Name: Hess's Law of Heat Summation
Uploaded: 2019-07-19
Duration: 4 min 58 s
Description: What about the enthalpy changes in a chemical reaction that can take place in more than one way? Watch this video to understand this important concept of the...

Question

Take it in turns to explain these terms to another learner: bond energy; enthalpy change; activation energy; standard conditions.

Accepted Answer

Bond energy:

Bond Energy, also known as average bond enthalpy or simply bond enthalpy, is a quantity that offers insight into the strength of a chemical bond. It is important to note that the bond energy of a chemical bond in a compound is the average value of all the individual bond dissociation enthalpies of the chemical bonds. For example, the bond energy of the carbon-hydrogen bond in a methane $({CH}_{4})$ molecule is equal to the average of the bond dissociation energies of each individual carbon-hydrogen bond. It can be calculated as follows.

${CH}_{4} + {BDE}_{1} \to {CH}_{3} + H . {CH}_{3} + {BDE}_{2} \to {CH}_{2} + H . {CH}_{2} + {BDE}_{3} \to CH + H . CH + {BDE}_{4} \to C + H . BE (C - H) = \frac{({BDE}_{1} + {BDE}_{2} + {BDE}_{3} + {BDE}_{4})}{4} .$

Enthalpy change:

Enthalpy is the measurement of energy in a thermodynamic system. The quantity of enthalpy equals to the total content of heat of a system, equivalent to the system’s internal energy plus the product of volume and pressure. A reaction to be driven forward is by having lower energy in the products than the reactants. This means that the change in enthalpy is negative. A reaction can still occur if the enthalpy change is positive, there just needs to be other things driving that reaction forward. Enthalpy change is the difference in enthalpy (energy) at the beginning of the reaction to the end of the reaction.

Activation energy:

Activation energy is defined as the minimum amount of extra energy required by a reacting molecule to get converted into product. It can also be described as the minimum amount of energy needed to activate or energize molecules or atoms so that they can undergo a chemical reaction or transformation. Activation energy is denoted by $E_{a}$ . It is usually measured in joules $(J)$ and or kilojoules per mole $(kJ {mol}^{- 1})$ or kilo calories per mole ( $k cal {mol}^{- 1}$ ).

Standard conditions:

Standard state conditions are defined by Standard Temperature & Pressure $(STP)$ with a temperature of $0 ° C or 273.15 K$ , Kelvin and a pressure of one atmosphere ( $1 atm$ ). At Standard Temperature & Pressure one mole of ideal gas occupies $22.4 L$ .

Take it in turns to explain these terms to another learner: bond energy; enthalpy change; activation energy; standard conditions.

Important Questions on Enthalpy Changes

Learn and Prepare for any exam you want !