X δt w the initial temperature of the water t i and

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x ΔT W ) The initial temperature of the water (T i ) and the maximum temperature reached after mixing (T f ) are used to calculated ΔT W (T f – T i ). Keep in mind that 100.0 mL of water weighs 100.0 grams and that s W of water is 4.18 J/ g∙ºC , we can modify the above equation to solve for s M . s M = (((100.0 x 4.18 x (T f – T i )) + (C cal x (T f – T i )))/ [mass M x (100.0 – T f )] Since we measure mass M , T i , and T f we can calculate the specific heat of the metal with the above equation. b. Heat of Reaction When chemical reaction occurs, heat is either evolved or absorbed by the reaction medium. We can represent a reaction that evolves heat as follows: Reactants → Products + Heat A reaction that absorbs heat would then be represented as follows: Heat + Reactants → Products
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3 Reactions that evolve heat are known as exothermic reactions and the heat that evolved is given, by convention, a negative sign. Reactions that absorb heat are known as endothermic reactions and the heat absorbed is given a positive sign. We will measure the heat evolved (or absorbed) as a solid solute dissolves in water. This particular heat of reaction is called, for obvious reason, the Heat of Solution and given the symbol ∆H S . To measure ∆H S for a particular solid, we will proceed as follows. We will place 100. mL of room temperature water into a coffee cup calorimeter. A preweighed sample of the solid is added to the water in the coffee cup calorimeter. o If the solid dissolves exothermically, the temperature of the water will rise. o If the solid dissolves endothermically, the temperature of the water will fall. The amount of heat evolves or absorbed will be calculated by the following equation: q = (mass x s x ∆T ) + (C cal x ∆T) o Mass is the sum of the mass of the water and the mass of the solid solute. o SH will be assumed to be the same as that of pure water, 4.18 J/ g∙ºC . o ∆T is the increase or decrease in the water’s temperature. To calculate ∆T, use the initial temperature of the water (T i ) and the maximum or minimum temperature reached after addition of the solute (T f ). The temperature change is then properly defined as: ∆T = T i – T f B y defining ∆T in this manner will give q the appropriate sign. For an exothermic reaction; T f will be greater then T i and ∆T will be negative , leading to a negative q. An exothermic reaction will also lose heat to the calorimeter and this heat be added back in to calculate q. If the reaction is endothermic T f will be less than T i and ∆T will be positive leading to a positive q. For an endothermic reaction we must also take into account heat gained from the calorimeter. q = (mass of solution x (T i – T f ) x 4.18 J/ g∙ºC) + (C cal x T i – T f )) To obtain ∆H S we must divide q by the number of moles of solute that actually dissolved.
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