Calculating Heat of Reactions. Calorimetry Process used for collecting heat data Calorimeter – device used Insulated cup containing water Reaction takes.

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Presentation transcript:

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 Heat absorbed in endothermic reaction lowers the water’s temp

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/g o C)

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

Example In a calorimeter containing 100 g of water at 21ºC, a reaction is carried out in which 3.5 g of NH 4 NO 3 decomposes into N 2 O and H 2 O. The temperature raises to 25ºC. Find the heat of reaction.

Example The reaction below is carried out in a calorimeter HCl + NaOH → NaCl + H 2 O 2.5g of NaOH reacts with excess HCl. The calorimeter contains 150g of water at 19 o C initially and the temperature rises to 24 o C during the reaction. Calculate the heat of the reaction.

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