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Calculating Heat of Reactions

Calculating Heat of Reactions. Calorimetry. Process used for collecting heat data Calorimeter – device used Insulated cup containing water Reaction takes place in the water Heat released in exothermic reaction raises the water’s temp

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Calculating Heat of Reactions

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  1. Calculating Heat of Reactions

  2. Calorimetry • Process used for collecting heat data • Calorimeter – device used • Insulated cup containing water • Reaction takes place in the water • Heat released in exothermic reaction raises the water’s temp • Heat absorbed in endothermic reaction lowers the water’s temp

  3. Calculating Heat • Heat released to or absorbed by the water in any reaction can be calculated by multiplying three factors Q = c x m x Δt Since it is water that is gaining or losing the heat in the calorimeter, c is the specific heat of water (4.18 J/goC)

  4. Heat (ΔH) of a Reaction • ΔH is always the J given off per mole of reactant (J/mol) • ΔH is Q divided by the number of moles that reacts • For exothermic reactions, ΔH is negative • For endothermic reactions, ΔH is positive

  5. Example • In a calorimeter containing 100 g of water at 21ºC, a reaction is carried out in which 3.5 g of NH4NO3 decomposes into N2O and H2O. The temperature raises to 25ºC. Find the heat of reaction.

  6. Example • The reaction below is carried out in a calorimeter HCl + NaOH → NaCl + H2O 2.5g of NaOH reacts with excess HCl. The calorimeter contains 150g of water at 19oC initially and the temperature rises to 24oC during the reaction. Calculate the heat of the reaction.

  7. Practice 1.5g of Na reacts with excess water according to the reaction below. 2Na + 2H2O → 2NaOH + H2 When the reaction is carried out in a calorimeter containing 125g of water at 18oC, the temperature rises to 28oC. Calculate the heat of the reaction.

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