Find solution
How to use Hess's Law to calculate enthalpy for the reaction?

I have a serious Chemistry question, please help?

  • Use Hess's law and the measured mean enthalpy changes for the NaOH-HCl and NH3-HCl reactions to calculate the enthalpy change to be expected for the reaction? NaOH + NH3Cl ------> NaCl + NH3 + H2O Compare your experimental value with the one you have just calculated. The correct value is only -3.9KJ/mol. Try to explain any disrepency between the eperimental and calculated values and between these values and the correct value?

  • Answer:

    (1) NaOH + HCl ===> NaCl + H2O /\H = x (2) NH3 + HCl ===> NH4Cl /\H = y x and y are minus-numbers that you got from your experiments. So write them in. You need NH3 on the right, so write (2) backwards and reverse the sign of /\H: (3) NH4Cl ===> NH3 + HCl /\H = -y (meaning it is now a plus-number) Add (1) and (3) together, cancel the HCl's, and add the /\H's also: (4) NaOH + NH4Cl ===> NaCl + H2O + NH3 /\H = x + -(-y)

steve_geo1 at Yahoo! Answers Visit the source

Was this solution helpful to you?

Other answers

(1) NaOH + HCl ===> NaCl + H2O /\H = x (2) NH3 + HCl ===> NH4Cl /\H = y x and y are minus-numbers that you got from your experiments. So write them in. You need NH3 on the right, so write (2) backwards and reverse the sign of /\H: (3) NH4Cl ===> NH3 + HCl /\H = -y (meaning it is now a plus-number) Add (1) and (3) together, cancel the HCl's, and add the /\H's also: (4) NaOH + NH4Cl ===> NaCl + H2O + NH3 /\H = x + -(-y)

steve_ge...

You can use Hess's law and the given reactions to find the change in enthalpy of the problem reaction. NaOH + HCl --> NaCl + H2O deltaH= known (lets call it x) NH3 + HCl ---> NH3Cl deltaH= known (lets call it y) We can add the products and reactants and their deltaH values using Hess's law to find the unknown deltaH. However, since HCl is not involved in our known equation we must get it to cancel out. Hess's law allows us to multiply and divide the equations by constants as long as we also multiply their deltaH by the same value. So we can flip the second reaction over and change the sign of the deltaH to get: NaOH + HCl --> NaCl + H2O deltaH= x NH3Cl ---> NH3 + HCl deltaH= -y now we can simply add the reactions to get: NaOH + HCl +NH3Cl --> NaCl + H2O+HCl + NH3 deltaH= x+(-y) but since HCl appears on both sides of the reaction, it cancels out to leave: NaOH + NH3Cl --> NaCl + NH3 + H2O deltaH= x-y I hope this is what you needed! Good luck!

lindsey

You can use Hess's law and the given reactions to find the change in enthalpy of the problem reaction. NaOH + HCl --> NaCl + H2O deltaH= known (lets call it x) NH3 + HCl ---> NH3Cl deltaH= known (lets call it y) We can add the products and reactants and their deltaH values using Hess's law to find the unknown deltaH. However, since HCl is not involved in our known equation we must get it to cancel out. Hess's law allows us to multiply and divide the equations by constants as long as we also multiply their deltaH by the same value. So we can flip the second reaction over and change the sign of the deltaH to get: NaOH + HCl --> NaCl + H2O deltaH= x NH3Cl ---> NH3 + HCl deltaH= -y now we can simply add the reactions to get: NaOH + HCl +NH3Cl --> NaCl + H2O+HCl + NH3 deltaH= x+(-y) but since HCl appears on both sides of the reaction, it cancels out to leave: NaOH + NH3Cl --> NaCl + NH3 + H2O deltaH= x-y I hope this is what you needed! Good luck!

lindsey

Related Q & A:

Just Added Q & A:

Find solution

For every problem there is a solution! Proved by Solucija.

  • Got an issue and looking for advice?

  • Ask Solucija to search every corner of the Web for help.

  • Get workable solutions and helpful tips in a moment.

Just ask Solucija about an issue you face and immediately get a list of ready solutions, answers and tips from other Internet users. We always provide the most suitable and complete answer to your question at the top, along with a few good alternatives below.