Definition of enthalpy change
Heat energy change at constant pressure
Definition of standard enthalpy change of formation of a compound
Enthalpy change when one mole of the compound is formed from its elements under standard conditions with all reactants and products in their standard States
Definition of standard enthalpy change of combustion of a substance
Enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions with all of the reactants and products in their standard States
The enthalpy change for endothermic reactions is …
Positive
The enthalpy change for exothermic reactions is…
Negative
Give examples of exothermic reactions
Combustion
Neutralisation
Give examples of endothermic reactions
Thermal decomposition
Explain what happens during an endothermic reaction
Heat energy is transferred from surroundings
Products have more enthalpy than reactants
Temperature of surroundings decrease
Explain what happens during exothermic reactions
Heat energy is transferred to the surroundings
Products have less enthalpy than the reactants
Temperature of surroundings increase
When writing standard enthalpy equations, what are solids?
Ionic compounds and metals and I2
When writing standard enthalpy equations, what are liquids?
Bromine
When writing standard enthalpy of formation equations it needs to be balance so…
One mole of substance is produced
When writing standard enthalpy of combustion equations it needs to be balanced so…
one mole of starting substance reacts with oxygen
When writing standard enthalpy of combustion equations what's produced?
Oxide
If hydrocarbon produces CO2 and H2O
What does it take 4.18 joules of energy do?
Raise the temperature of 1 g of water by 1°C (specific heat capacity)
What do calorimetry experiments do?
Measure the temperature change in a reaction
q =
mc∆T
q=mc∆T
q is
m is
c is
T is
Heat energy in J
Mass of substance heated in g
Specific heat capacity (4.18)
temp rise (K)
∆H =
q/n
∆H = q/n
∆H is
n is
Enthalpy change in kJmol-¹
Heat energy kJ
moles of limiting reactant (mol)
∆H =q/n
What must you do to the q value from the previous equation?
Change it to kJ by dividing by 1000
What must you do if the reaction is exothermic?
As a negative sign to the enthalpy change
How do you minimise heat loss?
Use polystyrene cup and lid rather than glass Beaker in non combustion experiments
Insulate the colorimeter in combustion experiments
Why is the value lower than the data value?
Heat loss to surroundings or container
Incomplete combustion (in combustion experiments)
Alcohol evaporate in rather than reacting (in combustion experiments)
Explain a method for a combustion calorimetry experiment
Use 50cm³ measuring cylinder to measure 50 cm³ of water
Transfer the water into a copper can and clamp the copper can so the base is just above the spirit burner
Use a two decimal place mass balance to measure the initial mass of the spirit burner containing the ethanol
Use a thermometer to record the initial temperature of the water
Place the spirit burner beneath the copper can and light the wick
Stir the water allowing the water temperature to increase by 35°C and then use the cover to extinguish the flame measuring the final temperature of the water
Measure the mass of the spirit burner to find the mass of the ethanol burned
Repeat steps 1 to 7 with spirit burners containing different alcohols
Calorimetry experiment for combustion why must you stir the water?
To make sure the heat energy is evenly distributed
Definition of mean bond enthalpy
The enthalpy needed to break the covalent bond into gaseous atoms averaged over different molecules
Why do we use an average of a range of different molecules to find the mean bond enthalpy for different bonds?
Bond enthalpies very slightly between molecules
Why will the enthalpy change values calculated using mean bond enthalpies be less accurate than using formation or combustion data?
Because the bond enthalpies aren't exact as they're calculated from bonds in different molecules
What type of process is breaking a bond in why?
Endothermic as is it requires energy
What type of process is forming a bond and why?
Exothermic is it gives out energy
Is the mean bond enthalpy negative or positive and why?
Positive as it's defined as the enthalpy needed to break the bond so endothermic
If all molecules are in the gaseous state, ∆H =
Total of bond energies broken — total of bond energies forms
When given formation data, ∆H =
Products - reactants
When given combustion data, ∆H =
Reactants - products
When given mean bond enthalpies, ∆H =
Bonds broken - bonds formed
State hess's law
The enthalpy change in a reaction is independent of the route taken
What type of reaction profile is this? Label
Exothermic
Enthalpy
Progress of reaction
Reactants
A.E
Enthalpy change
Products
What type of reaction profile is this? label
Endothermic
Why is the ∆H value calculated using mean bond enthalpies less accurate than using formation or combustion data?
Mena bond enthalpies not exact - calculated from bonds of different bonds and bond enthalpies vary slightly
Last changeda year ago
