Reaction(s) released heat. Reaction released the most heat per mole of reactant.
Answer (2)
Calculate the enthalpy change for the magnesium combustion reaction using Hess's Law: Δ H = − 450 + 82 + 286 = − 82 k J / m o l .
Identify that Reactions 1, 2, 3, and the magnesium combustion reaction all release heat ( Δ H < 0 ).
Determine that Reaction 1 releases the most heat per mole of reactant ( Δ H = − 450 k J / m o l ).
The magnesium combustion reaction has Δ H = − 82 k J / m o l , all reactions released heat, and Reaction 1 released the most heat. R e a c t i o n ( s ) 1 , 2 , 3 , M a g n es i u m co mb u s t i o n re l e a se d h e a t . R e a c t i o n 1 re l e a se d t h e m os t h e a tp er m o l eo f re a c t an t .
Explanation
Understanding the Problem We are given three reactions with their enthalpy changes ( Δ H ) and asked to find the enthalpy change for the magnesium combustion reaction: M g ( s ) + 1/2 O 2 ( g ) i g ha rro wM g O ( s ) . We also need to identify which reaction(s) release heat (exothermic) and which releases the most heat per mole of reactant.
Applying Hess's Law To find the enthalpy change for the magnesium combustion reaction, we will use Hess's Law. Hess's Law states that the enthalpy change of a reaction is independent of the path taken, so we can manipulate the given reactions to obtain the desired reaction and sum their enthalpy changes accordingly.
Manipulating the Reactions First, we need to manipulate the given reactions to obtain the desired reaction: M g ( s ) + 1/2 O 2 ( g ) i g ha rro wM g O ( s ) .
Reaction 1: M g ( s ) + 2 H Cl ( a q ) i g ha rro wM g C l 2 ( a q ) + H 2 ( g ) , Δ H = − 450 k J / m o l
Reaction 2: M g O ( s ) + 2 H Cl ( a q ) i g ha rro wM g C l 2 ( a q ) + H 2 O ( l ) , Δ H = − 82 k J / m o l
Reaction 3: H 2 ( g ) + 1/2 O 2 ( g ) i g ha rro w H 2 O ( l ) , Δ H = − 286 k J / m o l
We need to reverse Reaction 2 and Reaction 3:
Reversed Reaction 2: M g C l 2 ( a q ) + H 2 O ( l ) i g ha rro wM g O ( s ) + 2 H Cl ( a q ) , Δ H = + 82 k J / m o l Reversed Reaction 3: H 2 O ( l ) i g ha rro w H 2 ( g ) + 1/2 O 2 ( g ) , $\Delta H = +286 kJ/mol
Combining the Reactions Now, we add Reaction 1, reversed Reaction 2, and reversed Reaction 3:
M g ( s ) + 2 H Cl ( a q ) i g ha rro wM g C l 2 ( a q ) + H 2 ( g ) , Δ H = − 450 k J / m o l M g C l 2 ( a q ) + H 2 O ( l ) i g ha rro wM g O ( s ) + 2 H Cl ( a q ) , Δ H = + 82 k J / m o l H 2 O ( l ) i g ha rro w H 2 ( g ) + 1/2 O 2 ( g ) , Δ H = + 286 k J / m o l
The sum of these reactions is: M g ( s ) + 2 H Cl ( a q ) + M g C l 2 ( a q ) + H 2 O ( l ) \righarrow M g C l 2 ( a q ) + H 2 ( g ) + M g O ( s ) + 2 H Cl ( a q ) + H 2 ( g ) + 1/2 O 2 ( g ) .
Simplifying, we get M g ( s ) + 1/2 O 2 ( g ) \righarrow M g O ( s ) .
Calculating Enthalpy Change To calculate the enthalpy change for the magnesium combustion reaction, we sum the enthalpy changes of the manipulated reactions:
Δ H = − 450 + 82 + 286 = − 82 k J / m o l
Identifying Exothermic Reactions Now, we determine which reactions released heat (exothermic, Δ H < 0 ):
Reaction 1: Δ H = − 450 k J / m o l (released heat) Reaction 2: Δ H = − 82 k J / m o l (released heat) Reaction 3: Δ H = − 286 k J / m o l (released heat) Magnesium combustion: Δ H = − 82 k J / m o l (released heat)
All reactions released heat.
Identifying the Reaction with the Most Heat Released Finally, we identify the reaction with the most negative Δ H value to determine which reaction released the most heat per mole of reactant:
Reaction 1: Δ H = − 450 k J / m o l Reaction 2: Δ H = − 82 k J / m o l Reaction 3: Δ H = − 286 k J / m o l Magnesium combustion: Δ H = − 82 k J / m o l
Reaction 1 released the most heat per mole of reactant.
Final Answer Therefore, the magnesium combustion reaction has an enthalpy change of -82 kJ/mol. Reactions 1, 2, 3 and Magnesium combustion released heat. Reaction 1 released the most heat per mole of reactant.
Examples
Understanding enthalpy changes is crucial in many real-world applications. For example, when designing engines or power plants, engineers need to know how much energy is released or absorbed during chemical reactions to optimize efficiency and prevent overheating. Similarly, in the food industry, knowing the enthalpy changes of reactions helps in determining the nutritional value and shelf life of products. In everyday life, understanding exothermic reactions helps us use energy safely and efficiently, like when burning fuel for heating or cooking.
Reactions that release heat are called exothermic, and the enthalpy change (H) helps us measure this heat change. Among the reactions analyzed, Reaction 1 released the most heat per mole at -450 kJ/mol, while all reactions considered were exothermic. Thus, Reaction 1 is the one that releases the most heat per mole of reactant.
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