Chemical Energetics
Chemical energetics
We obtain our energy needs from the combustion of fuels, such as hydrocarbons.
Combustion/bond breaking
Burning natural gas in a plentiful supply/excess of air produces a large amount of energy.
During this process, complete combustion of methane, heat is given out. It is an exothermic reaction.
An exothermic reaction is a chemical reaction that releases heat energy into its surroundings.
methane + oxygen-carbon dioxide + water + heat energy
CH4(g) + 2O2(g) –> 2CO2(g) + 2H2O(l)+ heat energy
If only a limited supply of air is available then the reaction is not as exothermic and the poisonous gas carbon monoxide is produced.
This is the incomplete combustion of methane.
methane + oxygen carbon monoxide+ water + heat energy
2CH4(g) + 3O2(g) –> 2CO(g) + 4H2O(l)+ heat energy
The energy change above can be shown by an energy level diagram.
In the format:
NB: Activation energy is the minimum amount of energy required to compound to allow a chemical reaction.
When any reaction occurs, the chemical bonds in the reactants have to be broken(this requires energy). This is an endothermic reaction.
The bond energy is defined as the amount of energy in kilojoules(kj).
Endothermic reaction- A chemical reaction that absorbs heat energy from its surroundings.
NB: The H is the energy stored in chemical bonds.
The ΔH is the enthalpy change, the change in energy going from reactants to products/also called the heat of reaction.
Graph details
In an exothermic reaction, the energy level diagram has its products placed lower than the reactants as the energy is given off, it’s a negative change. The surrounding temperature increases as heat is being given out.
In an endothermic reaction, the energy level diagram has the products placed above the reactants as energy is absorbed, positive change. The temperature of the surroundings decreases as heat is absorbed.
You will be asked to find the enthalpy change in questions such as:
CH4(g) + 2O2(g) 2CO2(g) + 2H2O(l)
Using the data below, which tell us how much energy is needed to break a chemical bond and how much is given out when it forms, we can calculate how much energy is involved in each stage:
| BOND | BOND ENERGY |
| C-H | 435 |
| O=O | 497 |
| C=O | 803 |
| H-O | 464 |
| C-C | 347 |
| C-O | 358 |
(you aren’t required to memorize the bond energy, they are almost always given)
CH4(g) + 2O2(g)
Start with the reactants
Bond breaking
CH4-There are 4 C-H bonds in methane
hence 4435=1740 Kj
2O2-Your breaking 2 O=O(oxygen is diatomic that’s why we have double bonds)
hence 2497=994 kj
Add your reactants
1740+994=2734 Kj
Now the products
Bond formation
2CO2(g) + 2H2O(l)
2CO2 -making 2 C=O bonds
so, 2803=1606 kj
2H2O-making 4 O-H bonds
hence, 4464=1856 kj
Add them up:
1606+1856=3462 kj
Find the energy difference:
energy difference=energy required to- energy given out
break bond when bonds are made
=2734-3462
= -728 kj
The negative sign shows that it’s losing energy to the surroundings, which is an exothermic reaction.
Hydrogen is used as a fuel due to:
-being renewable
which means it cannot be used up and be made at a rate faster than the rate of use.
-is a clean fuel as when producing it only forms water as a waste product.
-can be produced from various sources, such as natural gas or biomass
-It is easy to store and transport
Fuel cells
Fuel cells just convert chemical energy into electrical energy.
You are only required to know the process of the reagents hydrogen and oxygen reacting in it/electrolysis of water.
Reaction:
As O2 gas is passed into the cathode region of the cell it is reduced:
O2(g)+2H2O(l)+4e–4OH–(aq)
The OH– ions formed are removed from the fuel cell by reacting with hydrogen:
H2(g)+2OH–(aq)2H2O(l) + 2e–
The electrons produced by this process pass around an external circuit to the cathode.
Radioactive isotopes such as 235U are also used as a source of energy as their nuclei are unstable and produce excess energy through radiation such as gamma rays.