In the given reaction, where A is converting into 2B without any enthalpy change, it implies that the reaction is exothermic. An exothermic reaction releases energy in the form of heat. Therefore, in a closed container, the temperature of the system will not stay the same due to the conservation of energy.
When the reaction starts, the exothermic nature of the reaction will cause the temperature of the system to increase. This is because the released energy from the reaction will be transferred to the surrounding environment, leading to a temperature rise.
The temperature increase will continue until the rate of heat transfer from the system to the surroundings is balanced by the rate of heat generation from the exothermic reaction. Eventually, the system will reach a new equilibrium temperature, which may be higher than the initial temperature of 25°C.
Therefore, in an exothermic reaction, the temperature of the system will change to accommodate the energy released during the reaction, rather than remaining constant.