How much heat is evolved Thermochemistry Question

In summary: Thanks again.In summary, the products of the reaction are: Ca(OH)2(s) + 64.8 kJ of heat. Taking the +64.8kJ into account, gives zero for ΔH. This means that the reaction is now balanced in terms of enthalpy. A product of the reaction is 64.8 kJ of heat (per mole), meaning that heat is given off. Hope that helps!
  • #1
anonymity
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"How much heat is evolved" Thermochemistry Question

This is a question from a general chemistry I quiz i had a few days ago. I got this question, and all other questions like it correct, but here's the thing.

Calcium oxide and water react in an exothermic reaction:
CaO(s) + H2O(l) → Ca(OH)2(s) DHrxn° = −64.8 kJ/mol

How much heat would be liberated when 7.15 g CaO(s) is dropped into a beaker containing 152 g H2O?

We have an exam tomorrow and I've been studying the majority of the weekend and I just started working on a thermochemistry worksheet that our instructor was kind enough to provide for us, and for every single "how much heat is evolved" or "how much heat would be released" question has an answer of negative _____.


The answer to this question would be positive, right? Because the negative enthalpy says that heat is released, and heat released = heat evolved = heat produced.

I think he's just trying to stress the concept of enthalpy but if this is the case i think he is definitely going about it in the wrong way > <


(PS: If i was allowed to see the answer that i recorded, this post would not be necessary, sorry)

Your Friend,
Anonymous
 
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  • #2


anonymity said:
The answer to this question would be positive, right? Because the negative enthalpy says that heat is released, and heat released = heat evolved = heat produced.
Yes.

Here is how I think of it. The products of the reaction are actually:
Ca(OH)2(s) + 64.8 kJ of heat​
If you think of it that way, then:
1. The reaction is now balanced in terms of enthalpy. Taking the +64.8kJ into account gives zero for ΔH
2. A product of the reaction is 64.8 kJ of heat (per mole), meaning that heat is given off.​

Hope that helps.
 
  • #3


yeah that helps. That's what i thought the whole time but my professor did it wrong on our homework answer sheet so i just had to make sure before the exam.

Thanks alot,

anonymous

edit:

felt that i needed to tell you that you devised a very clever way of thinking about that. I more-or-less just drilled "NEGATIVE ENTHALPY IS HEAT GENERATED" into my brain, but you created a working system. -.- cool.
 
  • #4


I have found it's easier to remember things when they make sense :smile:
 
  • #5


Dear Anonymous,

First of all, I want to commend you for your dedication to studying and understanding thermochemistry. It's a complex topic and it takes time and effort to fully grasp it.

To answer your question, yes, the answer to this question would be positive. As you correctly stated, the negative enthalpy indicates that heat is released in this reaction. However, when we are calculating the heat evolved in a reaction, we use the absolute value of the enthalpy change. This is because the enthalpy change is a state function and is independent of the path taken to reach that state. So whether the reaction is exothermic or endothermic, the absolute value of the enthalpy change is what we use to calculate the heat evolved.

I understand your confusion with the negative sign, as it may seem counterintuitive. But just remember that in this context, the negative sign is simply indicating the direction of the heat flow, not the magnitude.

I hope this helps clear up any confusion and good luck on your exam tomorrow!

Best regards,
 

1. What is heat?

Heat is a form of energy that is transferred from one object to another due to a difference in temperature. It is commonly measured in units of joules or calories.

2. How is heat evolved?

Heat can be evolved through various processes such as chemical reactions, phase changes, and nuclear reactions. In thermochemistry, heat is often evolved or released during exothermic reactions.

3. What is thermochemistry?

Thermochemistry is a branch of chemistry that deals with the study of heat and energy changes in chemical reactions. It involves the application of thermodynamics principles to understand and predict the behavior of chemical systems.

4. How is heat evolved measured?

Heat evolved can be measured using a calorimeter, which is a device that measures the change in temperature of a substance during a chemical reaction. The amount of heat evolved can also be calculated using the specific heat capacity and mass of the substances involved.

5. Why is it important to know how much heat is evolved?

Knowing how much heat is evolved in a chemical reaction is important because it can help us understand the energy changes that occur and how these changes affect the properties of the substances involved. This information is also useful in predicting the feasibility and efficiency of reactions, as well as in designing and optimizing industrial processes.

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