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.
CP20: Processing data
- Draw two lines; one horizontal at bottom of graph, the other diagonal across LHS of graph
- Extrapolate 2nd line and calculate temp change from first line.
- Then use Q=mcΔT
CP20: Sources of error
- Heat loss to surroundings
- Solution cools as it heats up
What is bond enthalpy?
enthalpy change when one mole of bonds in the gaseous state is broken.
Why are mean bond enthalpies used?
Bond enthalpies for a particular type of bond in different covalent environments are slightly different so a mean bond enthalpy is given.
What is the calculation for ΔH?
ΔH = bonds broken - bonds made
