Question

i.All decomposition reactions are endothermic. Why?
ii. Why do silver articles become black after sometime when they are exposed to air?

Answer 1

Hint: When bonds break in a compound, energy is required, whereas, when new bonds are formed, then energy gets released in the form of heat. Silver reacts with a hydrogen compound present in air to form a precipitate.

Complete step by step answer: In order to answer the question, we need to understand the thermodynamics of reaction. When a balanced chemical equation not only indicates the quantities of different reactants and products but also indicates the amount of heat evolved or absorbed ($\mathrm{\Delta }H$) in the reaction is known as the thermochemical equation. The physical state (along with allotropic states) of the substance in an equation is also specified. For example:
$$C_2{H}_{5}OH+3O_2\to 2C{O}_2+3H_{2}O\mathrm{\Delta }H=-1367kJmo{l}^{-1}$$
The equation represents an exothermic reaction shown by the negative sign of enthalpy change. It describes the combustion of liquid ethanol at constant pressure and temperature. Some important properties of the thermochemical equations are:
i. For exothermic reactions, $\mathrm{\Delta }H$is negative whereas for endothermic reactions, $\mathrm{\Delta }H$ is positive.
ii. Unless otherwise mentioned, AH values are for the standard state of the substance (i.e. 298 K and 1 bar pressure)
iii. The coefficients of different substances represent the number of moles reacted and formed for the heat change represented in the equation.
iv. The numerical value of $\mathrm{\Delta }H$ refers to the number of moles of substance specified by an equation. standard enthalpy change $\mathrm{\Delta }H$ will have units as $kJmo{l}^{-1}$
v. The physical state of different substances must be mentioned as the heat evolved or absorbed depends upon the physical state.
vi. If the reaction is reversed, the sign of $\mathrm{\Delta }H$ changes but the magnitude remains the same.
In a decomposition reaction, bonds of a large molecule break to form a small molecule and for this, energy is needed, which makes it endothermic.

ii.) The reactions involving exposure of silver in air are:
$$\begin{array}{rl}& Ag(s)+H_{2}S(g)\to A{g}_{2}S(black)\\ & \end{array}$$
This $A{g}_{2}S$ formation is the reason for black layer.

Note: It is to be noted that when a thermochemical equation is multiplied or divided by a constant term, then the enthalpy of the reaction also gets multiplied or divided accordingly.