Question Video: Comparing the Heat Transferred During Combustion and Freezing of Gasoline | Nagwa Question Video: Comparing the Heat Transferred During Combustion and Freezing of Gasoline | Nagwa

Question Video: Comparing the Heat Transferred During Combustion and Freezing of Gasoline Chemistry • First Year of Secondary School

When 10 mL of gasoline freezes at −57°C, the total amount of heat transferred is approximately 1 kJ. However, 300 kJ of heat is transferred when the same volume of gasoline is burned at room temperature. Why does combustion result in a greater transfer of heat than freezing? [A] Freezing is exothermic, while combustion is endothermic. [B] Bonds broken during combustion are weaker than those broken during freezing. [C] Bonds formed during combustion are stronger than those formed during freezing. [D] Combustion occurs at a higher temperature than freezing. [E] Freezing is endothermic, while combustion is exothermic.

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Video Transcript

When 10 milliliters of gasoline freezes at negative 57 degrees Celsius, the total amount of heat transferred is approximately one kilojoule. However, 300 kilojoules of heat is transferred when the same volume of gasoline is burned at room temperature. Why does combustion result in a greater transfer of heat than freezing? (A) Freezing is exothermic, while combustion is endothermic. (B) Bonds broken during combustion are weaker than those broken during freezing. (C) Bonds formed during combustion are stronger than those formed during freezing. (D) Combustion occurs at a higher temperature than freezing. Or (E) freezing is endothermic, while combustion is exothermic.

The focus of this question is on comparing the difference in heat transfer between the two processes, combustion and freezing, more specifically in regards to why the burning of gasoline, also known as combustion, demonstrates a greater transfer of heat than freezing does when equal volumes for each process are considered. To simplify the pertinent information provided, let’s use this table as it summarizes the information we will need to assist in answering the question.

In this question, we are comparing two different quantities of energy, one kilojoule of heat transferred in the freezing of 10 milliliters of gasoline and 300 kilojoules of heat transferred when combusting 10 milliliters of gasoline. Notice the substance type and the amount is the same, with the main differences being the process they are undergoing and the amount of heat that was transferred. The question is, why if we’re dealing with the same substance and the same amount for each process are the heat transfer values hugely different? To figure this out, let’s briefly discuss each process.

While, in reality, gasoline is a complex mixture of C4 to C12 hydrocarbons or compounds containing hydrogen and carbon atoms along with additives to enhance the fuel performance, for the purposes of illustrating the differences between the two processes, we will use the simplest hydrocarbon methane as a representative molecule. The process of combustion, like the combustion reaction of methane, involves substances reacting with oxygen. More specifically, energy is supplied such that the covalent bonds that hold the carbon and hydrogen atoms together in methane and the covalent bonds that hold the oxygen atoms together are broken in order for the carbon, oxygen, and hydrogen atoms to come together to form new covalent bonds and the new products, carbon dioxide and water.

The covalent bonds formed in the products are stronger than the covalent bonds broken in the reactants, which means the products will have a lower chemical potential energy than the reactants and will therefore be more stable. This will result in a release of energy or heat from the reaction into the surroundings and is why combustion reactions are good examples of exothermic reactions. We also see releases of energy during state changes or phase changes, such as in the physical process of freezing where the substances in question are going from a higher state of energy, known as the liquid phase, to a physical state of lower energy, known as the solid phase.

Using methane as a representative molecule for gasoline, when freezing occurs, the intermolecular forces between the solid-state methane molecules are stronger than the intermolecular forces between the liquid-state methane molecules. As a result of the methane molecules going from a physical state of higher energy, also known as the liquid state, to a physical state of lower energy, known as the solid state, a small amount of energy in the form of heat is released to the surroundings, which classifies freezing as an exothermic process. With combustion and freezing both being classified as exothermic processes, we can eliminate answers (A) and (E), which incorrectly indicate that either combustion or freezing is endothermic.

In addition, while it is likely that combustion does indeed occur at a higher temperature than freezing, it does not help us to compare the quantities of energy transferred. So, we can also eliminate answer choice (D). In comparing answer choices (B) and (C), one main distinction should be made when it comes to determining the meaning of breaking bonds or forming bonds as it refers to combustion and freezing. While we have mentioned that both combustion and freezing are exothermic processes, the way in which the energy or heat is released from each process stems from a different source. In answer choices (B) and (C), it mentions bonds are broken and formed during combustion.

This is true because combustion, being a chemical reaction, does involve breaking covalent bonds within reactant molecules and forming new covalent bonds within the products. More specifically, covalent bonds are a type of intramolecular force, which means combustion involves breaking and forming intramolecular forces. It also mentions in answer choices (B) and (C) that bonds are broken or bonds are formed during the process of freezing. As we stated earlier, in a physical process such as freezing, only intermolecular forces are broken and reformed, while the intramolecular forces, or the forces holding the atoms together in the methane molecules, are not broken or formed during the freezing process. As a result, intermolecular forces are the bonds referred to as being broken or formed in the freezing process.

That being said, with combustion and freezing both being an exothermic process, we know the products of the combustion reaction and the product of the freezing process have stronger bonds or forces than the reactants. So, we can eliminate answer choice (B). The reason option (C) is the answer is because if more energy is released when gasoline is burned, then the same volume being frozen, we know that the bonds formed in the products of combustion are stronger than those of the product formed during freezing.

This makes sense because intramolecular forces, like the ones formed in the combustion products, are stronger than intermolecular forces, like the ones formed between molecules upon freezing. So, why does combustion result in a greater transfer of heat than freezing? The answer is (C). Bonds formed during combustion are stronger than those formed during freezing.

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