5.1 Measuring energy changes

Explain why a reaction is endothermic or exothermic in terms of bond breaking and bond forming.

A reaction is endothermic when the energy absorbed to break bonds is greater than the energy released during bond formation. A reaction is exothermic when the energy released during bond formation is greater than the energy absorbed to break bonds.

What is an isolated system?

An isolated system is one which does not exchange energy or matter with the surroundings.

What is an open system?

An open system is one where both energy and matter are exchanged with the surroundings.

Is bond breaking/forming endothermic or exothermic?

Bond forming is exothermic. Bond breaking is endothermic.

Explain why bond breaking is an endothermic process.

Covalent bonds are formed due to electrostatic of attraction between shared electrons and bonded nuclei. Energy is required to overcome the forces of attraction, hence making it endothermic.

Why is bond enthalpy measured as an average?

Different molecules could contain the same type of bond with slight differences in the energy required to break the bonds, hence the need to have an average.

How can enthalpy changes be calculated?

Enthalpy changes from chemical reactions can be calculated from their effect on the temperature of their surroundings.

Define enthalpy.

Enthalpy is defined as the amount of energy contained within a system, for example bond energy.

All combustion reactions are exothermic/endothermic.

Exothermic

Define the term endothermic reaction and draw a profile diagram to represent the reaction.

In an endothermic reaction, the system gains/absorbs energy from the surroundings and as a result, temperature of the surroundings falls. ∆H = +ve

Define the term exothermic reaction and draw a profile diagram to represent the reaction.

In an exothermic reaction, the system loses/releases energy to the surroundings and as a result, the temperature of the surroundings increases. ∆H = -ve

Define standard enthalpy change of reaction (∆H).

It is defined as the amount of energy required or released when the reactants are transformed into products at standard state conditions.

Describe the limitations of finding the enthalpy change of combustion using a calorimeter.

Not all the heat produced by the combustion reaction is transferred to the water. Some is needed to heat the copper calorimeter and some has passed to surroundings. The combustion of the ethanol is unlikely to be complete owing to the limited oxygen available, as assumed by the literature value. The experiment was not performed under standard conditions.

Define temperature.

Temperature is defined as the average kinetic energy possessed by particles in a medium.

Define the term average bond enthalpy.

The average bond enthalpy is defined as the amount of energy required to break one mole of similar covalent bonds in different molecules.

Define the enthalpy change of formation.

The enthalpy change of formation of a substance is the energy required or evolved during the formation of one mole of a compound from its elements under standard state conditions.

What is the law of conservation of energy?

The law of conservation of energy states that energy can neither be destroyed nor created but can only be converted from one form to another.

Define specific heat capacity.

The specific heat capacity is defined as the amount of heat required to raise the temperature of one kilogram of a substance by one kelvin or one degree celsius. q = mcT

Define enthalpy change of combustion.

The standard enthalpy change of combustion is the amount of heat given out for the complete combustion of one mole of a substance in its standard state in excess oxygen under standard conditions.

Why are average bond enthalpy values not available for H-H bonds?

There can only be one type of H-H bond, thus there is no need for an average.

What is the enthalpy change of formation of elements at standard state?

Zero

What are standard state conditions?

1 atm (10^5 Pa) 25 ˚C

What is a closed system?

A closed system is one where energy is exchanged only with the surroundings.