Consider the following reaction. 2Mg solid plus O₂ gas react to form 2MgO solid plus 1200 kilojoules. What is the value of ΔH for the reverse reaction when six moles of MgO are consumed to produce oxygen gas and solid magnesium? A) Negative 3600 kilojoules, B) negative 1200 kilojoules, C) positive 1200 kilojoules, D) positive 2400 kilojoules, or E) positive 3600 kilojoules.
Before we tackle this question, let’s take a look at this reaction. We have solid magnesium and oxygen gas reacting to form magnesium oxide. We also have 1200 kilojoules of energy that’s being released as a result of this reaction. When energy is released as a result of a reaction, we call that reaction exothermic. An exothermic process has a negative value for the change in enthalpy, or ΔH, for the process.
The change in enthalpy is directly related to the amount of energy that’s absorbed or released as a result of a reaction. So, this reaction has an enthalpy change of negative 1200 kilojoules. This question is asking us about the enthalpy change for the reverse reaction. The reverse of this reaction is 1200 kilojoules plus two magnesium oxide reacting to form two magnesium plus O₂ gas. The question doesn’t want to just know the enthalpy change for the reverse reaction, but the reverse reaction when six moles of magnesium oxide are consumed.
Right now, we only have two moles of magnesium oxide being consumed, so we’ll need to multiply everything in this reaction by three in order to answer the question. This will give us 3600 kilojoules plus six magnesium oxide reacting to form six magnesium plus three oxygen gas. In this reaction, instead of having energy being released, the reaction needs to absorb energy in order to react.
When a reaction or process absorbs energy, we call that reaction endothermic. Endothermic reactions have a positive change in enthalpy. Since this reaction is absorbing 3600 kilojoules of energy, the change in enthalpy for this reaction will be positive 3600 kilojoules. This matches answer choice E. So, the value of ΔH for the reverse reaction when six moles of magnesium oxide are consumed is 3600 kilojoules.