The reaction profile diagram below shows that graphite is more stable than diamond. However, the conversion from diamond to graphite is very difficult. Which of the following statements explains why? (A) Diamond does not conduct electricity. (B) The energy required to break the covalent bonds between carbon atoms is low. (C) The reaction is exothermic, not endothermic. (D) The energy difference between diamond and graphite is very small. Or (E) the activation energy for the conversion of diamond into graphite is very high.
It is important to note that the phrase conversion from diamond to graphite is referring to the chemical reaction of transforming diamond into graphite, even though both are pure elemental-carbon-based covalent network structures that expand beyond what can be drawn here. To put it simply, the carbon atoms in diamond are each covalently bonded to four other carbon atoms, while the carbon atoms in graphite are only covalently bonded to three other carbon atoms. This difference in how the carbon atoms are bonded means converting diamond into graphite involves breaking the covalent bonds between carbon atoms in diamond to form new different covalent bonds between carbon atoms in graphite.
Because we are looking for a statement that explains why this chemical reaction is difficult, we can immediately eliminate the statement in answer choice (A). While this statement is true, electrical conductivity is a physical property, which is defined as a characteristic of a substance that can be observed or measured without changing the identity of the substance and is not an explanation for why this chemical reaction is difficult.
With the remaining statements relating to the reaction profile provided, let’s briefly discuss what this reaction profile tells us about the chemical reaction in question. We are told that graphite is more stable than diamond. The reaction profile correctly depicts this because the energy level for graphite, the product, is lower in energy than the energy level for diamond, the reactant. When the products are at a lower energy level than the reactants, this is due to energy being released, usually in the form of heat, which means that the conversion of diamond to graphite is exothermic and also means that answer choice (C) is true. However, confirming the reaction is exothermic does not explain why the reaction itself is difficult. So we can eliminate answer choice (C).
Continuing in comparing the energy levels of diamond and graphite, the difference in energy between the products and the reactants in this reaction, known as the enthalpy change or Δ𝐻, is not only negative because this is an exothermic reaction, but it is also small. Taking a look at answer choice (D), this statement is also true. But again, knowing this reaction releases a small amount of energy or heat does not explain why the reaction is difficult. Therefore, we can also eliminate answer choice (D) at this point.
The statements listed in answer choices (B) and (E) have to do with the portion of the reaction pathway in between the reactant and product energy levels. In a chemical reaction, bonds are broken in the reactant particles in order to form new bonds in the product particles. For this process to occur, a minimum amount of energy is required. This is known as the activation energy 𝐸 a, which is defined as the minimum amount of energy required by reactant particles to collide and react with each other, otherwise known as the initial increase or boost of energy needed to get the reaction started. If the chemical reaction is provided with an amount of energy denoted as 𝐸 that is smaller than the 𝐸 a, then the reactants won’t be able to transform into the products, and the reaction won’t proceed.
Looking at the 𝐸 a on the reaction profile, we can see that 𝐸 a is quite large or high for this reaction, which means it takes a lot of energy to break the covalent carbon bonds in diamond in order for new covalent carbon bonds in graphite to be formed. Based on this information, answer choice (B) is false, because the large 𝐸 a required for this reaction indicates the energy required to break the bonds between carbon atoms is quite large. As a result, we can eliminate answer choice (B), leaving the correct answer to be answer choice (E).
Therefore, which statement explains why the conversion of diamond to graphite is difficult? The answer is option (E). The activation energy for the conversion of diamond into graphite is very high.