# Understanding the heat of reaction

• Chemistry
• barryj
In summary, the conversation discusses the calculation of the heat of reaction for the equation CH4 + 2O2 -> CO2 + 2H2O. The heat of formation for CO2 and H2O is given, but data related to CH4 is needed to complete the calculation. The formula for computing the heat of reaction is also mentioned. It is clarified that the heat of formation is the enthalpy change when creating a mole of a substance from its elements. The importance of using elements in their standard state is also emphasized.

#### barryj

Homework Statement
given:
C + 2H2 -> CH4 What is the heat of reaction
Relevant Equations
C + O2 -> CO2 delta H = -393.5 kJ/mole
H2 + 1/2 O2 -> H2O delta H = -285.8 kJ/mole

CH4 + 2Os -> CO2 + 2H2O delta H = ?????

the book says delta H = -890.8 kL/mole
Since the heat of formtion of H2O is -285.8 and the heat of formation of CO2 is -393.5 I thought that the heat of reaction CH4 + 2O2
should be -393.5 + 2(-285.80 = -965.1

Where am I wrong?

This is way too messy to follow, can you please relist given equations and the target one?

Lets put it this way...
What is the heat of reaction for this equaiton.

CH4 + 2O2 -> CO2 + 2H2O delta H = ?

The heat of formation of CO2 is -393.5
The heat of formation of H2O is -285.8

Not enough information, you need data related to CH4 to answer.

The heat of formation is the enthalpy change when you create a mole of substance from it's elements (atoms), or vice versa.
So first you need to compute the enthalpy change from CH4 and O2 to C,H and O. Then you need the enthalpy change from C,H,O to CO2 and H2O.

ΔHrxn = ΣΔHf(products) - ΣΔHf(reactants)
So ΔHrxn = ΔHf(CO2) + 2ΔHf(H2O) - ΔHf(CH4)
(ΔHf(O2) is of course zero.)
(Oh, and by the way, heat of formation is from elements in their standard states - not from atoms when this is not the standard state. So from C(s), H2 and O2, not C(g), H and O.)

jim mcnamara
I now understand, Thanks all.

## 1. What is the heat of reaction?

The heat of reaction, also known as the enthalpy of reaction, is the amount of heat energy released or absorbed during a chemical reaction. It is a measure of the change in energy between the reactants and products.

## 2. How is the heat of reaction measured?

The heat of reaction is typically measured using a calorimeter, which is a device that can accurately measure heat changes. The reactants are placed in the calorimeter and the heat released or absorbed is measured using a thermometer or other heat-sensing device.

## 3. What factors affect the heat of reaction?

The heat of reaction is affected by several factors, including the nature of the reactants and products, the concentration of the reactants, the temperature and pressure of the reaction, and the presence of a catalyst. These factors can either increase or decrease the heat of reaction.

## 4. How does the heat of reaction relate to the overall energy of a system?

The heat of reaction is a measure of the change in energy of a system. A negative heat of reaction indicates that the reaction releases energy, while a positive heat of reaction indicates that the reaction absorbs energy. The overall energy of a system can be calculated by adding the heat of reaction to the initial energy of the system.

## 5. Why is understanding the heat of reaction important?

Understanding the heat of reaction is important for several reasons. It can help predict the direction of a reaction, determine the amount of energy needed for a reaction to occur, and optimize reaction conditions for maximum efficiency. It is also essential in various industries, such as pharmaceuticals and energy production, for developing and improving chemical processes.