Question

2. Look up a coffee-cup calorimeter in your textbook and describe the components of the apparatus. (2 points) Assuming that the calorimeter prevents any heat to be taken in or released, the amount of heat transferred between the system and the surroundings should be equal in value but opposite in sign. For example, if a hot piece of metal gives off 5 J of energy, than the surrounding water should take in 5 J of energy. (Remember exothermic processes have a “-“q while endothermic process have a “+” q).

q_system = - q_surroundings

With that being said, the transfer of heat during a chemical reaction can be concluded as

q_system (aka the reaction) = - q_surroundings (aka the solution)

and since we know that…

q = C x m x DT

we can conclude that…

c_rxn x m_rxn x DT_rxn= - (c_soln x m_soln x DT_soln)

(note that the “-“ sign is just to show that they are equal but opposite. The actual value of the rxn and solution depend on the reaction that took place)

Answer 1

Consider mixture of reactants in aqueous solution is placed in coffee cup. As the reaction occurs it releases energy which is transferred to the calorimeter. That is, heat released by reaction will be absorbed by calorimeter and there will be temperature rise in the calorimeter.

             q_rxn = -(q_cal) = -C_cal(change in temperature)

The reactant solutions if undergo neutralization form water, so if the heat absorbed by calorimeter and other products (excluding water) is ignored. The solution density and heat capacity can be taken just of water.

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calorimeter.

  q_rxn = -(q_cal + q_water)

1) If the temperature of calorimeter (and its contents) increases, the calorimeter (and its contents) should absorb energy. And this energy is released by the reaction. Thus, the sign of q_cal, q_contents is positive and the sign of q_rxn is negative.

2) If the temperature of calorimeter decreases, the calorimeter (and its contents) should release energy. And this energy is absorbed by the reaction. Thus, the sign of q_cal is negative and the sign of q_rxn is positive