lab 10

After having that completed your experiment and calculated your enthalpy of neutralization, you are told concentration of the NaOH solution you used was 5% less than indicated on its label. When you recalculate the enthalpy of neutralization, will the recalculated value be larger, smaller, or the same as you originally calculated?

A 5% decrease in the molarity of NaOH would decrease the moles of NaOH, making it the limiting reagent instead of HCl. The recalculated enthalpy would be larger than the original bc the heat of the reaction would be divided by a smaller amount of moles

enthalpy of reaction (heat of reaction)

Delta H or heat flow. Is defined as the amount of heat that would have to enter or leave a system in order to bring the temp back to its initial value.

enthalpy is path independent

Is dependent only on changes in a system between initial and final states. It Is calculated as the heat of reaction as a function of the #moles of limiting reagent created

heat capacity of the calorimeter or the calorimeter constant

The amount of heat in J required to raise the temp of the calorimeter by one degree kelvin. The calorimeter is not perfectly insulated, so we account for the heat absorbed by the calorimeter with the heat capacity.

Now consider the HCl you used in your experiment. If the concentration was 5% higher than that stated on its label, what will be the effect on the enthalpy of neutralization originally calculated?

The enthalpy would be less than the original because a 5% increase in HCl molarity would make NaOH the limiting reagent instead of HCl. Thus, the heat of reaction would be divided by a larger number of moles than the original, resulting in a larger delta H.

calculating specific heat capacity

The heat capacity of the calorimeter is determined in Part A of the experiment by measuring the temperature change that occurs when a known volume of hot water is added to a known volume of cold water. The heat lost by the hot water is equal to the heat gained by the cold water and the calorimeter. We assume that no heat is lost to the laboratory environment. To calculate the heat capacity, we need to make two assumptions: Any thermal energy transferred from the calorimeter to the outside environment is negligible. All dilute aqueous solutions have the same density (1.00 g/mL) and the specific heat capacity of water (4.184 J/g∙°C). The calorimeter was the same temperature as the cooler room temperature water.

adiabatic calorimeter

does not allow for the transfer of heat or matter between a thermodynamic system and its surroundings

neutralization reaction

exothermic and causes an increase in temp of the solution in the calorimeter.

endothermic

heat is absorbed and delta H is positive bc heat would have to enter the system in order to bring temp back to initial

exothermic

heat is released or evolved and delta H is negative bc heat would have to leave the system in order to bring the temp back to the original value

effect on enthalpy change if the temp probe read 1 degree Celsius lower than the true temp

if only the final temp is off by 1 degree, the enthalpy change would be smaller than the true value

why do you wash and dry the temp probe after you measure the temp of NaOh and before you measure the HCl

in order to avoid mixing the NaOH residue with the HCl and changing its concentration and temp

calorimetry

measure thermal energy changes in chemical reactions. constant pressure

standard enthalpy

q/n. standard enthalpy is higher for for strong acids with strong bases than standard enthalpies . Most organic acids only partially dissociate in aqueous solutions; energy is required to break the bond between the proton and its conjugate base, therefore lowering the measured enthalpy change.

graph of temp as a function of time

temp on y-axis and time on x-axis. large jump is change in temp used to calculate specific heat capacity

heat of reaction

the enthalpy change for a chemical reaction (delta H).

heat of neutralization

the enthalpy change when equal amounts of acid and base are mixed together to form 1 mole of H2O.

heat

the flow of thermal energy from an object at higher temp to one at lower temp

how do the two heats of reaction compare for the neutralization of NaOH with a strong acid vs a weak acid

the heat of reaction of the weak acid should be smaller because weak acids don't dissociate completely into individual ions, thereby releasing less heat during the reaction.