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Chemistry - Topic 5 - Energy Changes (Fuel Cells (Hydrogen-Oxygen Fuel…
Chemistry - Topic 5 - Energy Changes
Bond energy calculations
Energy must always be supplied to break bonds
Energy must always be
supplied
to break existing bonds - so bond breaking is an
endothermic
process.
Energy is
released
when new bonds are formed - so bond formation is an
exothermic
process.
During a chemical reaction,
old bonds are broken
and
new bonds are formed.
In
exothermic
reactions the energy
released
by forming bonds is
greater
than the energy used to
break
them. I
endothermic
reactions the energy
used
to break bonds is
greater
than the energy released by forming them.
Bond energy calculations - need to be practised
You can use these
known bond energies
to calculate the
overall energy change
for a reaction.
The overall energy change is the
sum
of the energies
needed
to break bonds in the reactants
minus
the energy
released
when the new bonds are formed in the products. You nee to
practise
a few of these, but the basic idea is really very simple.
Every
chemical bond has a particular
bond energy
associated with it. This
bond energy
varies slightly depending on the
compound
the bond occurs in.
Example
Find the
energy required
to break the original bonds
:
(1 x H-H) + (1 x Cl-Cl) = 436 kJ/mol + 242kJ/mol = 678 kJ/mol
Find the
energy released
by forming the new bonds
:
2 x H-Cl = 2 x 431 kJ/mol = 862 lJ/mol
The bond energies you need are:
H-H: +436 kJ/mol;
Cl-Cl: +242kJ/mol.
Find the
overall energy change
f or the reaction using this equation
:
Overall energy change = energy required to break bonds - energy released by forming bonds
678 kJ/mol - 862 kJ/mol = -184 kJ/mol
Using the bond energies given below, calculate the energy change for the reaction between H2 and Cl2 forming HCl.
H-H + Cl-CL ---> H-Cl H-Cl
More Exothermic and Endothermic Reactions
Reaction Profiles Show Energy Changes
The activation energy is the minimum amount of energy the reactants need to collide with each other and react. The greater the activation energy, the more energy needed to start the reaction — this has to be supplied.
This shows an endothermic reaction because the
products are at a higher energy than the reactants.
The initial rise in energy represents the energy needed to
start the reaction. This is the activation energy.
The difference in height represents the overall
energy change during the reaction per mole.
This shows an exothermic reaction — the products are at a
lower energy than the reactants. The difference in height represents the overall energy change in the reaction per mole.
Practical
Fuel Cells
Hydrogen-Oxygen Fuel Cells Involve a Redox Reaction
At the anode, hydrogen loses an electron. This is oxidation
H+ ions move to the cathode
Hydrogen to the anode, oxygen to the cathode
Oxygen gains an electron and reacts with the H+ to make water. This is reduction
The electrolyte is often potassium hydroxide
The electrons flow through an external circuit creating an electrical current.
The overall reaction gives 2H2 + 02 ----> 2H20
Hydrogen-Oxygen Fuel Cells Could be Used in Vehicles
It does not create any pollutants only water and heat.
However hydrogen is a gas,takes up space to store a rechargeable battery
The type of fuel cell mentioned above could solve the problem for cars when we run out of fuels
Hydrogen is explosive when mixed with air, hard to store safley
Often made with hydrocarbons which kind of defeats the point
Fuel Cells Use Fuel and Oxygen to Produce Electrical Energy
When fuel enters it oxidises and sets up a potential difference
Different types. An important one is 'hydrogen-oxygen fuel cell'
Fuel cells produce electrical energy by using energy supplied by the reaction of its supplements: fuel and oxygen
The fuel cell combines oxygen and hydrogen to produce water
Cells and Batteries
The Voltage of a Cell Depends on Many Factors
The bigger the difference in reactivity of the electrodes, the bigger the voltage of the cell.
You can predict what the voltage of a cell might be from information about the voltages of other cells.
Different metals react differently with the same electrolyte. This causes the charge difference (voltage) of the cell.
The electrolyte used in a cell will also affect the size of the voltage since different ions in solution will react differently with the metal electrodes used.
A battery is formed by connecting two or more cells together in series. The voltages of the cells in the battery are combined so there is a bigger voltage overall.
In Non-Rechargeable Batteries the Reactants Get Used Up
Once any one of the reactants is used up, the reaction can’t happen and so no electricity is produced.
The products can’t be turned back into the reactants, so the cell can’t be recharged.
Over time the ions in the electrolyte and the metal ions on the electrode get used up and turned into the products of the reaction.
In a rechargeable cell, the reaction can be reversed by connecting it to an external electric current.
The chemical reactions in some cells are irreversible.
Chemical Reactions in a Cell Produce Electricity
The electrolyte is a liquid that contains ions which react with the electrodes.
The chemical reactions between the electrodes and the electrolyte set up a charge difference between the electrodes.
An electrochemical cell is a basic system made up of two different electrodes in contact with an electrolyte
If the electrodes are then connected by a wire, the charge is able to flow and electricity is produced. A voltmeter can also be connected to the circuit to measure the voltage of the cell.
Exothermic and Endothermic Reactions
In an Exothermic Reaction, Heat is Given Out
As are neutralisation reactions
As are many oxidation reactions e.g sodium in water
Combustion is an example of this, as is burning fuels
Some hand warmers oxidise the iron in the air. This is exothermic
Self heating drinks also rely on exothermic reactions
Energy is transferred to the surroundings. Rise in temp
In an Endothermic Reaction, Heat is Taken In
These are much less common but include: The reaction of citric acid and sodium hydrogencarebonate and also thermal decomposition
Endothermic takes energy in from the surroundings
Energy is Moved Around in Chemical Reactions
If a product stors smore than the reactant, it must of drawn the difference in from the surroundings
Vice Versa
Chemical store a specific amount of energy. Different one stores different amounts
The overall amount of energy doesn't change.,all energy is conserved.
Bond Energies
Energy must always be supplied to break bonds.
Energy must be supplied to break the existing ones. Bondbreaking is endothermic
Energy is released when new bonds are formed, so formation is exothermic
During a chemical reaction, new bonds are formed
Endo-break is greather than forming. Exo-greater forming then breaking
Bond Energy Calculations
This varies depending on the compound
You can use known bond energies to calculate the overall change. The sum of energies needed tobrerakbonds
Every chemical bond has bond energy associated with it
The sum of energy needed to break them minus the energy relased
You cannot compare overall energy changes unless you know the numerical differences in thebond energies
