DF 1 - Developing fuels Flashcards
What is thermochemistry?
The study of energy and heat associated with chemical reactions. (Different fuels give out different amounts of energy when one mole is burnt.)
Are fuels energy sources?
They can be thought of as energy sources but they can’t release energy until they have combined with oxygen - so fuel-oxygen systems should be thought of as the energy sources
What is an exothermic reaction?
A reaction that gives out energy and heats its surroundings (increases in temp.)
What is an endothermic reaction?
A reaction that takes in energy and cools surroundings (decreases in temp.)
Draw an enthalpy level diagram for an exothermic reaction
x-axis: progress of reaction
y-axis: enthalpy
Reactants have more energy than the products
The energy is given out to surroundings so the enthalpy change is negative
Draw an enthalpy level diagram for an endothermic reaction
x-axis: progress of reaction
y-axis: enthalpy
Reactants have less energy than the products
Energy is taken in from the surroundings so the enthalpy change is negative
What 3 things need to be regulated in standard conditions?
Temperature, pressure and concentration
What is 0 degrees in kelvin?
273 K
What are the conditions for standard enthalpy change for a reaction?
Pressure: 100 kPa
Temperature: 298 K (25 degrees)
All substances in their standard states
Concentration: 1 mol/dm^3
What is the equation for enthalpy change?
Enthalpy change = mass x specific heat capacity x change in temp.
What is the specific heat capacity of water?
4.18 J per g per K
What is the units for enthalpy change?
kJ per mol
Why is enthalpy change of combustion (at standard conditions) less exothermic?
- Incomplete combustion
- Loss of energy to surroundings (biggest factor)
- Experiment not done at standard conditions
How do improve the accuracy of the reaction?
- Add a wind shield
- Lid
- Insulation (e.g wool around beaker)
What does a bomb calorimeter do?
It burns fuel electrically with oxygen in a pressurised vessel so that complete combustion can happen. Energy is transferred to the surrounding water where temperature rise is measured.
What does a bomb calorimeter do?
It burns fuel with oxygen so that complete combustion can happen
What does a bomb calorimeter do?
It burns fuel with oxygen so that complete combustion can happen
What is it made up of?
It is made up of:
- bomb
- platinum resistance thermometer
- thermostat (to control heat)
- stirrers
- thermostatted outer tank of water
- calorimeter full of water
What is the standard enthalpy change of combustion?
It is the enthalpy change that occurs when one mole of a substance is burnt completely in oxygen under standard conditions in standard states
What is standard enthalpy change of formation?
It is the enthalpy change when one mole of a compound is formed from its elements under standard conditions in standard states
Is the enthalpy change of combustion positive or negative?
Negative - it is an exothermic reaction so it loses energy to the surroundings
What is the standard enthalpy change of neutralisation?
It is the enthalpy change when one mole of H+ ions react with one mole of OH- ions to form one mole of water under standard conditions and in solutions containing 1 mol/dm^3
Is a neutralisation reaction endothermic or exothermic?
Exothermic - energy is given out
