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there was a reaction in my book. It is as follows:

$\ce{PbO + H2-> Pb + H2O}$

My question is Hydrogen is below lead in the reactivity series. Then how is this reaction possible?

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The activity series you're referring to is constructed for ambient conditions where the temperature is $25°C$ and water is a liquid. But, of course, thermodynamic properties can depend on temperature and especially on phase. At higher temperatures where water is ordinarily a gas, it has a lot more entropy than the liquid. That makes oxidation of hydrogen to water more thermodynamically favorable than what appears in your ambient temperature series.

For high temperature reactions where water mixes with hydrogen in the gas phase, the entropy effect pushes hydrogen up the series so it roughly matches iron.

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The enthalpy of formation of H2O is significantly larger than the enthalpy of formation of PbO, so this reaction is exothermic. Reactivity series are not perfect predictors. Results depend on what an element is being reacted with.

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  • $\begingroup$ you contradicted your initial comment that the reaction is endo-thermic. $\endgroup$ – 0tyranny 0poverty Feb 22 '18 at 6:06
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Look at it from a redox reaction perspective. I'm not sure what the parameters the reaction is taking place under but the Pb is going from Pb2+ to just Pb. This does make sense because lead oxide is commonly used as an oxidizing agent and hydrogen gas is considered to be a powerful reducing agent. Hope this helps

Just verified this with my advisor. It's an odd reaction but it does work. All the best

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  • $\begingroup$ Lead oxide which is commonly used as an oxidizing agent is another lead oxide, $\ce{PbO2}$. $\endgroup$ – Ivan Neretin Oct 15 '15 at 12:56

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