Topic 4- Energetics Flashcards
What is enthalpy
A chemical reaction where there is normally a change in energy.
What is enthalpy change
The heat change in a reaction at a constant pressure.
What are the standard conditions?
100kPa - Pressure
298K or 15 degrees- Temperature
What does endothermic mean?
Heat energy in
What does exothermic mean?
Heat energy lost
How does bond breaking work?
To break a bond energy must be absorbed.
Therefore endothermic and must always be positive.
How does bond making work?
Energy is released
Therefore exothermic must always be negative
Equation for enthalpy change!
Total energy to break bonds- total energy to make bonds
Calorimetry- fuel. Working out the enthalpy change of combustion!!
. Weigh the fuel, before and after burning, work out the mass of fuel burnt.
- Energy of fuel is transferred to water, some lost to the surroundings.
Make sure to place a lid on top- prevents heat loss.
Also add a wind shield, this prevents draught/ moving flame/ drying out.
Calorimetry- solution
- Polystrene cup to prevent heat loss.
- Add acid first- measure temperature.
- Add an alkali/ solid, stir, measure temperature change.
What is Hess’s Law
The total enthalpy change of a reaction is independent of the route taken.
What is the enthalpy of formation?
Enthalpy change when 1 mole of a substance is produced from its elements under standard conditions.
What is the enthalpy of combustion?
Enthalpy change when 1 mole of a substance is burnt completely in oxygen under standard conditions.
