First of all, U IS in fact internal energy. It represents the sum of the energy of the system (but since there's no such thing as 0 energy, we usually discuss delta U). Second, a simple application of the first law of thermodynamics says, if the system is adiabatic, delta U = 0. Period. It really is no more complicated than that. If no energy is being transferred between the system and surroundings, the system will neither increase nor decrease in energy. In your example, where there is a reaction, the relevant quantity is the enthalpy of reaction (delta H). The energy balance equation is delta U = Q + W + delta H + (integral)Cp*dT This accounts for heat transfer with the surroundings (Q), expansion/contraction work (W), the reaction (delta H) and the temperature of the system (last term). Since we have a fixed container and it's adiabatic, Q and W are both 0. delta U is 0 as well, so in conclusion, the change in energy due to reaction causes a change in temperature. Read more: http://www.physicsforums.com First of all, U IS in fact internal energy. It represents the sum of the energy of the system (but since there's no such thing as 0 energy, we usually discuss delta U). Second, a simple application of the first law of thermodynamics says, if the system is adiabatic, delta U = 0. Period. It really is no more complicated than that. If no energy is being transferred between the system and surroundings, the system will neither increase nor decrease in energy. In your example, where there is a reaction, the relevant quantity is the enthalpy of reaction (delta H). The energy balance equation is delta U = Q + W + delta H + (integral)Cp*dT This accounts for heat transfer with the surroundings (Q), expansion/contraction work (W), the reaction (delta H) and the temperature of the system (last term). Since we have a fixed container and it's adiabatic, Q and W are both 0. delta U is 0 as well, so in conclusion, the change in energy due to reaction causes a change in temperature. Read more: http://www.physicsforums.com

First of all, U IS in fact internal energy. It represents the sum of the energy of the system (but since there's no such thing as 0 energy, we usually discuss delta U). Second, a simple application of the first law of thermodynamics says, if the system is adiabatic, delta U = 0. Period. It really is no more complicated than that. If no energy is being transferred between the system and surroundings, the system will neither increase nor decrease in energy. In your example, where there is a reaction, the relevant quantity is the enthalpy of reaction (delta H). The energy balance equation is delta U = Q + W + delta H + (integral)Cp*dT This accounts for heat transfer with the surroundings (Q), expansion/contraction work (W), the reaction (delta H) and the temperature of the system (last term). Since we have a fixed container and it's adiabatic, Q and W are both 0. delta U is 0 as well, so in conclusion, the change in energy due to reaction causes a change in temperature. Read more: http://www.physicsforums.com