【High School Chemistry】
It was found that many students used Geiss's law to calculate enthalpy change, accustomed to all the known thermochemical equations, converted into target reaction equations, and then calculated the enthalpy of the target reaction, which is unscientific and too time-consuming in the exam.
For example, the real question of this national selection, through observation, found that the reactants/products in the target reaction equation only appear once in equations 1, 2, and 3, at this time, only the position and coefficient of the reactants and products need to be put together, and the total enthalpy change can be calculated, and there is no need to match the position and coefficients of all substances and the process of re-appointment.
Analysis: The reactant As2O5 coefficient is correct, but it should be placed on the left, so △H3 becomes -△H3; the reactant H2O coefficient is added to 3, and then placed on the left, so △H2 is to become -3△H2; the product H3AsO4 coefficient is added to 2, and the position is unchanged, so △H1 becomes 2△H1. After the coefficients and positions of the reactants and products of the target reaction are determined, it is possible to directly add up the enthalpy of each change regardless of other substances to obtain △H=2△H1-3△H2-△H3.
However, what to do when the reactants/products of the target reaction appear more than once in a known system of equations?
For example, in this question, it was observed that in the target reaction, Fe2O3, Fe and CO2 have only appeared once each, so it is only necessary to grasp these three substances. The coefficients and positions of Fe2O3 and Fe are both correct, and △H1 is unchanged; the position of CO2 is correct, but the coefficient is increased by 3, so △H2 becomes 3△H2. So △H = △H1 + 3 △ H2, that is, although C and O2 appear in reaction 3, reaction 3 is not related to the reaction of interest.
Of course, the type of problem type of Guys' law is relatively simple, and this method can only speed you up a bit.