell if a reaction is endothermic from its enthalpy number by seeing if it’s positive. When a reaction absorbs more energy than it releases, it’s endothermic, thus giving us a positive enthalpy number!
Hope that helps :)
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Re: endothermic/exothermic processes
Enthalpy is a state property, meaning that the change in enthalpy is only dependent on starting and ending conditions. The pathway, then, does not matter. Adding heat in the middle is just one step in the path to create an overall negative change.
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Re: endothermic/exothermic processes
As long as there is a negative change in enthalpy and the reaction gives off heat, it is exothermic. If parts of the reaction have a positive change in enthalpy it does not matter as long as the overall change is positive after using Hess's Law to add up the change in enthalpy for all parts of the reaction. We only use the final change in enthalpy for our calculations.
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