## Problem 8.99

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Maria1E
Posts: 61
Joined: Sat Jul 22, 2017 3:01 am

### Problem 8.99

Hydrochloric acid oxidizes zinc metal in a reaction that produces hydrogen gas and chloride ions. A piece of zinc metal of mass 8.5 g is dropped into an apparatus containing 800 mL of .500M HCl(aq). If the initial temperature of the hydrochloride acid solution is 25 degrees celsius, what is the final temperature of this solution? Assume that the density and molar heat capacity of the hydrochloric acid solution are the same as those of water and that all the heat is used to raise the temperature of the solution.

I have two questions about this problem.
1) How are we supposed to know when the enthalpy of formation of a certain product or reactant is 0?
2) In the final calculation, it is stated that -20 kJ = (800 g)(-4.184 J/Cxg)(delta T). Where is the 800 grams from?

Cynthia Tsang
Posts: 52
Joined: Fri Sep 29, 2017 7:07 am
Been upvoted: 1 time

### Re: Problem 8.99

For the first question, standard energy of formations are 0 when a compound is in its most stable, standard state. The definition of standard enthalpy of formation is the amount of energy required to create a 1 mole of a substance from its original standard state. Examples include O2, H2, I2. Br2, however, does not have a standard enthalpy formation of 0.

Cynthia Tsang
Posts: 52
Joined: Fri Sep 29, 2017 7:07 am
Been upvoted: 1 time

### Re: Problem 8.99

For the second question, the 800g comes from the 800 mL of water. Recall that water has a density of 1g/mL so this is used to change the volume to mass. Densities for other substances will be provided, but that's a good one to remember.

Return to “Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)”

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