Topic 8: Energetics 1 Flashcards
Key definitions, CP19, CP20, CP21, bond enthalpies
Equation to calculate energy
Q=mcΔT
Standard enthalpy change of combustion (ΔcH°)
The enthalpy change when one mole of a fuel is combusted completely in oxygen under standard conditions (1atm and 298
Standard enthalpy change of formation (ΔfH°)
Enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.
Standard enthalpy change of neutralisation (ΔnH°)
The enthalpy change when an acid and an alkali react together under standard conditions to give one mole of water.
Standard enthalpy change for a reaction (ΔrH°)
Enthalpy change when substances react under standard conditions in quantities given by the equation for the reaction.
Standard enthalpy change of atomisation (ΔaH°)
Enthalpy change when one mole of gaseous atoms is formed from the element in its standard state.
What is enthalpy?
the measure of the heat content of a substance
CP19: Enthalpy of combustion: Steps
- Measure 100 cm3 of water into a can.
- Accurately weigh a ‘burner’ containing one of the alcohols.
- Arrange the apparatus in such a way that the burner can be used to heat the water in the can.
- Take the temperature of the water before you start heating.
- Use your burner to heat the water. Stop heating when you have heated the water enough to give a reasonable temperature rise.
- Take the final temperature of the water and reweigh the burner.
CP19: How to calculate the standard enthalpy change of combustion (ΔcH°)?
- Use Q=mcΔT to find the value for Q
- Use ΔcH°=Q/1000/n
(to calculate n use M=Mr x n)
CP19: Sources of error?
- heat loss to surroundings
- Not standard conditions
- Incomplete combustion
leads to a calculation which is less exothermic than the data book
CP21: Enthalpy of neutralisation: Steps
- Clean and rinse a 25cm3 glass pipette and then pipette out 25cm3 of the acid into a polystyrene cup (within a glass beaker).
- Record the initial temperature of the alkali solution as accurately as you can. Put the thermometer through the hole of a lid.
- Clean and rinse a 25cm3 glass pipette and then pipette out 25cm3 of the alkali into the polystyrene cup.
- Place the lid and thermometer back on the cup and keep swirling whilst monitoring the temperature.
- Record the maximum temperature reached.
CP21 graph + data processing
- extrapolate from ascending and descending part of curve.
- the intersection is the point of maximum temperature.
CP21: sources of error
- Heat loss to surroundings
- Solution cools as it heats up
CP21: Improving accuracy
- Add lid to polystyrene cup
- Add insulation to beaker
- use pipette / burette to measure.
CP20: Energy changes during displacement: Steps
- Using a 25 cm3 pipette, measure 50 cm3 of the copper(II) sulfate solution into the polystyrene cup held securely in a beaker.
- Place the thermometer in the copper(II) sulfate solution. Gently stir the solution with the thermometer, start the timer and record the temperature to at least the nearest 0.5°C in the table below.
- Continue to stir the solution gently, recording its temperature every half minute for 3.0 minutes.
- At exactly 3.5 minutes, add the zinc powder to the copper(II) sulfate solution, stirring as you do so. Do not attempt to record the temperature at 3.5 minutes.
- Record the temperature of the solution, to at least the nearest 0.5°C, from minute 4.0 to minute 9.0 at half minute intervals. Continue to stir the solution throughout.
