Chapter 9 - Enthalpy Flashcards
Enthalpy changes, Measuring enthalpy changes, Bond enthalpies and Hess' law and enthalpy changes.
What is the law of the conservation of energy?
Energy cannot be created or destroyed, just transferred from one form to another.
Define enthalpy
The heat content that is stored in a chemical system.
Define enthalpy change
The difference between the enthalpy of the products and the reactants.
Define exothermic
A chemical system where energy is transferred from the system to the surrounding
Define endothermic
A chemical system where energy is transferred to the system from the surrounding
Define activation energy
The minimum energy required for a reaction to take place.
What are the standard conditions?
Standard pressure - 101kPa
Standard temperature - 298K
Standard concentration - 1mol/dm^3
Standard state - The physical state of a substance under standard conditions
Define standard enthalpy change of formation
The enthalpy change that takes place when one mole of a compound is formed from its constituent elements under standard conditions, with all reactants and products in their standard states.
Define standard enthalpy change of combustion
The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.
What is the equation used to measure an energy change?
Q=mcΔT Q - energy change with surroundings (J) m - mass (g) c - specific heat capacity (J/g/K) ΔT - change in temperature (K)
How is enthalpy change worked out from the energy change?
ΔH = -Q/n
ΔH - enthalpy change (J/mol)
Q - energy change with surroundings (J)
n - number of moles (mol)
Define standard enthalpy change of neutralisation
The enthalpy change when an acid and an alkali react together under standard conditions give one mole of water(H20) , with all reactants and products in their standard states.
What are the causes for less energy being transferred than expected when working out ΔH(c)
Heat loss to the surroundings
Incomplete combustion of ‘alcohol’
Evaporation of ‘alcohol’ from the wick
Non-standard conditions
How can heat loss be accounted for using a graph of temperature against time?
Extrapolate the cooling curve back to when it was added.
Enthalpy change of neutralisation always has the same value, what is it?
-57.5 kJ/mol
Define average bond enthalpy
The energy required to break one mole of a specified type of bond in a gaseous molecule.
What are some properties of bond enthalpies?
Energy is always required to break bonds
Bond enthalpies are always endothermic
Bond enthalpies always have a positive enthalpy value
What kind of energy and enthalpy changes are bond formation?
Exothermic and releases energy
Define the specific heat capacity
is the energy required to raise the temperature of 1g of a substance by 1K
Determination of an enthalpy change of combustion:
Describe the experiment for spirit burner
1) using a measuring cylinder, measure out 150cm^3 of water. Then pour it into a beaker and record the initial temperature to the nearest 0.5C.
2) add methanol to the spirit burner. Then weigh it.
3) place the spirit burner under the beaker. Then light it. Then stir the water whilst burning with a thermometer.
4) after 3 minutes, extinguish the flame. Immediately, record the maximum temperature reached by the water.
5) re weigh the spirit burner containing the methanol. Assume that the wick has not been burnt
what is endothermic in terms of bonds
where energy is required to break bonds
what is exothermic in terms of bonds
where energy is released when bonds are formed
Formula for calculating enthalpy changes from average bond enthalpies
Delta r H = the sum of bond enthalpies in reactants - the sum of bond enthalpies in products
The enthalpy change for any element in its standard state is
0
Limitation of calculating the enthalpy change of reaction from average bond enthalpies
As you are using average bond enthalpies, the actual energy involved in breaking and making bonds would be slightly different.
Also calculations need all species to be in gaseous molecules. For example if ur product produces H20(l) then you it would have to be H20(g). This means your calculated enthalpy change of reaction is not a standard enthalpy change
Determination of an enthalpy change of reaction, the experiment.
- Determined by using plastic cups made of polystyrene foam (which are cheap,waterproof and light weight), which offer some insulation against heat lost to the surroundings.
- Record temperature change using thermometer
- Heat energy calculated using mcΔT
Define the standard enthalpy change of reaction?
the enthalpy change that accompanies a reaction in their molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states.
Methods for reducing heat loss in the determination of an enthalpy change of a reaction?
Adding a lid to the polystyrene cup
Adding insulation around the polystyrene cup
Methods for reducing heat loss in the determination of an enthalpy change of combustion?
Adding a lid to the beaker
Using draft shields around the apparatus
