Can you help me with this enthalpy problem?

Question

Consider the reaction:

2H2+O2==>2H2O (change in enthalpy -572J)

 

A. How much heat is evolved for the production of 1.00 mol H2O?

 

B. How much heat is evolved when 186g O2 is reacted with excess hydrogen?

 

C. How much heat is reacted when 4.03 g of hydrogen is reacted with excess hydrogen.

 

*These are not the exact numbers in my book, I just want to learn how to do the steps.

 

Answer 1

A.  The information provided tells you that 572 J is evolved in the production of TWO moles H₂O.  So, you should be able to figure out the heat evolved in the production of ONE mole H₂O.

 

B.  How many moles of O₂ is 186 g of O₂?  186 g x 1 mol/32 g = 5.81 moles.  From the equation, you know that 1 mole O₂ generates 572 J of heat.  So heat evolved from 5.81 moles is 5.81 x this amount.

 

C.  How much heat is reacted?  You probably mean how much heat is evolved.  How many moles of H₂ is 4.03 g of H₂?

4.03 g H2 x 1 mole/2 g = 2.015 moles H₂.  TWO moles H2 generates 572 J of heat, so how much is generated by 2.015 moles H₂?  Just multiply.