1. Exothermic reactions and the fire triangle Shooting

2.  Shooting was originally a functional activity used for hunting, to provide food, and as an offensive and defensive method.  Although still unfortunately often linked to acts of violence, shooting is now a recognised sport and leisure activity when carried out in a safe and controlled environment.  The National Rifle Association (NRA), founded in 1959, is the main governing body of the sport in Britain and has its base at Bisley, a world-renowned shooting venue. Background

3. Shooting involves the propulsion of a bullet from a barrel. This propulsion can come from a variety of different sources, which include:  Stored kinetic energy from spring loaded guns.  High air pressure in inside air guns, in which compressed air propels a bullet out of the hamber to release the build up of pressure.  From a combustion reaction, such as when gunpowder is ignited. It is this final way in which a bullet can be fired that really links the sport of shooting to chemistry, although all the methods have direct links to science as a whole. Shooting and the link to Chemistry

4. In order for a fire to take place there must be three inter - relating components present. These can be represented in a diagram known as the fire triangle, which can be seen on the next slide.  The first component which must be present is a fuel to burn. This is often, but not always, carbon based.  The second component which must be present is oxygen.  The third component which must be present is a source of ignition (or heat). This could be a spark or a flame for example. Once the fuel has started to burn the energy released from the burning fuel will continue to supply the energy needed for the burning to continue. The fire triangle

5. Or ignition source When one or more of the three components is no longer present the fire will stop.

6. Combustion reactions are reactions with oxygen that are normally accompanied by a release of energy.  Typically heat and light energy are given off during a combustion reaction – a fire being a common example.  If sound energy is also released the combustion reaction is known as an explosion.  When gunpowder is ignited a combustion reaction (often in the form of a controlled explosion) takes place. It is the release of energy from the reaction that will propel a bullet from a gun, when gunpowder is used to fire bullets. Combustion reactions

7. Gunpowder, also known as black powder, is a granular mixture of charcoal, sulfur and potassium nitrate, KNO 3 (which is also known as saltpetre). The mixture, when ignited, is explosive and burns rapidly producing volumes of gases and hot solids which can be used as a propellant in firearms. The composition of modern day gunpowder is quite different to traditional gunpowder, but the traditional powder composition was developed as long ago as 1780. Traditional gunpowder

8. The proportions, by weight, of the mixture of substances in traditional gunpowder vary slightly depending on the purpose of the powder but are normally roughly, 15% charcoal (softwood), 10% sulfur, and 75% potassium nitrate. Each component has a specific purpose as follows:  The nitrate, typically potassium nitrate, supplies the oxygen for the reactions which take place when the powder is ignited.  The charcoal provides carbon and other fuel sources for the reaction.  The sulfur lowers the temperature of ignition and increases the speed of combustion whilst also acting as an additional fuel source. Traditional gunpowder

9. The reactions which take place when gunpowder is ignited are extremely complicated and difficult to represent in a chemical equation in full. However the following is a widely accepted simple chemical equation which is commonly used to represent the combustion of gunpowder: 2KNO 3 (s) + S(s) + 3C(s) K 2 S(s) + N 2 (g) + 3CO 2 (g) A still simplified equation that represents the reaction slightly more accurately is: 10KNO 3 (s) + 3S(s) + 8C(s) 2K 2 CO 3 (s) + 3K 2 SO 4 (s) + 6CO 2 (g) + 5N 2 (g) Traditional gunpowder

10. When gunpowder is ignited energy is produced. More energy is produced in the reaction than is needed for the reaction to take place. Chemists refer to this type of reaction as being exothermic. The chemical equation for an exothermic reaction can be represented as follows: Reactantsproducts + energy Some reactions require more energy to take place than is produced from the reaction. These types of reaction are referred to as being endothermic by chemists. They can be represented as follows: Reactants + energyproducts Exothermic and endothermic reactions

11.  It is extremely difficult to measure the absolute amount of energy in a chemical system. Chemists therefore measure the enthalpy change, ∆H.  ∆H = energy used in bond breaking reactions – energy released in bond making products.  By definition the enthalpy change has a negative or minus value: ∆H < 0. In an exothermic reaction a negative ∆H is produced as a larger value (the energy released in the reaction) is subtracted from a smaller value (the energy used for the reaction) The opposite is true, i.e. a positive ∆H value, for an endothermic reaction. Enthalpy change, ∆H

12. Energy diagrams are often used to show if a reaction is endothermic or exothermic. Below is an example of an energy diagram for an exothermic reaction: Energy diagram for the complete combustion of carbon. The information above tells a chemist that burning 12 g of carbon in oxygen produces 393.5 kJ. Enthalpy change, ∆H

13. An example of an energy diagram for an endothermic reaction can be found below: Energy diagram for the formation of hydrogen iodide. The information in this energy diagram tells a chemist that a net value of +52 kJ of energy is required for the reaction to take place. As the ∆H is positive the reaction is endothermic. Enthalpy change, ∆H

14. Using data tables chemists can work out if a reaction will be exothermic or endothermic. Here is an example using the formation of hydrogen chloride from reacting hydrogen and chlorine together. H 2 (g) + Cl 2 (g) 2HCl(g) Using relative atomic masses, 2 g of hydrogen react with 71g of chlorine to form 73 g of hydrogen chloride. The data book states that it requires +436 kJ of energy to break the H-H bonds in 2 g of hydrogen molecules and +242 kJ of energy to break the Cl-Cl bonds in 71 g of chlorine molecules. Working out ∆H for a reaction from data tables

15. The data book also tells us that −431 kJ of energy are released when 36.5g of hydrogen chloride is formed. As we form 2HCl from 2 g of H 2 and 71 g of Cl 2 then 73 g of HCl are formed. If it takes −431 kJ per 36.5g of HCl produced then the energy required to produce 2HCL is twice as much, or − 862 kJ. The energy change is therefore: +436 kJ (energy required to break H-H bonds in 2 g of hydrogen molecules) + +242 kJ (energy required to break Cl-Cl bonds in 2 g of chlorine molecules) + −862 kJ (energy released in the formation of 2 g of HCl) = − 184 kJ. Or: (+436 kJ) + (+242 kJ) + ( − 862 kJ) = − 184 kJ As the ∆H for the total reaction is negative then the reaction is exothermic. Working out ∆H for a reaction from data tables

16. Source of fuel – required as something has to burn in order for the fire to take place.  Source of oxygen – required to allow the combustion reaction causing the fire to take place.  An initial ignition or source of heat to provide the energy for the reaction to start. 1.What are the three inter-related components which make up the fire triangle in order for fire to take place? Explain the role of each component.

17. 2.What are the three most common components of traditional gunpowder? Charcoal, sulfur and potassium nitrate (often known as salt petre). 3.What is the difference between an exothermic reaction and an endothermic reaction? An exothermic reaction is a reaction whereby more energy is produced and released from the reaction than is required in order for the reaction to take place. An example of which is a combustion reaction. An endothermic reaction requires more energy for the reaction to take place than is created.

18. 4.Using the following data, is the formation of water from hydrogen and oxygen an exothermic or an endothermic reaction? Bond energies in kJ/mol: H-H is 436, O=O is 496 and O-H is 463. Show your workings. The reaction is exothermic producing a net enthalpy change of +484 kJ (242 kJ per mole of hydrogen burned or water formed). Workings – Chemical equation is 2H 2 (g) + O 2 (g) 2H 2 O(g) Bonds broken (energy taken in/needed): (2 x H-H = 2 x 436) + (1 x O=O = 1 x 496) = 1368 kJ (answered continued on next slide)

19. Bonds made (energy produced): (4 x O-H = 4 x 463) = 1852 kJ The overall energy change for the reaction is therefore: −1368 kJ − −1852 kJ = +484 kJ (or 242 kJ per mole of hydrogen burned or water formed). As the enthalpy change is positive for the overall reaction the reaction is exothermic.

20. 5.Draw an energy diagram for the reaction in Q4. Energy diagram for the formation of water from hydrogen and oxygen. The information in this energy diagram tells a chemist that a net value −484 kJ of energy is required for the reaction to take place. As the ∆H is negative the reaction is exothermic.