3.3 Energy Change And Calculations Flashcards
What is released when substances burn?
Energy
How can the energy released from a substance be measured?
Calorimetry
What is calorimetry used for?
The energy values on the back of food packets
What is simple calorimetry?
Heating water from the substance being burned with the temperature increase showing energy released
How do you calculate energy transferred from a substance?
Q = M x C x ∆T
Energy transferred = Mass (of water) x Specific heat capacity x Change in temperature
What are the two major sources of error in a practical when calculating heat loss?
Heat lost to surroundings is not accounted for
Incomplete combustion/reaction as lack of oxygen present
What is used in industry to produce accurate food energy values?
Bomb calorimeter
Why is a bomb calorimeter adapted for accurate energy values?
Constant supply of oxygen - no incomplete combustion
Temperature prove takes frequent readings - more accurate
Insulating jacket minimises heat loss
Magnetic stirrer allowing for constant stirring and consistent results
What is exothermic?
Heat energy goes from the chemical reaction into the surroundings
What is endothermic?
The heat energy goes from the surroundings into the chemical reaction
What is the change of energy in exothermic? Why?
Negative
Energy is lost to the surroundings so the chemical reaction is cooler
What is the change of energy is endothermic? Why?
Positive
Energy is taken into the chemical reaction from the surroundings so the chemical reaction is hotter
When bonds are broken is it exo or endo?
Endothermic
When bonds are made is it exo or endo?
Exothermic
Describe the energy proportions in exothermic diagrams?
Energy released from forming bonds is greater than
Energy needed to break the existing bonds
Describe the energy proportions in endothermic diagrams?
Energy needed to break the existing bonds is greater than
Energy released from forming new bonds
What is the activation energy?
Where bonds are broken
What do catalysts do?
Lower the activation energy
So the energy needed to break the bonds is lower
What happens on a energy level diagram when a catalyst is used?
The curve upwards becomes shallower
How do you calculate change in energy for a reaction?
Total energy needed to break all bonds in reactants
Take
Total energy released when new bonds are made in the products
What will the question give you when calculating change in energy?
The bond energies
Eg
A C-H bond = +412kj/mol
What will help you calculate change in energy?
The display formula to count the bonds
DON’T forget the balancing numbers when counting the bonds
What is the issue with fossil fuels?
They are non-renewable and cause pollution
What can be used as an alternative fuel?
Hydrogen
Why is hydrogen used as an alternative fuel source?
Burns easily
Releases large amounts of energy
Produces only H2O
What are the disadvantages of using hydrogen?
Difficult to acquire and store
