Energetic Flashcards
(21 cards)
What is enthalpy change
Heat or energy change under constant pressure
What is the standard enthalpy conditions?
Tem = 298K Pressure = 100 kPa
What is Exothermic? What are there units in terms of delta?
Heat given out = ΔH (Negative)
What is Endothermic? What are there units in terms of delta?
Heat taken in = ΔH (Positive)
Give examples of Exothermic reactions?
Reaction to form water: 2H(2) + O(2) —> 2H(2)O
Combustion of fuels
Give examples of Endothermic reactions?
Thermal decomposition: CaCO(3) —> CaO + CO(2)
Not a Flashcard
Working out enthalpy change
Knowing Hess Cycle and knowing how to use Hess law to work out enthalpy change
When is a reaction considered endothermic?
When a reaction which involves making bonds releases more energy than energy released when breaking bonds
When is a reaction considered exothermic?
When a reaction which involves breaking bonds releases more energy than energy released when forming bonds
What is calorimetry used for?
It can be used to figure out how much heat is given out by a reaction by measuring temperature change
It can be used to measure what is not as enthalpy change
Why is hard to obtain accurate results in calorimetry?
This is because during the experiment, you always lose heat ti the surrounding
If liquid is often too volatile, some fuel may be lost during the experiment by evaporation
Sometimes when you burn fuel some may go through incomplete combustion (meaning less energy given out)
{May result in accurate results}
How can you improve accuracy in calorimeter?
By carrying out the experiment in insulated conditions
How can you work out Enthalpy of Combustion of a flammable liquid?
1) You burn the the flammable liquid in an apparatus known as calorimeter
2) As the fuel burns, it heats the water. You can then work out the heat energy: that has been adsorbed by the water if you know the mass of the water…… the temperature change ΔT and the specific heat capacity of the water.
{q (heat change) = m (mass in g) x C (specific heat capacity x ΔT (temp. change in °C)
Why do we use the reading from water when working out Enthalpy of combustion in a calorimetry?
This is because all the energy is adsorbed by the water - which allows us to work out exactly
What does hess law state?
The ΔH for a reaction is independent of the route taken (so enthalpy change is the same regardless of the route)
What is enthalpy of formation?
ΔfH⦵ when enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
What is Standard enthalpy of combustion?
ΔcH⦵ is the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions (298k and 100kPa)
Why don’t we find the standard enthalpy formation for elements?
Because these elements are in there standard elemental state
Why is enthalpy change of formation considered as useful?
Allows us to calculate enthalpy changes that you can’t find directly
What is enthalpy? What is the symbol?
Energy stored in a compound (symbol H)
What is energy store the sum of?
Is the sum of the potential energy stored by electrostatic forces within and between the particles.
And the sum of kinetic energy stored by the movement of particles from place to place (transitional place) and when bonds in them vibrate and rotate
