Video Transcript
The chemical equation for the decomposition of magnesium carbonate is as follows. MgCO3 solid plus 117 kilojoules react to produce MgO solid plus CO2 gas. Based on this chemical equation, which of the following statements is true? (A) 117 kilojoules of energy is released when a single molecule of MgCO3 decomposes. (B) 117 kilojoules of energy is needed to make one mole of MgCO3 decompose. (C) 117 kilojoules of energy is needed to make 42 grams of MgCO3 decompose. (D) 117 kilojoules of energy is needed to make a single molecule of MgCO3 decompose. Or (E) 117 kilojoules of energy is released when one mole of MgCO3 decomposes.
The question tells us that the reaction is for the decomposition of magnesium carbonate. As MgCO3 is the only starting material, it must be magnesium carbonate. The magnesium carbonate decomposes into MgO, which is magnesium oxide, and CO2, carbon dioxide. Although there is only one starting material, the energy value, 117 kilojoules, is also on the left side of the chemical equation. This means that 117 kilojoules of energy is needed for the reaction to occur.
Options (B), (C), and (D) all state that the energy is needed for the reaction, whilst options (A) and (E) state that the energy is released upon the decomposition. This is not true. Thus, neither option (A) nor option (E) can be the answer to this question.
We can read this equation as with 117 kilojoules of energy, one mole of magnesium carbonate decomposes to produce one mole of magnesium oxide and one mole of carbon dioxide. If we look at the possible answer choices that we have remaining, option (B), 117 kilojoules of energy is needed to make one mole of magnesium carbonate decompose, appears to be the correct statement. But to confirm, let’s check options (C) and (D).
Option (C) states that 117 kilojoules of energy is needed to make 42 grams of magnesium carbonate decompose. The chemical equation doesn’t tell us about the mass of magnesium carbonate, only the number of moles, which is one. We can calculate the number of grams in one mole of magnesium carbonate using the atomic masses of magnesium, carbon, and oxygen. We would need to multiply the atomic mass of magnesium, which is 24.3 grams, by the number of equivalents of magnesium in magnesium carbonate, which is one. We would need to do the same for carbon and oxygen and add all of the values up. This gives a value of 84.3 grams. This means that one mole of magnesium carbonate is equivalent to 84.3 grams. It is not equivalent to 42 grams. So option (C) is not the answer to this question.
Option (D) is 117 kilojoules of energy is needed to make a single molecule of magnesium carbonate decompose. We’ve already established that it’s one mole, not one molecule. But also it doesn’t really make sense as an option, as magnesium carbonate is an ionic compound. Ionic compounds have large repeating lattice structures. So you cannot have a molecule of an ionic compound. Therefore, option (D) cannot be the answer to this question.
So the answer to the question “Based on this chemical equation, which of the following statements is true?” is (B). 117 kilojoules of energy is needed to make one mole of magnesium carbonate decompose.
