2.1 Flashcards
What is enthalpy, H
Heat content of a system at constant pressure
What is enthalpy change
Head added to a system at constant pressure
Equation to measure enthalpy change
Enthalpy of products - enthalpy of reactants
Exothermic reaction
- releases energy to surroundings
- temperature rise
- enthalpy change is negative
Endothermic reactions
- absorb energy from surroundings
- temperature drop
- enthalpy change is positive
What is the principle of conservation of energy
energy cannot be created or destroyed only changed from one form to another.
Conditions for standard enthalpy change
- all substances in their standard states
- 298 K (25 c)
- 1 atm (101000 Pa)
What is the standard enthalpy change of formation
the enthalpy change when one mole of a substance is formed from its constituent elements in their standard states under standard conditions.
What is the standard enthalpy change of formation of all elements in their standard state
0 kJ mol-1
Standard enthalpy change of formation of water
H2 (g) + 1/2 O2 (g) —–> H2O (l) /\fH = -286 kJ mol-1
What is the standard enthalpy change of combustion
enthalpy change when one mole of a substance is completely combusted in oxygen under standard conditions.
What is the standard enthalpy change of combustion of methane
CH4 (g) + 2O2 (g) —-> CO2 (g) + 2H2O (l) /\cH = -891 kJ mol-1
Equation for the standard enthalpy of reaction
/\rH = sum/\fH(products) - sum/\fH(reactants)
What is Hess’s law
the total enthalpy change for a reaction is independent of the route taken from the reactants to the products
What is bond enthalpy
the enthalpy required to break a covalent X-Y bond into X atoms and Y atoms, all in the gas phase
What is average bond enthalpy
Average value of the enthalpy required to break a given type of covalent bond in the molecules of a gaseous species
How to calculate bond enthalpy change
BURP
Sum of reactants - sum of products
What can be done to measure enthalpy due to not being able to measure it directly
Measure the heat transferred to its surroundings
What is a calorimeter
Insulated container that helps calculate the chemical change of a reaction
• change in temperature inside the calorimeter caused by the enthalpy change is measured with a thermometer
What information is needed to calculate the enthalpy change when doing it physically
- temperature change
- mass
- specific heat capacity of contents of calorimeter
Equation that represents the relationship between the temperature change and the amount of heat transferred
q= mc/\t
c- specific heat capacity
m- mass of solution in cup (volume)
What is done to obtain the maximum temperature change
allowances are made for heat lost to the surroundings
• temperatures of the solution are taken for a short period before mixing and some time after mixing
• graph of temperature against time is plotted and maximum temperature is obtained by extrapolating the graph back to the mixing time
Look
Equation to calculate the enthalpy change of reaction per mole
/\H = -q/n
reactant that is not in excess has to be accurately measured - mass of solid or concentration of solution must be known
What is done when determining an enthalpy change of combustion
Known mass of fuel is burnt in air to heat a known mass of water and the temperature change in the water is recorded
Main points to note when determining an enthalpy change of combustion
- suitable gap between the base of the metal container and the top of the spirit burner
- accurately measured the amount of water added
- use an accurate thermometer to measure initial temperature and record the steady value obtained
- weigh the spirit burner with fuel and record initial mass
- adjust the gap between the metal container and the spirit burner if necessary after lighting the wick
- allow the fuel to heat the water to a suitable temperature (about 20 c)
- extinguish the flame and record final maximum temperature
- allow spirit burner to cool thoroughly before re-weighing and recording final mass
Why is the value of enthalpy change of combustion much lower than the book value?
- some energy transferred from burning fuel is lost in heating the apparatus and the surroundings
- fuel isn’t completely combusted
Simplest type of calorimeter
Coffee cup calorimeter
Examples of indirect determination of enthalpy change
- magnesium oxide with carbon dioxide to magnesium carbonate (/\H of reaction between Mg and acid and MgCO3 and acid are separately measured)
- Formation of MgO (adding Mg and MgO separately to HCl)
- Formation of MgCO3 (adding Mg and MgCO3 separately to HCl)
- Decomposition of sodium hydrogencarbonate (adding sodium hydrogencarbonate and sodium carbonate separately to HCl)
Main points to note in the practical work of indirect determination of an enthalpy change
• measure an excess volume of acid using a burette or pipette and place in a polystyrene cup
• use accurate thermometer to measure initial temperature of acid, record the temperature when a steady value has been obtained
• accurately weigh the solid in powder (fast as possible) form in a suitable
container
• add all solid to the cup and stir the mixture well and start a stopwatch
• keep stirring with the thermometer and record the temperature regularly (every 30s)
• stop recording temperature when is has fallen for about 5 minutes
• re-weigh the weighing container to ensure correct mass of solid added is recorded
• plot a graph of temperature against time to calculate maximum temperature of the mixture might have reached
• calculate heat transferred
• calculate enthalpy change
• repeat with other solid
• use Hess’s law to calculate the required enthalpy change
Direction of arrows in an enthalpy cycle of formation
From the common elements to the reactants and products
Direction of arrows in enthalpy cycle of combustion
From reactants and products to the common combustion products
Why are average bond enthalpies used - why aren’t they all the same for a specific bond?
Actual value of the bond enthalpy for a particular bond depends on the structure of the rest of the molecule.
The bond enthalpy of a given bond is similar in a wide range of compounds but the average is calculated using the values from many different compounds
