1. EXPLAINING CHEMICAL ENERGY CHANGES

2. EXPLAINING CHEMICAL ENERGY CHANGES In other words, breaking bonds is endothermic while making bonds is exothermic. In many chemical reactions, some bonds (in reactant molecules) are broken and new bonds are formed to make the product molecules. Two examples are:

3. When hydrogen burns in oxygen to form water, effectively H—H bonds in hydrogen gas and O—O bonds in oxygen gas break and H—O bonds form to make water. The reaction does not actually occur by each reactant breaking up completely into atoms with the atoms then recombining to form product molecules, but that is the net effect. 2 When methane burns in oxygen to form carbon dioxide and water, C—H bonds in methane and O—O bonds in oxygen gas are broken and C—O and H—O bonds are formed to make carbon dioxide and water respectively. The enthalpy change for a reaction, H, will be the energy required to break the necessary bonds in reactant molecules minus the energy released when the new bonds are formed: Hydrogen/oxygen reaction

7. http://www.chemistry.co.nz/hess_law.htm Henri Hess (1802 - 1850) Germany Hess' Law states "the heat evolved or absorbed in a chemical process is the same whether the process takes place in one or in several steps" Also known as the law of constant heat summation. X Y A B C a b c d e

8. http://www.saskschools.ca/curr_content/chem30/modules/module3/lesson5/lesson5p2.html How to estimate the heat of propane burning in oxygen Useful weblink

9. http://www.saskschools.ca/curr_content/chem30/modules/module3/lesson5/theatform.html

10. TEMPERATURE, QUANTITY OF HEAT AND HEAT CAPACITY 7.4

11. If two objects or samples of material are brought into contact, heat will flow from the hot object to the cold one until the temperature of the two objects is equal. When the temperature is uniform throughout both objects or samples of material, we say that thermal equilibrium has been reached.

12. Quantity of heat Amount of heat (or quantity of heat ) is different from temperature: two objects can be at the same temperature but contain very different amounts of heat. Consider a red-hot pin and a red-hot horseshoe—both have the same temperature but they contain very different quantities of heat. This is demonstrated by the experiments in Figure 7.3. Three well-insulated beakers each contain 100 g of water at 25.0°C. To the fi rst is added 10 g of water at 100°C, to the second is added 20 g of water at 100°C, and to the third is added 20 g of copper at 100°C. After mixing, the final temperatures are 32.0°C, 37.5°C and 26.5°C respectively.

13. Experiment demonstrating that quantity of heat depends upon mass, (a) and (b), and nature of substance, (b) and (c)

14. 3 Statements page; Specific heat capacity is therefore measured in joules per kelvin per gram, J K –1 g –1. Specific heat capacities of some common liquids

15. names, formulae + structures

16. High specific heat capacity of water Specific heat capacities allow us to calculate the quantities of heat that fl ow from one object or substance to another. From the above definition of specific heat capacity, when an object or a sample of a substance undergoes a change in temperature, the quantity of heat involved q is given by: q = m C ∆ T