Energy And Chemical Reactions Flashcards
Definition of exothermic reaction
Give out energy to their surroundings So bonds can be made This means the temperature increases Delta h is negative E.g. Combustion reactions
Definition of endothermic reaction
Take in energy
So bonds can be broken
Decrease in temperature
Delta h is positive
Enthalpy profile for endothermic reaction
Energy
/--,--\ products
| ---.--
React|
Ants.|
| .= Delta h.
------, . Reaction progress. ,=activation energy
Enthalpy profile for exothermic reaction
Energy
Re /--.--\
---- .,
|
| .=activatione
| products. ,=delta h
, ---------
Reaction timeDelta h equation (change in energy )
E=mc/\t
Energy =massxspecific heat capacityxchange in temperature
Bond delta h equation
Bb-bf
Bonds broken -bonds formed
What is a fuel cell
It’s something that coverts chemical energy to electrical energy
It converts the energy released by the combustion of a fuel directly in electrical energy
Advantages of fuel cells
Doesn’t need to be recharged
Doesn’t create any pollutants
H2o only product h2 is renewable no co2
Disadvantages of fuel cells
Hydrogen is highly flammable
Hydrogen is difficult to store
What is oxidised and what is reduced in a fuel cell reaction
Hydrogen is oxidised and oxygen is reduced
Equation of fuel cell
2h2+o2—>2h2o
What is used for the fuel cells electrolyte
Acids or alkalis
Two main uses of fuel cells
Cars and spacecrafts
Advantages of a fuel cell car
Not rechargeable Larger range(good for long distance driving) Both electrical and fuel cell cars have low emissions of green house gases
Disadvantages of fuel cell car
High cost of platinum catalysts for fuel cells
Hydrogen is highly flammable
Hydrogen is difficult to store
Electrical car disadvantages
It’s rechargeable
Meaning it’s better for shorter distance driving (smaller range)
Batteries are heavy and expensive
Half equation of hydrogen
H2—>2h2++2e-
Hydrogen is oxidised because it’s at the negative electrode where it loses electrons
Half equation for oxygen
O2+4h++4e- —>2h2o
Oxygen is reduced because it’s at the positive electrode where it gains electrons
Fuel cells info
Fed with oxygen and hydrog to produce water
Hydrogen goes to neg electrode where it’s oxidised
Oxygen goes to pos electrode where it’s reduced
The oxygen reacts to hydroxide ions and hydrogen reacts with hydroxide ions to form water
Example of random error
Not measuring water properly
Systematic error
Forgetting to keep the lid on
Half equation for first stage of rusting
O2+2h2o+4e->4OH-
