3. Physical Chemistry

what are exothermic and endothermic reactions?

exothermic - reactions that release thermal energy

endothermic - reactions that take in thermal energy

how would you do a calorimetry experiment to find the enthalpy change of a reaction in solution?

METHOD:

• set a calorimeter with a polystyrene cup, plastic lid, and thermometer

• add a fixed volume of one reagent to it and take the initial temperature

• add an excess amount of the second reagent and stir continuously

• record the maximum temperature and calculate the temperature change

• calculate the energy released using:

  Q = mcΔT

  Q = the heat energy change, J

  m = the mass of the substance being heated, g

  c = the specific heat capacity, J/g/°C (4.18 J/g/°C)

  ΔT = the temperature change, °C

how would you do a calorimetry experiment to find the enthalpy change of a combustion reaction?

what errors could there be?

METHOD:

• measure a fixed volume of water into a copper can

• weigh the spirit burner using a balance

• measure the initial temperature of the water

• burn the fuel and stir the water

• wait until the temperature has risen by approximately 20 ^oC and extinguish the flame

• record the final temperature of the water and re-weigh the spirit burner

• calculate the energy released using:

  Q = mcΔT

ERRORS

• not all of the heat produced by the combustion reaction heats the water

  • some is lost to the surroundings

  • some is absorbed by the calorimeter

• to minimise heat losses, the calorimeter should not be too far above the flame and have a lid

• shielding can be used to reduce draughts

• the main errors are:

  • heat losses

  • incomplete combustion

what is the equation for heat energy changes?

Q = mcΔT

Where:

• Q = the heat energy change, J

• m = the mass of the substance being heated, g

• c = the specific heat capacity, J/g/°C

• ΔT = the temperature change, °C

which is positive and negative in exothermic and endothermic reactions?

exothermic is negative

endothermic is positive

what is the equation for energy released per mol of fuel?

ΔH = Q/n

units are kJ/mol

how would you draw an energy level diagrams for both exothermic and endothermic reactions?

do exothermic and endothermic reactions involve making or breaking bonds?

MEXO-BENDO

Exothermic: making

Endothermic: breaking

During reactions, energy must be taken in to break bonds and energy is released when new bonds are formed. Whether a reaction is endothermic or exothermic overall depends on the difference between the energy needed to break existing bonds and the energy released when new bonds are formed.

what are the energy levels in exothermic reactions?

• the energy released when new bonds are formed is greater than the energy taken in to break bonds

• the change in energy is negative since the reactants have more energy than the products

• therefore an exothermic reaction has a negative ΔH value

what are the energy levels in endothermic reactions?

• the energy needed to break existing bonds is greater than the energy released when new bonds are formed

• the change in energy is positive since the products have more energy than the reactants

• therefore an endothermic reaction has a positive ΔH value

what is the equation for enthalpy change in terms of bonds?

Enthalpy change (ΔH) = Energy taken in - Energy given out

PRACTICAL: Investigating Temperature Changes

METHOD

• using a measuring cylinder, place 25 cm³ of the NaOH solution into the calorimeter

• measure and record the temperature of the solution

• add 5 cm³ of the dilute HCl and stir

• measure and record the highest temperature reached by the mixture

• repeat steps 1 – 4 increasing the amount of acid added by 5 cm³ each time

RESULTS

• record your results in a table with volume of acid and temperature

• the larger the difference in temperature, the more energy is absorbed or released