Solving Enthelpy Problem: ΔH = 12.9 kJ

  • Thread starter jools111
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In summary, the conversation is about a lab involving diluting NaOH and calculating the net ionic equation and ΔH value. The individual is confused about the positive value for ΔH and asks for clarification. Another person explains that the sign can be changed since the heat was gained by the solution and lost by the NaOH. They also remind the individual to include ions and charges in the reaction equation. Various online calculators are suggested for further help.
  • #1
jools111
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Homework Statement



I am working on a lab that has me a little confused:

I diluted 5.5g of NaOH into 200ml of water to disassociate the sodium ions. In the equation below you can see my temperature results.

Write the net ionic equation for the reaction, and note the value of ΔH.

NaOH ---> Na + OH

Homework Equations



ΔH = m•ΔT•Q
ΔH = (200 g + 5.5 g)(final temp - initial temp)(4.18 J/g°C)
ΔH = (205.5 g)(41 °C - 26 °C)(4.18 J/g°C)
ΔH = 12.9 kJ

The Attempt at a Solution



Because it's an exothermic reaction (It created heat...), I know ΔH should be negative. Why is it coming up positive? I'm lost here... I'm pretty sure I've done everything correctly. Thanks!
 
Last edited:
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  • #2
Don't worry. You have correctly calculated amount of heat that was absorbed by the solution - it was gained by the solution, so it was 'lost' by the NaOH. Just change the sign.

Beware, correct reaction equation is

NaOH -> Na+ + OH-

you can't ignore ions/charges.
 
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  • #3
Borek said:
Don't worry. You have correctly calculated amount of heat that was absorbed by the solution - it was gained by the solution, so it was 'lost' by the NaOH. Just change the sign.

Beware, correct reaction equation is

NaOH -> Na+ + OH-

you can't ignore ions/charges.

--
buffer calculator, concentration calculator
pH calculator, stoichiometry calculator

So would the correct answer to the question be negative? I'm a little confused. I understand the logic, but can I just arbitrarily change the symbol like that?
 

1. What is enthalpy and why is it important?

Enthalpy is a measure of the total energy of a system and its surroundings. It is important because it helps us understand and predict the direction and extent of chemical reactions and physical processes.

2. What does the value of ΔH = 12.9 kJ mean?

The value of ΔH = 12.9 kJ represents the change in enthalpy for a specific reaction or process. In this case, it means that the system has gained 12.9 kilojoules of energy.

3. How is enthalpy related to heat and temperature?

Enthalpy is directly related to heat and temperature. Heat is the transfer of energy between two objects with different temperatures, and enthalpy is the measure of this energy transfer. Temperature, on the other hand, is a measure of the average kinetic energy of particles in a system.

4. Can the value of ΔH be negative?

Yes, the value of ΔH can be negative. A negative value indicates that the system has released energy to its surroundings, which is often seen in exothermic reactions.

5. How can we use ΔH to calculate the heat of a reaction?

We can use the equation q = ΔH to calculate the heat of a reaction, where q represents the heat and ΔH is the change in enthalpy. This allows us to determine the amount of energy transferred during a chemical reaction.

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