enthalpy Flashcards
enthalpy, H
measure of heat energy in a chemical system
The chemical system refers to the atoms, molecules or ions making up the chemicals
enthalpy change
- difference in enthalpy between reactants and products (ΔH)
ΔH= H(products) - H (reactants)
ΔH can be positive or negative, depending on whether the products contain more or less energy than the reactants - usually in kJ/ mol
law of conservation of energy
energy cannot be created or destroyed
system, surroundings and universe
system- chemicals, reactants and products
surroundings- apparatus eg thermometer, the laboratory
universe- everything. Includes both system and surroundings
exothermic reactions
- energy transferred from the system to the surroundings
- ΔH negative
- temp of surroundings increases
endothermic reactions
- energy transferred from the surroundings to the system
- ΔH is negative
- temperature of surroundings decreases
standard conditions
- standard pressure- 100kPa (close to pressure of one atmosphere 101kPa)
- standard temperature (298K)
- standard concentration is 1 mol/dm^3- relevant to solutions only
- standard state- physical state of substance under standard conditions
standard enthalpy change of reaction, ΔH°r
enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in standard states
eg Mg + 1/2 02 —-> MgO has an enthalpy change of -602
2Mg + O2 ——–> 2MgO and now has an enthalpy change of double the first reaction, of -1204 kJ/mol
standard enthalpy change of formation ΔH°f
The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in standard states
- in equation compound must always have one mole so ratio should be written as such
- all elements have an enthalpy change of formation of 0kJ/ mol
standard enthalpy change of combustion ΔH°c
- enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in standard states
- when a substance completely reacts with oxygen, the products are the oxides of the elements in the substance
enthalpy change of neutralisation ΔH°neut
energy change that accompanies the reaction of an acid by a base to form one mole of H20 (l), under standard conditions, with all reactants and products in their standard states
- the value of ΔH°neut is the same for all neutralisation reactions as involves the reaction of H+ and OH- to form one mol of H20
energy change
q= mcΔT
m- mass of materials that are changing temperature, usually measured in grams
c- specific heat capacity of surroundings. In most experiments you will be measuring the temperature change of water or aqueous solutions, for water c= 4.18 J/g/K
ΔT- temperature change determined from thermometer readings
specific heat capacity, c
- energy required to raise the temperature of 1kg of a substance by 1 K
determining enthalpy change of combustion
-liquid fuels such as methanol can be easily burnt using spirit burners. You can measure the enthalpy change of combustion by placing a spirit burner under a beaker of water ( where initial temp has been measured) , light the burner ( after measuring its initial mass) and burn the methanol whilst stirring the water.
- after about 3 mins extinguish the flame and immediately record the max temp reached by the water
- re-weigh the spirit burner containing the methanol. Assume that the wick hasnt been burnt
- then work out the mass of fuel burnt and the temperature change of the water. Then use q=mcΔT to work out the energy change. For m use the mass of water which can be calculated as 1cm^3= 1g
Then calculate the amount of mol of methanol burnt and divide this by q to find the enthalpy change in kJ/ mol
how accurate are experimental values of enthalpy changes of combustion
-heat is lost to the surroundings other than water, this includes the beaker but mainly the air surrounding the flame
- incomplete combustion of methanol. May be some incomplete combustion with CO, and Carbon being produced instead of CO2. You would see a black layer of soot on the beaker
- evaporation of methanol from the wick. The burner must be weighed as soon as possible after extinguishing the flame, otherwise some methanol may have evaporated. Spirit burners usually have a cover to reduce this error
- non-standard conditions. The conditions for the experiment are unlikely to be identical to standard conditions
All but the last of these reasons would lead to a value of ΔHc, that is less exothermic than expected
- use of draught screens and an input of oxygen gas could minimise errors from heat loss and incomplete combustion
cooling curves
- to correct extrapolate the cooling curve back to when the zinc was initially added. Draw a vertical line from the time that the solutions were mixed to the extrapolated cooling curve
determination of enthalpy change of a reaction ΔH°r
- carries out in plastic cups made of polystyrene foam, which offer heat loss against insulation
- when carrying out reactions between aqueous solutions, the solution itself is the immediate surroundings
eg add an excess of zinc powder to copper sulfate. The mixture should be stirred until a max temperature is obtained - calculate q, assume density of solution is same as water and c= 4.18
-then work out moles of CuSO4 that reacted and work out in kJ/mol
determination of enthalpy change of neutralisation
- mix two solutions eg NaOH and HCl and measure the temperature rise.
- calculate q, assuming density is 1g/cm^3 and c=4.18. Use the total volume of the solution to work out m
- calculate the mol of each that reacted
- then use molar ratio to work out mol H20 and divide this by q to get it in kJ/mol
average bond enthalpy
energy required to break one mole of a specified type of bond in a gaseous molecule
- energy always required to break bonds
- bond enthalpies always endothermic
- so always have positive values
limitations of average bond enthalpies
- actual bond enthalpies can vary depending on the chemical environment of the bond
- average bond enthalpy is calculated from the actual bond enthalpies in different environments
- need all species to be in gaseous molecules, so calculation of ΔHr is not a STANDARD enthalpy change. To work out standard enthalpy change take into account the enthalpy change for the gases condensing into liquids ( or solid)
bond breaking and making
- bond breaking is endothermic as energy is required ΔH is pos
- bond making is exothermic as energy is released ΔH is neg
Hess’ law
if a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route
two rules to help with Hess’s Law
when using enthalpy change of formation:
ΔrH= ∑ΔfH products - ∑ ΔfH reactants
when using enthalpy change of combustion:
ΔrH= ∑ ΔcH reactants - ∑ ΔcH products
