GEN CHEM | Enthalpy & Thermochemial Equations

Enthalpy

• represented as H

• energy change

• quantifies as the heat flow into or out of the system in a process that occurs at constant pressure

• tells us about the “heat content of a system”

• state function (path independent)

• depends only on reactants and products

Enthalpy of Reactions

• represented as ∆H_rxn

• heat transferred in a chemical reaction held at constant pressure

• the difference between the enthalpies of the products and enthalpies of the reactants

• ∆H = H(products) - H(reactants)

Enthalpy of Endothermic Processes

• H(products) > H (reactants)

• ∆H is positive

Enthalpy of Exothermic Processes

• H(products) < H (reactants)

• ∆H is negative

Thermochemical Equations

equations that show the mass relationships between the substances involved in a reaction, alongside its corresponding enthalpy change

Rules in Interpreting Thermochemical Equations

1. Specify the physical states of all reactants and products as they can influence the resulting enthalpy

2. Reversing a reaction entails that the sign of ∆H reverses as well

3. If you multiply both sides of the equation by a factor of n, then multiply ∆H by the same factor of n.

4. The stochiometric coefficients, which are the numbers beside each reactant and product, refer to the number of moles of a substance.

Standard Enthalpy (∆Hº_rxn or ∆Hº_f)

• enthalpy of a reaction carried out at 1 atm, as substances are said to be at the standard state at this point

• ∆Hº_f of any element in its standard state is therefore 0

Formulas for Standard Enthalpy of Reaction (Summation)

∆Hº_rxn = ∑(n∆Hº_f products) - ∑(n∆Hº_f reactants)

or

In formula [aA + bB → cC + dD]

where a, b, c, d = stoichiometric coefficients and A, B, C, D = reactants/products

∆Hº_rxn = [c∆Hº_f (C) + d∆Hº_f (D)] - [a∆Hº_f (A) + b∆Hº_f (B)]

Hess’ Law

• The overall enthalpy change in converting reactants to products is the same, regardless if the reaction took place in one step or in a series of steps

• The sum of the ∆Hº_rxn of the sub-processes is the sum of the overall reaction

Calculating ∆Hº_rxn with Hess’ Law

1. Balance the chemical reaction

2. Manipulate the sub-step processes to ensure that the products and reactants are the same with the 1-step process. Reverse the sub-step processes if necessary,

3. Maipulate the coefficients to ensure it is the same with the 1-step process.

4. Cancel the substances found in both reactants and products.

5. Cancel the substances in both reactants and products.

6. Add the ∆Hº_rxn of the sub-step processes to get ∆Hº_rxn of the 1-step process.