Calculating Delta H of A_2 Reaction with B_2

In summary, the conversation discusses a chemical reaction between a hypothetical substance A2 and B2. The equation for the reaction is given, along with the enthalpy change. The conversation also includes information about the molar masses of A and B, and the amount of water in a calorimeter. The individual has attempted to solve for the enthalpy change, but is unsure if the calculation was correct. They are seeking clarification on how to properly account for the coefficient of A2 in the calculation.
  • #1
Dooh
41
1
Ok, I am stuck half way through this problem. here's what i got:

a hyopthetical substance A_2 reacts with B_2:

3 A_2 + 4 B_2 --> 2 A_3 B_4

delta H = ? kcal/mole A_2

1 mole of A = 56.4g
1 mole of B = 29.6g

when 15.6g of A_2 and an excess of B_2 react in a calorimeter that contains 0.00186 mL of water, the water temp changes from 23/5 celsius to 86.7 celsius. What is the delta H of the reaction in kcal/mole?

I approached this by first solving for q , then divide it by mole. But I am not sure if i solved for the mole correctly because i multiplied 56.4g by 6 since its "3 A_2". Also, after i get the answer, am i suppose to divide it by 3 because in the reaction, the coefficient for A_2 was 3, that doesn't match with what the answer wanted, which is kcal/ mole A_2. (coefficient is 1) Any help?
 
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  • #2
Hi, did you notice that A is bimolecular, as in A2? In addition, you are right to include its coefficient in calorimetric calculation, since 3 moles of A2 reacts with an excess of B2. One mole of A2 is equal to 2*56.4=112.8 grams. Be careful...
 
  • #3
You know that [itex]q_{reaction} = -(q_{solution} + q_{calorimeter}) [/itex]. [itex]q_{solution}[/itex]refers to water and you should know what [itex]q_{calorimeter} [/itex]represents. In the end simply divide [itex]q_{reaction}[/itex] by the molar quantity of the limiting reagent, than use factor labeling using the coefficient ratio of [itex]\frac{moles lim reagent}{moles rxn} [/itex].
 

1. What is the purpose of calculating Delta H for a reaction between A2 and B2?

The purpose of calculating Delta H is to determine the change in enthalpy for a chemical reaction. This value can indicate whether a reaction is exothermic (releases heat) or endothermic (absorbs heat).

2. How do you calculate Delta H for a reaction between A2 and B2?

Delta H can be calculated by subtracting the enthalpies of the products from the enthalpies of the reactants. This value can be obtained from experimental data or calculated using Hess's Law and standard enthalpy of formation values.

3. What factors can affect the accuracy of Delta H calculations?

The accuracy of Delta H calculations can be affected by experimental errors, such as measuring equipment and human error. It can also be affected by the purity of the reactants and the completeness of the reaction.

4. Can the value of Delta H change over time?

The value of Delta H is a constant for a specific reaction at a given temperature and pressure. However, it can change with different conditions, such as temperature and concentration of reactants.

5. How is Delta H related to the spontaneity of a reaction between A2 and B2?

The sign of Delta H (positive or negative) can indicate the spontaneity of a reaction. If Delta H is negative, the reaction is exothermic and spontaneous. If Delta H is positive, the reaction is endothermic and non-spontaneous.

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