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# Enthalpy change

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1. The equation for the equilibrium between nitrogen and oxygen may be written intwo ways.

N2(g) + O2(g) <=> 2NO(g)
½N2(g) + ½O2(g) <=> NO(g)

The standard enthalpy change for the reaction as shown in the first equation is+180 kJ mol–1.

What is the standard enthalpy change for the reaction as shown in the secondequation?

A +45 kJ mol–1
B +90 kJ mol–1
C +180 kJ mol–1
D +360 kJ mol–1

I thought it was C as the answer is in the unit of per mole. But the correct answer is B.

2. (Original post by mystreet091234)
The equation for the equilibrium between nitrogen and oxygen may be written intwo ways.

N2(g) + O2(g) <=> 2NO(g)
½N2(g) + ½O2(g) <=> NO(g)

The standard enthalpy change for the reaction as shown in the first equation is+180 kJ mol–1.

What is the standard enthalpy change for the reaction as shown in the secondequation?

A +45 kJ mol–1
B +90 kJ mol–1
C +180 kJ mol–1
D +360 kJ mol–1

I thought it was C as the answer is in the unit of per mole. But the correct answer is B.

It's exactly what you just said. The enthalpy change is the energy released per mole.

If 180kJ is required to react 1 mole of N2 and 1 mole of O2 together,
then 90kJ is required to react 0.5 mole of N2 and 0.5 mole of O2 together

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