1.4- Energetics Flashcards
Give the definition of 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
What happens in an exothermic and an endothermic reaction
In an exothermic change energy is transferred from the system to the surroundings, the products have less energy than the reactants
In an endothermic change, energy is transferred from the surrounding to the sytem they require an input of heat energy, the products have more energy than the reactants
What is the standard enthalpy change of formation
The standard enthalpy change of formation of a compound is the enthalpy change when 1 mole of the compoun is formed from its elements under standard conditions with reactants and products being in their standard states
What is the standard enthalpy change of combustion
The standard enthalpy of combustion of a substance is defined as the enthlapy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions, with all reactants and products being in their standard states
What are standard conditions
. 100 kPA
. 298 K
. Solutions all at 1 mol dm3
. All substances should have their normal state at 298 K
Give the formula for energy change
Q = MC^T
Q is in joules, M is in grams, C is the heat capacity, T is in either C or K
Outline the method for calorimetry
. Wash the equipment with the solutions to be used
. Dry the cup after washing]
. Put polysterene cup in a beaker for insulation and support
. Measure out desired volumes of solution with volumetric pippetes and transfer them to insulated cup
. Clamp themometer into place making sure the themometer bulb is immersed in solution
. Measure the intial temperatures of the solution or both solutions if 2 are used, do this every minute for 2-3 minutes
. 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 the before and after weighing method.
.Stir mixture
.Record temp evey minute after the addition for several minutes
. Extrapolate the temp curve back to the time the reactants were added together , as a slow reaction will cause the exact temp not to be obtained
What are errors in the calorimetry method
.Energy transfer from surroundings
.Approximation in a specific heat capacity of the solution, the method assumes all of the solutions have the heat capacity of water
. Neglecting the specific heat capacity of the calorimeter, we ignore the energy that is absorbed by the apparatus
. Reaction or dissolving may be incomplete or slow
. density of soltion is taken to be the same as water
Give the errors of measuring the enthalpies of combustion using calorimetry
. Energy losses from calorimeter
.Incomplete combustion of the fuel
.Incomplete transfer of energy
.Evaporation of fuel after weighing
. Heat capacity of calroimeter not included
.Measurements not carried out under standard conditions
What is hess’s law
Hess’s law states that total enthalpy change for a reaction is independent of the route by which the chemical change takes place
What way do formation arrows and combustion arrows go
. For formation the arrows go up to the reactants and products
. For combustion the arrows go down from the reactants and products
Give the formulas for formation and combustion enthalpy of reactions
Formation-
^H reaction = ^H products - ^H reactants
Combustion-
^H reaction = ^H reactants- ^H products
Give the definition of mean bond energies
The mean bond energy is the enthalpy change needed to break the covalent bond into gasesous atoms, averaged over different molecules
Give the formula for mean bond energies, and what way the arrows go
The arrows go down from the reactants and products
^H reaction = Bond energies broken - Bond energies made
Explain the enthalpy of combustion in a homologous series
For successive members in a homologous series such as alkanes and alcohols there is a constant rise in the size of the enthalpies of combustion as the numebr of carbon atoms increases
