Energetic 1 - Topic 8 Flashcards
Define enthalpy change
Enthalpy change is the amount of heat energy taken in or given out during any change in a system provided the pressure is constant
Draw reaction profile of endothermic and exothermic reaction
Definition for standard enthalpy change of formation
The standard enthalpy change of formation of a compound is the enthalpy change when 1 mole of the compound is formed from its elements under standard conditions (298K and 100kpa), all
reactants and products being in their standard states
Definition of Standard enthalpy change of combustion
the enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions (298K and 100kPa), all reactants and products being in their
standard states
What the standard conditions
- 100 kPa pressure
- 298 K (room temperature or 25oC)
- Solutions at 1mol dm-3
- all substances should have their normal state at 298K
Definition of enthalpy change of reaction
the enthalpy change when the number of moles of reactants as specified in the balanced equation react together
What is the standard enthalpy change of neutralisation
the enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water
How to calculate energy change
energy change = mass of solution x heat capacity x temperature change
How to calculate energy change
energy change = mass of solution x heat capacity x temperature change
Core practical for measuring enthalpy change
1) washes the equipment (cup and pipettes etc) with the solutions to be used
2) dry the cup after washing
3) put polystyrene cup in a beaker for insulation and support
4) Measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup
5) clamp thermometer into place making sure the thermometer bulb is immersed in solution
6) measure the initial temperatures of the solution or both solutions if 2 are used. Do this
every minute for 2-3 minutes
7) At minute 3 transfer second reagent to cup. If a solid reagent is used then add the
solution to the cup first and then add the solid weighed out on a balance - If using a solid reagent then use ‘before and after’ weighing method
8) stirs mixture (ensures that all of the solution is at the same temperature)
9) Record temperature every minute after addition for several minutes.
Why is it difficult to obtain the temperature rise if the reaction is too slow and how can we counteract this in our core practical
- as cooling occurs simultaneously with the reaction
- to counteract take readings at regular time intervals and extrapolate the temperature (curve/line back to the time the reactants were added together) and also take the temperature of the reactants for a few minutes before they are added together to get a better average temperature.
Errors in the core practical
- loss of energy to surroundings-
- approximation in specific heat capacity of solution. The method assumes all
solutions have the heat capacity of water. - reaction or dissolving may be incomplete or slow.
- density of solution is taken to be the same as water
How to work out enthalpy change
Q/moles or energy (J) / moles (of reactant that is not in excess)
When calculating the energy of enthalpy change what do we assume the mass is equal to
The TOTAL volume of water/solutions (or the density of water)
What is the specific heat capacity of water
4.18 J g-1 Kg -1
