8: Energetics I Flashcards
What are the standard conditions
- Pressure 100 kPa
- Temperature 298 K
- Standard states (solid, liquid or gas)
Define the standard Enthalpy change of reaction and its sign
The Enthalpy change when the reactants in the stoichiometric equation react to give the products under standard conditions (exo/endo)
Define the standard Enthalpy change of formation and its sign
The Enthalpy change when one mole of compound is formed from its elements under standard conditions (exo/endo)
Define the standard Enthalpy change of combustion and its sign
The Enthalpy change when one mole of substance is burnt in excess oxygen under standard conditions (exo)
Define the standard Enthalpy change of neutralisation and its sign
The Enthalpy change when one mole of water is formed by reacting an acid and alkali under standard conditions (exo)
What is the standard Enthalpy of formation of an element in its standard state
0
What are factors of exothermic reactions
- Heat energy is given off by the reactions to the surroundings
- Products have less energy than reactants
- Energy of system decreases
- Energy and temperature of surroundings increases
- Negative delta H
- Thermodynamically feasible
- Kinetically hindered
What is the exothermic Enthalpy level diagram
What are the factors of endothermic reactions
- Heat energy is absorbed by the reaction from the surroundings
- Products have more energy than the reactants
- Temperature and energy of the surroundings decreases
- Energy of the system increases
- Positive delta H
What is the endothermic Enthalpy level diagram
Describe the experiment measuring Enthalpy changes
- Add the reaction mixture to a polystyrene cup
- Place a plastic lid over the top
- Check the initial temperature
- Check the final temperature
What is calorimetry
The measurement of Enthalpy changes in chemical reactions, done in calorimeters (e.g. polystyrene cup)
What is the equation for heat transferred
What is the equation for Enthalpy change
What is Hess’s Law
The total Enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same
What law of conservation of energy inspired Hess’s law
Energy cannot be created or destroyed, it can only change form (in a closed system the total amount of energy present is always constant)
Why is Hess’s Law used
To calculate Enthalpy changes that can’t be found experimentally (e.g. using a calorimeter)
How to calculate Hess’s Law
- Arrows drawn away/towards products or reactants based on the definition of the Enthalpy change it is calculating
- To calculate delta H, follow the arrows (e.g. -delta H1+delta H2)
Define bond (dissociation) Enthalpy
The energy required to break a bond
Is breaking bonds endothermic or exothermic
Endothermic
Is making bonds endothermic or exothermic
Exothermic
What happens to bond Enthalpy when the reaction is exothermic
- It means that more bonds are being made than broken
- Products are more stable than reactants
What is the endothermic bond Enthalpy profile
- More bonds are being broken than made
- Products are less stable than reactants
What is average bond Enthalpy
The energy needed to break one mole of bonds in a gaseous molecule averaged over similar compounds
What is the equation for bond Enthalpy
- Remember definitions and moles
