8. Energetics I

Pressure: 1atm, 100 kPa Temperature: 298K Concentration: 1 moldm-3

The heat energy change measured at a constant pressure –> Symbol = ΔH

For exothermic reaction, enthalpy change is negative (temperature increases) So for endothermic reaction, enthalpy change is positive (temperature decreases)

In an exothermic reaction, energy is transferred from the system (chemicals) to the surroundings. The products have less energy than the reactants as energy is let out

• The energy released to form bonds in products is greater than the energy required to break bonds in reactants so ΔH is negative • Bond forming is exothermic. • This releases energy to the surroundings.

In an endothermic change, energy is transferred from the surroundings to the system (chemicals).

The enthalpy change when the number of moles of substances shown by an equation as written react under standard conditions (298K, 100kPa), with all reactants and products in the states given.

The enthalpy change when one mole of a substance forms from its constituent elements under standard conditions (298K, 100kPa), with all reactants and products in their standard states eg: 2C (s) + 2.5H2 (g) –> C2H5

The enthalpy change when one mole of a substance burns completely in oxygen under standard conditions (298K, 100kPa), with all reactants and products in their standard states eg: C2H5OH (l) + 3O2 (g) –> 2CO2 (g) + 3H20 (l)

The enthalpy change when one mole of water is produced in a neutralisation reaction between an acid and an alkali under standard conditions (298K, 100kPa), (not standard states because all will be aqueous) NaOH (aq) + 0.5H2SO4 (aq) –> 0.5Na2SO4 (aq) + H2O (l) Value is always exothermic (negative)

The enthalpy change for a given reaction is the same, independent of the route by which this is achieved (provided the temperatures, pressures, and physical states of products and reactants are the same).

Hess cycles are used to measure the enthalpy change for a reaction that cannot be measured directly by experiments. It may be because: It is hard to measure the temperature change of solids Impossible to add the exact amount/ mol of water to hydrate a salt Some endothermic reactions require a lot of heat to start reaction which affects enthalpy value Instead alternative reactions are carried out that can be measured experimentally.

The energy required to break one mole of bonds, in the gaseous state.

The energy required to break one mole of bonds, in the gaseous state, averaged across a range of compounds.

Q = mcΔT Energy transferred (J) = mass (same as volume) x specific heat capacity x temperature change

ΔH = Q/n Enthalpy change (KJ) / number of moles of the thing that was added

–> put polystyrene cup in a beaker for insulation and support –> Measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup –> measure the initial temperatures of the solution or both solutions if 2 are used using a thermometer. Do this every minute for 2-3 minutes –> At minute 3, transfer second reagent to cup. –> If using a solid reagent then use ‘before and after’ weighing method –> stir mixture ( mixing ensures that all of the solution is at the same temperature) –> Record temperature every minute after addition for several minutes

The reaction is not instantaneous so the best fit line allows for the effect of cooling during the reaction

Plot a graph of time on x axis. Temperature on y axis. The temperature change is the difference between the two EXTRAPOLATED lines after second reagent was added

–> place a lid on the polystyrene cup to reduce heat loss –> measure the temperature more often to give a more precise extrapolation maximum temperature change –> use a pipette / burette to measure the solution as it has less uncertainty in volume measurement Or use a thermometer with more gradations / more precise thermometer to give a more precise temperature change

1) if spirit burner is uncapped, fuel will evaporate, making it appear as though more fuel has been combusted 2) not all energy is transferred to water (calorimeter and air) 3) not all of fuel underwent complete combustion 4) experiment may not have been under standard conditions 5) energy lost to surroundings 6) assuming solutions have specific heat capacity and density of water 7) reaction/dissolving may be incomplete or slow 8) using mean bond enthalpies which assume reactants (the alcohol) are in the gaseous state when they are actually liquids

Exothermic

ΔH= sum of bond enthalpies of reactants (bonds broken) - sum of bond enthalpies of products (bonds formed)

Endothermic