# Solving Chemistry Enthalpy Homework: Step-by-Step Guide

• laurenM
In summary, the enthalpy change for the reaction of 6.000g of C2H6 to give 2C2H4(g) and 2H2(g) is -6.41 kJ, which is found by dividing the given enthalpy by 30g to account for the 6 grams of C2H6 used in the reaction.
laurenM

## Homework Statement

Given 2C2H6 $\rightarrow$2C2H4(g) 2H2(G) $\Delta H$=-64.2kJ

$\Delta H$ for the reaction of 6.000g of C2H6(g) to givev C2H4(g) andH2(g) is

## Homework Equations

$\Delta H$ = $\Sigma products$ - $\Sigma reactants$
q=ct
keeping the units
The enthalpies are not given and are not to be looked up

## The Attempt at a Solution

I understand that i have to divide ΔH by 2 and multiply by 6g
but that gives me a unit of kJ*g so i need to divide by grams to get the units right... my solution book tells me to divide by 30g but i am unaware as to where that comes from

You are given the enthalpy for a reaction of one mole. How much does a mole weigh in grams of this material? That would be my thinking, but the book has a slightly different number than I would expect

Book is right.

BOYLANATOR said:
You are given the enthalpy for a reaction of one mole.

Enthalpy is given for two moles, not for one. But generally speaking you are on the right track, it is abut finding number of moles.

The answer from the book is-6.41 kJ. I found the mols of C2H6 and used the ratio to figure out the sum of mass of reaction and products to get to get 12.00g this gives me the answer of -16.05 kJ.
=-64.2/2 to get it to a 1 to 1 ratio
=-32.1*6 multiply by 6.00g
=-192.6/12
=-16.05 kJ
if you divide by 30 instead of 12 you get the right answer i just don't get where it comes from

I don't follow your logic here. You worked out the enthalpy would be -32.1 kJ for a one mole reaction. That's ok.

But in this case it's not one mole of C2H6, it's 6 grams. What fraction of a mole is 6 grams?

Just make sure that the units cancel properly:

$$(32.1 \frac{kJ}{mole})(\frac{1mole}{30grams})(6grams)=? kJ$$

## 1. What is enthalpy?

Enthalpy is a thermodynamic property that measures the total energy of a system, including both its internal energy and the energy required to create or maintain its surroundings.

## 2. How do I calculate enthalpy change?

To calculate enthalpy change, you need to know the initial and final enthalpy values of the system and the amount of heat exchanged during the process. The enthalpy change can be calculated using the formula ΔH = Hfinal - Hinitial.

## 3. What are the units for enthalpy?

The units for enthalpy are joules (J) or kilojoules (kJ) in SI units. In some cases, it may also be expressed in calories (cal) or kilocalories (kcal).

## 4. What is the difference between endothermic and exothermic reactions?

In an endothermic reaction, energy is absorbed from the surroundings, resulting in a positive enthalpy change. In contrast, an exothermic reaction releases energy into the surroundings, resulting in a negative enthalpy change.

## 5. How do I use enthalpy to predict the spontaneity of a reaction?

The sign of the enthalpy change can be used to predict the spontaneity of a reaction. If the enthalpy change is negative, the reaction is exothermic and spontaneous. If the enthalpy change is positive, the reaction is endothermic and non-spontaneous. A reaction with a positive enthalpy change can become spontaneous at high temperatures.

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