For which one of the following reactions is the value of δh°rxn equal to δh°f for the product?

a) 2ca (s) + o₂ (g) → 2cao (s)
b) c₂h₂ (g) + h₂ (g) → c₂h₄ (g)
c) 2c (graphite) + o₂ (g) → 2co (g)
d) 3mg (s) + n₂ (g) → mg₃n₂ (s)
e) c (diamond) + o₂ (g) → co₂ (g)

The reaction D has the value of ΔH°rxn equal to ΔH°f for the product.

Explanation:

The ΔH°f for product is equal to ΔH°rxn when the reagents are in their elemental state (ΔH°f = 0) and form one mole of product.

We have to find the reagents that are in their elemental state and that only form one mole of product:

A) 2Ca (s) + O₂ (g) → 2CaO (s)

The reagents are in their elemental state but the reaction forms two mole of product.

B) C₂H₂ (g) + H₂ (g) → C₂H₄ (g)

C₂H₂ (g) is not in its elemental state.

C) 2C (graphite) + O₂ (g) → 2CO (g)

Graphite and Oxygen are in their elemental state but the reaction forms two mole of product.

D) 3Mg (s) + N₂ (g) → Mg₃N₂ (s)

Magnesium and Nytrogen are in their elemental state and the reaction forms one mole of product.

E) C (diamond) + O₂ (g) → CO₂ (g)

Diamond is not in its elemental state.

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answer : one can design an experiment using a liquid and an additive that can be added to the liquid and observe the chemical changes in the reaction species.

one would need a beaker which is filled with a known amount of the liquid in which thermometer is placed to measure the temperature changes; after addition of the additive to the liquid solution one can note down the temperature change and find whether the reaction is endothermic or exothermic in nature. by subtracting the initial temperature value with the final temperature one can find the temperature changes.

various combinations of liquid and additive can be used. liquid in this case can be a known quantity of water and additives can be epsom salt or without epsom salt the experiment can be conducted to notice the changes. water and ice, hydrogen peroxide and dry yeast, vinegar and baking soda,etc.