combustion and reactivity

Fuels are substances from which energy can be transfered to make something happen.

Combustion is the scientific word for burning.

Heat is needed to start combustion.

The 3 things that are needed for combustion are heat, fuel and oxygen, which are known as the fire triangle.

Without these 3 componants, a fire cannot start or will be extinguished.

Hydrocarbons are fuels with molecules which contain only hydrogen and carbon.

Fossil fuels such as petrol and diesel are hydrocarbons.

During combustion, they react with oxygen to form carbon dioxide and water.

Hydrocarbons are named acording to the number of carbon atoms in each molecule.

Each carbon atom can have 4 other atoms bonded to it.

Coal (hydrocarbon) + oxygen -> carbon dioxide + water

Oxidation occurs when a substance reacts with oxygen to form an oxide.

Some examples are the combustion of hydrocarbons, and metals reacting with oxygen to form metal oxides.

Mass of reactants = mass of products.

Metal + oxygen -> metal oxide

Non-renewable fuels cannot be replaced at the rate that humans use them up. Examples are coal oil and natural gas.

Renewable fuels can be produced quickly enough to replace what has been used up. an example is biofuels made from plants and animal waste.

Hydrogen + oxygen -> water

Independant variables are variables that you choose/change. An example is the type of fuel you want to test in an experiment.

Dependant variables are what you measure to determine the result of the experiment, and depends on the independant variable. An example is the temperature of water after heating it with different fuels.

Control variables are variables that have to stay the same and could affect the results of the experiment. An example is the amount or starting temperature of water.

Physical changes do not involve a change in mass; the particles are just being rearranged.

Melting and boiling are physical changes where the particles gain energy to break the molecular bonds between them.

Freezing and condensing are physical changes where the particles lose energy as they change state.

Expansion occurs when substances are heated because the molecules have more energy so take up more space as they vibrate or move around more.

Dissolving occurs when a solid is added to a liquid to make a solution. The amount of solid (solute) that can be added before the solution is saturated increases with temperature.

Substances change shape at different temperatures.

When carbon burns in plenty of air, only carbon dioxide (CO2) is produced. This is complete combustion.

If there is not enough oxygen, incomplete combustion occurs. This results in:

    - Carbon dioxide (CO2) from carbon reacting completely with oxygen

    - Carbon monoxide (CO) from carbon reacting partly with oxygen

   - Soot particles from carbon that has not reacted with oxygen

Coal + oxygen -> carbon dioxide + water

Impurities in fossil fuels, such as sulfur, also react with oxygen when heated.

Many of the products of burning fuels in engines are pollutants, which can harm living things and damage the environment.

For example, carbon monoxide is poisonous and can kill, and soot particles can damage lungs and trigger asthma.

Acid rain is caused by sulphur dioxide and nitrogen dioxide. They harm living organisms, and the damage can be reduced by using lime to nuturalise it.

Diesel vehicles have filters to capture soot.

Most vehicles have a catalytic converter in their exhaust systems. This causes carbon monoxide to react with more oxygen to form carbon dioxide.

Nitrogen oxides are also broken down into nitrogen and oxygen.

Chemical changes are hard to reverse and a new substance is made.

The new substances that are made have different properties than the original substance.

The reactivity series allows us to predict how metals will react. 

For example, potassium is more reactive than zinc.

More reactive metals will have a larger temperature change.

Lithium + water -> lithium hydroxide + hydrogen

Rust forms when iron reacts with water and oxygen to form hydrated iron (III) oxide.

Rusting is an oxidation reaction. The rust will flake away, leaving the iron beneath it exposed.

Metals used to protect less reactive metals are called sactificial metals.

The reactivity series allows us to predict how metals will react.

A more reactive metal will displace a less reactive metal from a compound.

An example of this is Carbon + Copper oxide -> Carbon dioxide + Copper. Here the Carbon is more reactive than the copper so it displaces it.

In the blast furnace, iron ore (III) oxide is heated with carbon (coke). As the carbon burns, it produces carbon dioxide.

The carbon dioxide reacts with more carbon in the coke to form carbon monoxide. 

As carbon is more reactive than iron, the carbon monoxide displaces the iron from iron (III) oxide. We say that the iron (III) oxide has been reduced as the oxygen has been removed.

The carbon monoxide has been oxidised as oxygen has been added to form carbon dioxide.

When energy is transferred to the surroundings, this is an exothermic reaction, and the temperature of the surroundings increases.

Some exothermic reactions require activation energy in the form of heat to begin, for example a flame or a spark.

When energy is taken in from the surroundings, it is an endothermic reaction and the temperature of the surroundings decreases.

The rate of a chemical reation is how fast the reactants are changed into products.

A catalyst is a substance that speeds up a reaction without being used up.

Catalysts are reaction spacific.

They decrease the activation energy of a reaction so less energy is needed.

Gas pressure is caused by the force of particles hitting the walls of a container.

If you increase the temperature, the pressure will increase because the particles move faster and so hit the walls of the container with more force and more often.

Increasing the number of particles inside a container and decreasing the volume of the container will also increase the pressure because the particles are closer together and hit the walls of the container more often.