# How Do Delta U and Delta H Compare in a Constant-Volume Calorimetry?

• zmike
In summary, the conversation discusses the calculation of delta U and delta H for the complete burning of 1 mole of gas in a constant-volume bomb calorimeter at 298 K, with 5150 kJ of heat being evolved. The equations H=U+W and dU=dH are mentioned, and it is determined that W=0 and dU=dH. The final conclusion is that q=-5150 kJ and therefore, the calculation is complete.
zmike

## Homework Statement

1 mole of a gas is burned completely burned in a constant-volume bomb calorimeter at 298 K, 5150 kJ of heat is evolved. Calculate the values delta U and delta H

H = U + W

## The Attempt at a Solution

I was lead to believe that you had to calculate number of moles difference delta n by my professor but that lead to no where.

Since constant volume, W = 0?? or does it?

if so then U would be = to qp then shouldn't q=5150 and U = H?

help??

I think...
yes ... you are right that the W=0 and dU=dH but "q=-5150 kJ" since heat is evolved..

so,

dU = q-W
so dU = q

The legend said:

If W=0, no work is done

Borek said:
If W=0, no work is done

First, it is important to clarify that the term "confusing thermodynamic q" is not appropriate. Thermodynamics is a complex and fundamental field of science, and it is important to approach it with patience and a willingness to learn. With that in mind, let's break down the problem at hand.

The problem states that 1 mole of gas is burned completely in a constant-volume bomb calorimeter at 298 K, resulting in the evolution of 5150 kJ of heat. This means that the reaction is exothermic, and the heat released is equal to the change in enthalpy (delta H) of the reaction. The value of delta H can be calculated by dividing the heat released by the number of moles of gas (1 mole).

However, the problem also asks for the values of delta U and delta H. These values are related by the equation H = U + W, where H represents enthalpy, U represents internal energy, and W represents work. In this case, since the reaction takes place in a constant-volume bomb calorimeter, there is no work being done (W = 0), so the equation simplifies to H = U. This means that the value of delta H is also equal to the change in internal energy (delta U) of the reaction.

To summarize, the values of delta U and delta H for this reaction are both equal to 5150 kJ. It is important to understand the concepts of enthalpy and internal energy, as well as the relationship between them, in order to solve thermodynamic problems effectively. I suggest reviewing these concepts and practicing with similar problems to strengthen your understanding.

## 1. What is thermodynamic q and how does it relate to thermodynamics?

Thermodynamic q is a variable that represents the transfer of heat into or out of a system. It is a crucial component of the first law of thermodynamics, which states that the change in internal energy of a system is equal to the heat added to the system minus the work done by the system.

## 2. How is thermodynamic q different from heat?

Thermodynamic q and heat are often used interchangeably, but they have subtle differences. While heat refers to the transfer of thermal energy between two bodies, thermodynamic q specifically refers to the heat added to or removed from a system during a thermodynamic process.

## 3. What are the units of thermodynamic q?

The units of thermodynamic q depend on the unit system being used. In the International System of Units (SI), the unit of q is joules (J). In the British thermal unit (BTU) system, the unit of q is BTU. In both cases, q represents energy.

## 4. How is thermodynamic q calculated?

Thermodynamic q is calculated using the equation q = mcΔT, where m is the mass of the substance, c is the specific heat capacity of the substance, and ΔT is the change in temperature. This equation is derived from the first law of thermodynamics and is used to determine the amount of heat transferred during a thermodynamic process.

## 5. Can thermodynamic q be negative?

Yes, thermodynamic q can be negative. A negative q value indicates that heat is being removed from the system, while a positive q value indicates that heat is being added to the system. The sign of q depends on the direction of heat flow and does not affect its calculation.

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