Topic 8 Energetics I

The attractive force of the nuclear charge outweighs any shielding effect.

The enthalpy change measured at 100kPa & (usually) 298K when the number of moles of substances in the equation as written react.

Directly proportional to the absolute temperature of the substance.

Kinetic energy & potential energy (a measure of how strongly particles attract & repel each other).

Measure of the total energy of a system. This can’t be determined directly, but the enthalpy change for a physical or chemical process can be determined.

The heat energy transferred between the system and the surroundings at a constant pressure.

Activation energy is only shown on an enthalpy profile diagram.

Enthalpy, H on the vertical y-axis. Progress of reaction on the x-axis (this is not always necessary). Give the value for delta H, incl. the sign. Give the formula & state symbols of reactants and products on each level. Draw an arrow from reactants to products.

H products - H reactants

Down arrow, delta H is negative.

Up arrow, delta H is positive.

The enthalpy change at 100kPa & (usually) 298K when one mole of a substance is completely burnt in oxygen. (Ensure only one mole of the substance being burnt is represented in the equation.)

Weigh a spirit burner containing the liquid. Add a known volume of water to a copper can. Measure the temperature of the water. Light the burner. Constantly stir the mixture. When the temperature of the water has reached ~ 20°C above its initial temperature, extinguish the flame and immediately reweigh the burner. Measure the final temperature.

Mass of water. Divide by the moles of the liquid burnt. Include a sign.

• Some of the heat energy produced in the burning is transferred to the air not the water. • Incomplete combustion, which would cause soot to form on the bottom of the can. • Some of the heat energy is transferred to the copper can not the water. • The conditions are not standard, e.g., producing water vapour not liquid water.

Th enthalpy change measured at 100kPa & (usually) 298K when one mole of water is produced by the neutralisation of an acid by an alkali.

Use a pipette with a safety filler to place 25 cm^3 of 1 mol dm^-3 acid in a polystyrene cup. Measure the temperature of the acid. Use a pipette with a safety filter to place 25 cm^3 of alkali into a beaker. Measure the temperature of the alkali. Add the acid to the alkali and measure the maximum temperature reached.

Some heat energy will be transferred to the thermometer and the polystyrene cup. Temperature is only given to 2 s.f., so give answers to 2 s.f.. Measurement uncertainties.

The mass is the sum of the 2 volumes, assuming density is 1 g cm^-3. Divide by the moles of limiting reagent, which equals the amount of water formed.

Delta rH = Sum of bonds broken - sum of bonds made If delta H reaction is given instead, rearrange to find the missing bond enthalpy required.

Bond enthalpies are measured in the gaseous state.

The enthalpy change when one mole of a bond in the gaseous state is broken.

The enthalpy change when one mole of a bond, averaged out over many different molecules, is broken.

Bond enthalpy