Collision course – Which car?
The UK public has become very dependent on cars as a means of transport. There are nearly as many cars in the country as there are people (about 60 million). As fossil fuels decline in supply, government needs to take a decision about the kind of car to recommend to the UK public for the first half of the 21st century.
Do we stay with petrol/diesel do down the petrol-battery (hybrid) powered route or hydrogen?
The Low Carbon Trust is biased in favour of hydrogen power or hybrid cars. This is because these use less carbon-based fuels (none at all in the case of cars powered by hydrogen) and use technology that reduces the emission of greenhouse gases. Using low carbon fuels at an early stage means that fossil fuel supplies can be made to last longer.
The fuel Suppliers Association (FSA) is biased in favour of retaining petrol cars as the main car type. This is because change to hydrogen or battery-powered cars is seen as very costly and may cause job losses among fuel suppliers as garages change their nature and become fewer in number.
How will cars be fuelled in the future?
Hydrogen and a petrol-battery hybrid can be used to power cars as well as the conventional petrol/diesel fuelled cars.
Low carbon-technology (LCT) vehicles are essential to help reduce emissions and conserve fossil fuel supply
The reaction between hydrogen and oxygen is a suitable reaction that can be used in hydrogen fuel cells and has the benefit of having water as its only emission.
You can actually do an experiment at home to break up water molecules by electrolysis.
Electrolysis of water is the decomposition of water (H2O) into oxygen (O2) and hydrogen gas (H2) due to an electric current being passed through the water.
An electrical power source (12V battery for instance) is connected to two electrodes, or two plates (typically made from some inert metal such as platinum, stainless steel or iridium) which are placed in the water. Hydrogen will appear at the cathode (the negatively charged electrode, where electrons enter the water), and oxygen will appear at the anode (the positively charged electrode). Assuming ideal faradaic efficiency, the amount of hydrogen generated is twice the number of moles of oxygen, and both are proportional to the total electrical charge conducted by the solution.
Electrolysis of pure water requires excess energy in the form of overpotential to overcome various activation barriers. Without the excess energy the electrolysis of pure water occurs very slowly or not at all. This is in part due to the limited self-ionization of water. Pure water has an electrical conductivity about one millionth that of seawater. Many electrolytic cells may also lack the requisite electrocatalysts. The efficiency of electrolysis is increased through the addition of an electrolyte (such as a salt, an acid or a base) and the use of electrocatalysts. If you are doing this at home you could use common salt.
The process can be reversed to produce water.
How much energy comes from making water?
- The reaction releases energy in the form of heat.
- The energy comes from making bonds between hydrogen and oxygen.
- The water is produced as a vapour.
- The equation for the reaction is 2 H2 + O2 2H2O
- 4 –O‑H bonds are formed in the reaction.
- The enthalpy change of the formation of water is 4 x 463 = 1852, hence -1852 kJ mol-1 as Bond enthalpy values (kJ mol-1) are O-H = 463, O=O = 498 and H-H = 432
- 1 x O=O, 2 x H-H are the bonds broken.
- 498 + (2x 432) = +1362 kJ mol-1 is the energy released.
- Enthalpy change for the reaction between hydrogen and oxygen to make water = bonds broken ‑ bonds formed 1362 – 1852 = – 245 kJ mol-1
- The energy released is used to make the car move.
- The water goes out through the exhaust pipe.
- Advantages ‑ low pollution, energetically favourable, clean. Disadvantages ‑ refuelling system is not in place yet, hydrogen storage is more difficult, probably less power per mole compared with petrol. Also at the moment we rely on fossil fuels to produce the electricity to electrolyse the water to produce the hydrogen in the first place.
The new Honda FCX Clarity fuel cell vehicle is unveiled during the Los Angeles Auto Show in Los Angeles, Calif., on November 14, 2007.
Hydrogen fuel cells are relatively expensive to produce. As of October 2009, Fortune magazine estimated the cost of producing the Honda Clarity at $300,000 per car. Many designs require rare substances such as platinum as a catalyst. In 2010, a new nickel-tin nanometal catalyst was tested to lower the cost of fuel cells.
A hydrogen car uses hydrogen as its primary source of power for locomotion. There are two main methods: combustion or fuel cell conversion. Fuel cell conversion is what has been in the news most recently and is expected to power the cars of the future.
In fuel cell conversion, the hydrogen is turned into electricity through fuel cells, which then powers electric motors. The proton exchange membrane fuel cell (PEMFC) is one of the most promising technologies. It transforms the chemical energy liberated during the electrochemical reaction of hydrogen and oxygen to electrical energy. A stream of hydrogen is delivered to the anode side of the membrane-electrode assembly. At the anode side it is catalytically split into protons and electrons.
The newly formed protons filter through the polymer electrolyte membrane to the cathode side. The electrons travel along a circuit to the cathode side of the membrane-electrode assembly. This creates the current output of the fuel cell. Meanwhile, a stream of oxygen is delivered to the cathode side of the membrane-electrode assembly. At the cathode side oxygen molecules react with the protons permeating through the polymer electrolyte membrane and the electrons arriving through the external circuit for form water molecules.
V. Kind, Contemporary chemistry for schools and colleges. London: Royal Society of Chemistry, 2004.
Why we need to get rid of fossil fuel powered cars
There are two main reasons. Firstly we are running out of fossil fuels. This means that the petrol and diesel we get from oil will become scarce.
The second reason is the pollution that petrol and diesel cars produce such as unburnt hydrocarbons, carbon monoxide, carbon dioxide, nitrogen oxides and sulphur dioxide.
A petrol engine is an internal combustion engine with spark-ignition, designed to run on petrol (gasoline) and similar volatile fuels.
In most petrol engines, the fuel and air are usually pre-mixed before compression (although some modern petrol engines now use cylinder-direct petrol injection). The pre-mixing was formerly done in a carburetor, but now it is done by electronically controlled fuel injection, except in small engines where the cost/complication of electronics does not justify the added engine efficiency. The process differs from a diesel engine in the method of mixing the fuel and air, and in using spark plugs to initiate the combustion process. In a diesel engine, only air is compressed (and therefore heated), and the fuel is injected into very hot air at the end of the compression stroke, and self-ignites.
The four strokes refer, in the four stroke engine, to intake, compression, combustion (power) and exhaust strokes that occur during two crankshaft rotations per power cycle. The process by which the car is propelled forward.
Petrol driven cars can be quite cheap if you opt for a second hand model but the cost of petrol is increasing.
By 2050, measures are supposed to be implemented where the European Union roads will only be routes of transit for electric vehicles only.
The Toyota Prius is the world’s top selling hybrid electric vehicle, with cumulative global sales of over 3 million units by June 2013.
A hybrid vehicle is a vehicle that uses two or more distinct power sources to move the vehicle. The term most commonly refers to hybrid electric vehicles (HEVs), which combine an internal combustion engine and one or more electric motors. However other mechanisms to capture and utilize energy are included.
A hybrid will still produce carbon dioxide and other pollutants but not as much as the general petrol driven car.
The purchase price for most hybrid cars are 15%-25% higher than for their non-hybrid petrol and diesel equivalents. Typically, for new car, the additional purchase price is in the range of £2000-£4000 on a £15,000 model, and £3000-£5000 on a £20,000 model. Although the running costs are less.
RSC , available from: http://www.rsc.org/Education/Teachers/Resources/Aflchem/resources/52/index.htm [accessed 3 February 2011]