Heat of rxn per mol H2O formed in acid-base reaction

In summary, the heat of reaction per mole of water formed in the acid-base reaction is -55kJ/mol, assuming a total mass of 150g for the solution. This is calculated by using the equation ΔHrxn = qrxn = -mcΔT and taking into account the mass and specific heat of the reactants and products.
  • #1
Nuchem
3
0

Homework Statement


Calculate the heat of reaction per mole of water formed in the acid-base reaction. Assume that the total mass of the solution is 150g.

75mL of 2.0M NaOH added to 75mL of 2.0M HCl in a styrofoam cup. Average initial temp = 295.65K. final temp = 308.8K

Homework Equations


ΔHrxn = qrxn = -mcΔT


The Attempt at a Solution


qrxn = -(141.234g)(4.186J/gK)(308.8K-295.65K) = -7.8kJ

based on: .15 mol of HCl (5.5g) and .15 mol NaOH (6.0g) reacted to form .15 mol H2O (2.7g) and .15 mol NaCl (8.8g) therefore since the entire reaction is aqueous and the total mass of the solution is 150g: 150g -8.8g (the amount of the product that is not water) = 141.2g

in a simpler train of thought i decided 2.70g of water were actually formed and therefore:
qrxn = -(2.70g)(4.186J/gK)(308.8K-295.65K) = -1.49*102J

I feel that i am grossly over thinking this, however, and I need a bit of advice
 
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  • #2
Everything gets heated - so you can't ignore product and assume only water mass is important.

Simplest approach is to assume you have 150 mL of solution before and after the reaction and assume density and specific heat capacity of water.

That's not exactly true, but close enough. Note that density of neither solution was exactly 1 g/mL, so mass of the solution is not 150 g. But you had the same problem with your approach.
 
  • #3
Alright so "per mole of water formed" isn't really important in the calculations? Assuming the specific heat and density of the products is equal to water (I am actually instructed to do this at a later point) and assuming the mass of the solution is 150g (also instructed to do so) you're saying qrxn = -(150g)(4.186J/gK)(308.8K-295.65K) = -8.3kJ per mole of water formed?
 
  • #4
Nuchem said:
Alright so "per mole of water formed" isn't really important in the calculations??

It is important, just not when you calculate amount of heat that evolved in this particular experimental setup. And you calculated this amount of heat correctly - now just check how many moles of water were produced to be able to calculate heat per mole.
 
  • #5
Oh! Okay that's a lot more clear now, thank you. So I would take the -8.3kJ and divide by moles of water (.15 seeing that HCl + NaOH → H2O + NaCl is 1:1:1:1) to get
-8.3kJ/.15mol = -55kJ/mol
 
  • #6
I have not checked the numbers, just skimmed - and what you did looks OK now.
 

What is the heat of reaction per mole of water formed in an acid-base reaction?

The heat of reaction per mole of water formed in an acid-base reaction is the amount of energy released or absorbed when one mole of water is produced. It is a measure of the heat released or absorbed during the chemical reaction between an acid and a base.

How is the heat of reaction per mole of water formed calculated?

The heat of reaction per mole of water formed is calculated by dividing the amount of heat released or absorbed during the reaction by the number of moles of water produced. This can be determined experimentally by measuring the temperature change of the reaction or calculated using thermodynamic data.

What factors can affect the heat of reaction per mole of water formed in an acid-base reaction?

The heat of reaction per mole of water formed can be affected by a variety of factors, including the strength and concentration of the acid and base, the reaction temperature, and the presence of catalysts. The heat of reaction can also vary depending on the specific acid and base used in the reaction.

Why is the heat of reaction per mole of water formed important in acid-base reactions?

The heat of reaction per mole of water formed is important because it provides information about the energy changes that occur during the reaction. It can help determine the efficiency of a reaction, as well as the stability and strength of the products. This information is crucial in understanding and optimizing acid-base reactions in various industries, such as chemical manufacturing and pharmaceuticals.

How does the heat of reaction per mole of water formed compare between exothermic and endothermic acid-base reactions?

In an exothermic acid-base reaction, the heat of reaction per mole of water formed will be negative, indicating that energy is released during the reaction. On the other hand, in an endothermic acid-base reaction, the heat of reaction per mole of water formed will be positive, indicating that energy is absorbed during the reaction. This difference in heat of reaction can also affect the rate and direction of the reaction.

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