How can the enthalpy change be determined for a reaction

Related questions How are enthalpy changes expressed in chemical equations? How does enthalpy change with pressure? How do you calculate standard molar enthalpy of formation? Why is enthalpy a state function?

Why is the enthalpy of formation of oxygen zero? Why can enthalpy not be measured directly? How does pressure affect enthalpy? How does enthalpy relate to internal energy? How does entropy effect enthalpy? Therefore, they are used as standard states or reference points for calculations of different thermodynamic properties of these elements. Tin : White tin on the left is the most stable allotrope of tin, and is used as its standard state for thermodynamic calculations.

The standard state should not be confused with standard temperature and pressure STP for gases, or with the standard solutions used in analytical chemistry. Graphite : Graphite is the most stable state of carbon and is used in thermochemistry to define the heat of formation of carbon compounds. The standard enthalpy of formation, or standard heat of formation, of a compound is the change in enthalpy that accompanies the formation of one mole of the compound from its elements in their standard states.

For example, the standard enthalpy of formation for carbon dioxide would be the change in enthalpy for the following reaction:. The standard enthalpy of reaction is the enthalpy change that occurs in a system when a chemical reaction transforms one mole of matter under standard conditions. The change in enthalpy does not depend upon the particular pathway of a reaction, but only upon the overall energy level of the products and reactants; enthalpy is a state function, and as such, it is additive.

In order to calculate the standard enthalpy of a reaction, we can sum up the standard enthalpies of formation of the reactants and subtract this from the sum of the standard enthalpies of formation of the products. Stated mathematically, this gives us:. Calculate the standard enthalpy of reaction for the combustion of methane:. In order to calculate the standard enthalpy of reaction, we need to look up the standard enthalpies of formation for each of the reactants and products involved in the reaction.

These are typically found in an appendix or in various tables online. For this reaction, the data we need is:. Next, we sum up our standard enthalpies of formation. This law states that if a reaction takes place in several steps, then the standard reaction enthalpy for the overall reaction is equal to the sum of the standard enthalpies of the intermediate reaction steps, assuming each step takes place at the same temperature.

Since enthalpy is a state function, the change in enthalpy between products and reactants in a chemical system is independent of the pathway taken from the initial to the final state of the system. Negative enthalpy change for a reaction indicates exothermic process, while positive enthalpy change corresponds to endothermic process.

However, we can see that the net reaction is a result of A being converted into B, which is then converted into C, which is finally converted into D.

Turning graphite into diamond requires extremely high temperatures and pressures, and therefore is impractical in a laboratory setting. The change in enthalpy for this reaction cannot be determined experimentally. The most basic way to calculate enthalpy change uses the enthalpy of the products and the reactants. If you know these quantities, use the following formula to work out the overall change:. The addition of a sodium ion to a chloride ion to form sodium chloride is an example of a reaction you can calculate this way.

Inserting these values gives:. When a substance changes from solid to liquid, liquid to gas or solid to gas, there are specific enthalpies involved in these changes. The enthalpy or latent heat of melting describes the transition from solid to liquid the reverse is minus this value and called the enthalpy of fusion , the enthalpy of vaporization describes the transition from liquid to gas and the opposite is condensation and the enthalpy of sublimation describes the transition from solid to gas the reverse is again called the enthalpy of condensation.

Imagine that you heat ice from Kelvin until it melts, and then heat the water to K. The enthalpy change for the heating parts is just the heat required, so you can find it using:. The specific heat of ice is So the calculation takes place in a few parts. First, the ice has to be heated from K to K i. For 5 moles of ice, this is:. Calculations for vaporization are the same, except with the vaporization enthalpy in place of the melting one.