1. Energetics Flashcards
How can a reaction be endothermic? How can a reaction be exothermic?
(a) If more energy is put into breaking bonds than given out when new bonds are formed
(b) If more energy is given out when new bonds are formed than used to break bonds
Name some examples of exothermic reactions.
- Burning fuels
- Neutralisation
Name some examples of endothermic reactions.
- Thermal decomposition
- Heating blue copper sulfate (CuSO4.5H20): The bonds attaching water molecules to the copper sulfate must be broken to produce white anhydrous copper sulfate
If a reaction is exothermic in one direction, what will it be in the other direction?
Endothermic
What is heat energy measured in?
KJ/mol
What is a fuel’s energy density?
Energ y given out per gram. Iit is ver important in considering its efficiency
Why is hyrogen difficult to use as a fuel?
Although it has a very high energy density when burnt, it is difficult to store and take up a lot of space because it is a gas
What is elthalpy change at standard state and how is it shown?
ΔH°298- standard conditions are 298K and 100kPa of pressure
Temperature and pressure can both affect elthalpy change so it is important to state the conditions under which an experiment takes place
How can pressure affect elthalpy change?
Some energy is needed to overcome the atmospheric pressure
Why should state symbols always be used when considering enthalpy changes?
It requires energy to turn a liquid into a gas
What is the standard enthalpy of formation (ΔH°f)?
The enthalpy change when one mole of a sustance is formed from its elements under standard conditions and will all products and reactants in their standard states
What is the standard enthalpy of combustion (ΔH°c)?
The enthalpy chnge when oe le of a substance is burned completely in oxygen under standard conditions and with all products and reactants in their standard states
What is temperature?
Tthe average kinetic energy of particles in a system. This is measured with a thermometer
What is heat?
The total energy of all particles. This depends on how many particles there are, unlike temperature. Heat always flows from high –> low temperature
What is the equation used to measure heat energy change?
Q=mcΔt
m = g c = J/g/K t = K
What is the equation used to measure enthalpy change?
ΔH = Q/mols
How could we improve a simple calorimetry experiment?
- Add a draught screen
- Add a lid
- Add an insulator
Look over different types of calorimeters in your textbook.
Done!
Why can polystyrene cups often be used in calorimetry experiments?
They have a low SHC and are good insulators
Read notes and see diagram on ‘allowing for heat loss’.
Done!
What does Hess’ Law state?
The elthalpy change for a chemical reaction is the same regardles of the route taken from reactants to products. We can show this in a thermochemical cycle.
Do practice with Hess’ Law!
What is the equation to find out the enthalpy of formation?
ΔHf = ΔHp- ΔHr
What is the equation to find out the elthalpy of combustion?
ΔHc = ΔHr - ΔHp
