Question

You will use Hess’ Law inthis experiment to determine the heat of reaction(ΔH) for a reaction that is very difficult to measure directly:

The heat of combustion of magnesium ribbon, which is represented by:
(1) Mg(s)+ 1/2O2(g) ------> MgO(s)
Reaction (1) can be obtained by combining equations (2), (3), and (4) using Hess’ Law
(2) MgO(s)+ 2 HCl(aq) -------> MgCl2(aq)+ H2O(l)
(3) Mg(s)+ 2 HCl(aq) -------> MgCl2(aq)+ H2(g)
(4) H2(g)+ 1/2O2(g) ------> H2O(l)
Question:
A. Combine reactions(2), (3), and (4) to obtain reaction (1) using Hess’ law.
B. Determine (calculate) the ΔH values for reactions (2), (3) and (4).

Answer 1

Answer: The
`\Delta H_1` for the reaction is equal to
`(\Delta H_3+\Delta H_4-\Delta H_2)`

Step-by-step explanation:

Hess’s law of constant heat summation states that the amount of heat absorbed or evolved in a given chemical equation remains the same whether the process occurs in one step or several steps.

According to this law, the chemical equation is treated as ordinary algebraic expressions and can be added or subtracted to yield the required equation. This means that the enthalpy change of the overall reaction is equal to the sum of the enthalpy changes of the intermediate reactions.

The given chemical reaction follows:

(1)
`Mg(s)+(1)/(2)O_2(g)\rightarrow MgO(s)`
`\Delta H_1=?`

The intermediate balanced chemical reaction are:

(2)
`MgO(s)+2HCl(aq.)\rightarrow MgCl_2(aq.)+H_2(g)`
`\Delta H_2`

(3)
`Mg(s)+2HCl(aq.)\rightarrow MgCl_2(aq.)+H_2(g)`
`\Delta H_3`

(4)
`H_2(g)+(1)/(2)O_2(g)\rightarrow H_2O(l)`
`\Delta H_4`

The expression for enthalpy of the reaction follows:

`\Delta H_1=[1* (-\Delta H_2)]+[1* \Delta H_3]+[1* \Delta H_4]`

Hence, the
`\Delta H_1` for the reaction is equal to
`(\Delta H_3+\Delta H_4-\Delta H_2)`