# what is the significance of delta U and delta H? I want a brief answer

Δ H is defined as change in enthalpy of system during any reaction. It is equal to heat change at constant pressure.

Δ U is defined as internal energy of system . It is equal to heat change at constant volume
All the system have some basic value of internal energy until there is no heat transferred from the system or to the system. But after the transfer of the heat , the internal energy of the system can either increase or decrease which can be given by Δ H. Generally it is calculated by the formula
Δ H = Δ U + P Δ V

Heat of reaction at constant volume is equal to the internal energy of the Substance. When Volume of the reaction is kept constant then energy given to the reaction is absorbed by the reactant and it increases the internal energy of the reactant.
On the other hand, heat of reaction at constant pressure is actually the overall energy change in the system( Enthalpy). When energy is absorbed by the reaction at constant pressure, then some amount of energy is also used to increase the volume of the system and some of the energy is absorbed in form of internal energy of the system , so it is overall energy change of the system.
Significance is that by knowing heat change of the system and all other parameters we can calculate internal energy of the system.

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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
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