CH 110 Pre-Lab revised Fall 2017 HESS\' LAW AND THE FIRST LAW OF THERMODYNAMICS

Question

CH 110 Pre-Lab revised Fall 2017 HESS' LAW AND THE FIRST LAW OF THERMODYNAMICS 1. (a) IN YOUR OWN WORDS, explain how Hess' Law (of constant heat summation) applies to the 3 chemical reactions that will be studied in this experiment. (Note: answers copied from any source will NOT receive credit) (b) Is energy released or absorbed for an exothermic reaction? (c) What is the sign (positive or negative) for the AH of an exothemi c reaction ? - (d) Use the heat capacity of the calorimeter given and the equation in the middle of p. 38 to calculate the heat gained by the calorimeter if the AT for a reaction was 5.86 C

Explanation / Answer

Q1

Hess law may be explained as --> the arrangement and multiplication/inversion of the equations is valid, always if we ensure the total enthalpy change changes as the operations involved

This implies --> increase in 2x factor, implies 2x in enthalpy

inversion of reaction --< Hrxn must be negative, since it is now inverted

b)

exothermic --> negative Hvalue, which implies Hfinal < Hinitial, the energy is released as heat

c)

as stated above

enthalpy change is negative for exothermic

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