Calculate the enthalpy of formation of water.Is the reaction an exothermic or endothermic reaction?

The reaction is as follows:

2H₂ + O₂ -> 2H₂O

This means that for every 2 molecules of Hydrogen gas, one molecule of Oxygen is needed to make 2 molecules of water

To calculate the enthalpy of this reaction, we use the equation:
 

Enthalpy of reaction =  (Enthalpy of products) - (Enthalpy of reactants)
 

The Enthalpy of Formations for each molecules are:

H₂ =0kJ/mol

O₂ =0kJ/mol

H₂O=-285.83kJ/mol

From the equation given earlier, we can put the numbers in and get

Enthalpy= -285.83-(0+0)

This means Enthalpy of formation of water=-285.83kJ/mol

So how can we tell if its endothermic or exothermic?

It's really quite simple. If there is a negative (minus) sign, the reaction is exothermic, and if there is a positive (plus) sign, the reaction is endothermic!

Therefore we can see, our answer has a negative sign so is exothermic!