# Is the kinetic energy change of a system (q) equal to the enthalpy change of a system (∆H)?

When silver nitrate and hydrochloric acid are put together in a container, they form silver chloride. If you're given information about the reactants (including temperature change, heat capacity, and mass of reactant aqueous molecules), then you can find the kinetic energy change of the system.

What I want to know is: does the $$q$$ value you find for the system equal to the $$\Delta H$$ value for the system? Worded differently, does the kinetic energy change of the system equal to the enthalpy change of the system? Is this one of those obvious questions that most people understand instantly but people like me who lack common sense don't?

Under the restrictions of constant $$p$$, and no non-$$pV$$ work, we have:

$$q_p=\Delta H$$

q is the heat flow into the system (postive for flow in, negative for flow out). However, q is not necessarily the change in the system's kinetic energy. For instance, suppose you have an endothermic chemical reaction at constant temperature. Heat will flow into the system. However, some or all of that heat flow could go into increasing the potential energy of the chemical bonds. And if the endothermic chemical reaction is accompanied by an increase in volume, some of that heat flow will go into the work needed to expand the system against the external pressure.

Indeed, if the temperature is constant, and the heat capacity of the reactants and products are the same, then there will be no change in the kinetic energy of the system. I.e., in that case, none of the heat flow will go into changing the kinetic energy of the system.

does the q value you find for the system equal to the ΔH value for the system?

It seems that the reaction is undergoing at constant pressure. In that case, q value is indeed equal to the ΔH value for the system.

Worded differently, does the kinetic energy change of the system equal to the enthalpy change of the system?

q value is not the same as 'kinetic energy change'. q is energy exchanged in the form of heat.

When system intakes heat, its internal energy can change or remain the same.

Internal energy is actually Total Energy = Kinetic Energy + Potential Energy. So, when Internal Energy changes, any or both of KE & PE can change. KE remains the same in an isothermal process.

It is right if you consider that kinetic energy may have different forms: it can be either vibration, or rotation or translation energy.