![]() ![]() The amount of heat gained or lost by a sample (q) can. The heat capacity of 1 gram of a substance is called its specific heat capacity (or specific heat), while the heat capacity of 1 mole of a substance is called its molar heat capacity. The hot gasses (in the form of steam) have to release energy into the environment in the form of heat to cool to the point that they can form liquid water, meaning that the formation of H 2O is exothermic. Heat capacity is the amount of heat required to change the temperature of a given amount of matter by 1C. This makes sense - H 2 and O 2 are gasses, while H 2O, the product, is a liquid. Since the sign is negative, we know that our reaction is exothermic. In our example, our final answer is -13608 J.Beware strongly exothermic reactions - these can sometimes signify a large release of energy, which, if rapid enough, can cause an explosion. The larger the number itself is, the more exo- or endo- thermic the reaction is. On the other hand, if the sign is negative, the reaction is exothermic. dHH final - H initial or dHH(products) - H(reactants) Tables of enthalpies are generally given as dH values. If the sign of your final answer for ∆H is positive, the reaction is endothermic. You will not need to be able to calculate the enthalpy directly in chemistry, we are only interested in the change in enthalpy, or dH. One of the most common reasons that ∆H is calculated for various reactions is to determine whether the reaction is exothermic (loses energy and gives off heat) or endothermic (gains energy and absorbs heat). Determine whether your reaction gains or loses energy. Therefore, the change in energy, and so the H value, can be calculated from E m x c x T where m is the mass of water present (kilograms), and c 4.18 kJ Kg. ![]()
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