2S(s) + 2O₂(g) → 2SO₂(g) #ΔH_"c"# = -593.6 kJįinally, we add equations A, B, and C to get the target equation. We use Equation 2 to eliminate the S(s), but we have to double it to get 2S(s). Equation 1 contains C(s), so we write it as Equation B below.ī. Now we eliminate C(s) and S(s) one at a time. We have to reverse equation 3 and its ΔH to put the CS₂ on the left. It contains the first compound in the target (CS₂). Write down the target equation, the one you are trying to get. If enthalpy change is known for each equation, the result will be the enthalpy change for the net equation.ĭetermine the heat of combustion, #ΔH_"c"#, of CS₂, given the following equations.
This is accomplished by performing basic algebraic operations based on the chemical equation of reactions using previously determined values for the enthalpies of formation.Īddition of chemical equations leads to a net or overall equation.
Hess' law allows the enthalpy change (ΔH) for a reaction to be calculated even when it cannot be measured directly. In other words, if a chemical change takes place by several different routes, the overall enthalpy change is the same, regardless of the route by which the chemical change occurs (provided the initial and final condition are the same). The law states that the total enthalpy change during a reaction is the same whether the reaction is made in one step or in several steps. The heat of combustion for the reaction is -1075.0 kJ. color(green)("2S"("s") + "2O"_2("g") → "2SO"_2("g") ΔH_f = "-593.6 kJ")#įinally, we add the three equations to get the target equation, cancelling things that appear on opposite sides of the reaction arrows. We will use equation 2, but we will have to double it and its #ΔH# to get Equation 5. That would be equation 1, since we have already used equation 3. First, we find an equation that contains #"C"("s")"#. We have to eliminate these one at a time. This equation contains #"C"("s")# and #"S"("s")#, neither of which is in the target equation. That would be equation 3, but we must reverse equation 3 and its #ΔH# to get the #"CS"_2# on the left in Equation 4. Now we need to organize the given equations so that they add up to give the target equation.Ī good place to start is to find one of the equations that contains the first compound in the target equation ( #"CS"_2#). Write down the three equations you must use to get the target equation. Write down the target equation (the one you are trying to get). What is the value for the heat of combustion, #ΔH_c#, of the following reaction? You can use any combination of the first two rules.You must then multiply the value of #ΔH# by the same constant. You can multiply the equation by a constant.There are a few rules that you must follow when manipulating an equation. So, you can calculate the enthalpy as the sum of several small steps. \( \newcommand\) above.Hess's law states that the total enthalpy change does not rely on the path taken from beginning to end.
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