Energetics Flashcards
Define enthalpy change.
Enthalpy change is the amount of heat energy taken in or given out during any change in a system provided the pressure is constant.
Examples of exothermic reaction.
- Combustion of fuels
- Oxidation of carbohydrates
- The reaction of water with quicklime (calcium oxide)
Example of endothermic reaction.
- Decomposition of calcium carbonate (limestone)
- Photosynthesis
- Dissolving certain ammonium salts
Define the standard enthalpy change of reaction, ∆Hr
Standard enthalpy change of reaction is the enthalpy change when the amounts of reactants specified in the stoichiometric equation react completely to give products under standard conditions of 298k and 101kPa.
Define standard enthalpy change of formation, ∆Hf
It is the enthalpy change when one mole of a compound is formed from its elements under standard conditions of 298k and 101kPa.
What is the standard enthalpy change of formation of any element?
Zero.
Define the standard enthalpy change of combustion, ∆Hc
It is the enthalpy change when one mole of a substance is burnt in excess oxygen under the standard conditions of 298k and 101kPa.
Define enthalpy change of neutralization, ∆Hneut
The enthalpy change when one mole of water is formed by the reaction of an acid with an alkali under the standard conditions of 298K and 101KPa.
Energy transfer as heat is given by the equation:
q = mc∆T
q is heat transferred in Joules
m is mass of water in grams
c is specific heat capacity of in J/g/°C
∆T is the temperature change
What is specific heat capacity?
The energy needed to raise the temperature of 1g of a substance by 1°C
When calculating energy transferred as heat, what assumptions do we make?
- 1cm³ of water has a mass of 1g
- The solution has the same specific heat capacity of water
What is meant by Enthalpy?
The chemical energy stored in a system that can be converted into heat is known as enthalpy. It is denoted by “H”
