The synthesis reaction to form boron hydride is: [tex]2 B( s )+3 H _2(g) \rightarrow B _2 H _2(g), \Delta H =+36 KJ$[/tex]. What is the enthalpy value for the manipulated reaction: [tex]$1 / 2 B_2 H _2(g) \rightarrow B ( s )+3 / 2 H _2(g)$[/tex] ?
A. -18 KJ
B. -36 KJ
C. +18 KJ
D. +72 KJ
Answer (1)
Reverse the given reaction, which changes the sign of Δ H from + 36 KJ to − 36 KJ.
Multiply the reversed reaction by 2 1 .
Multiply the enthalpy change of the reversed reaction by 2 1 : 2 1 × ( − 36 KJ ) = − 18 KJ .
The enthalpy change for the manipulated reaction is − 18 KJ .
Explanation
Understanding the Problem We are given the synthesis reaction of boron hydride: 2 B ( s ) + 3 H 2 ( g ) i g h t ha r p oo n u p B 2 H 2 ( g ) with an enthalpy change of Δ H = + 36 KJ. We need to find the enthalpy change for the reaction 2 1 B 2 H 2 ( g ) i g h t ha r p oo n u pB ( s ) + 2 3 H 2 ( g ) .
Reversing the Reaction First, we reverse the given reaction. When a reaction is reversed, the sign of the enthalpy change is also reversed. So, for the reaction B 2 H 2 ( g ) i g h t ha r p oo n u p 2 B ( s ) + 3 H 2 ( g ) , the enthalpy change is Δ H = − 36 KJ.
Multiplying the Reaction Next, we multiply the reversed reaction by 2 1 . When a reaction is multiplied by a factor, the enthalpy change is also multiplied by the same factor. So, for the reaction 2 1 B 2 H 2 ( g ) i g h t ha r p oo n u pB ( s ) + 2 3 H 2 ( g ) , the enthalpy change is Δ H = 2 1 ( − 36 KJ ) = − 18 KJ .
Final Answer Therefore, the enthalpy value for the manipulated reaction 2 1 B 2 H 2 ( g ) → B ( s ) + 2 3 H 2 ( g ) is − 18 KJ .
Examples
Enthalpy changes are crucial in chemical engineering for designing and optimizing chemical reactions. For instance, in designing a reactor for producing boron hydride, understanding the enthalpy change helps determine the energy input or removal required to maintain the reaction at a desired temperature. This ensures efficient and safe operation, preventing overheating or insufficient reaction rates. By manipulating reactions and understanding their enthalpy changes, engineers can optimize processes for maximum yield and energy efficiency.
