Enthalpy Flashcards
Describe what is meant by enthalpy
Enthalpy change (∆H) is the heat energy change
measured under conditions of constant pressure.
Enthalpy has the symbol H
Enthalpy refers to all the heat energy that is stored in a chemical system (all the chemicals present in the reaction)
Enthalpy difficult to determine
So scientists determine change in enthalpy
Change in temp of a reaction can be used to calculate the enthalpy change
Neutralisation releases heat energy so it is an
Exothermic reaction
What is the equation for enthalpy change
Enthalpy change = enthalpy of the products - the enthalpy of the reactants
🔼H = H (products) - H (reactants)
How to repreeent enthalpy changes
Enthalpy changes can be represented on a profile
Diagram
draw and label an enthalpy profile diagram for endothermic reacton
enthalpy of products > enthalpy of reactants
enthalpy change is positive
arrow head points upwards
Tells us that the chemical system has taken in heat energy from the surroundings
Causing temperature of the surroundings to decrease
draw and label an enthalpy profile diagram for an exothermic reaction
enthalpy change is negative
shown with a downward pointing arrow
tells us heat energy was released from the chemical system to the surroundings
Activation energy is always shown using an upward pointing arrow - which runs from the enthalpy of the reactants to peak of the curve
Also write reactants and products formulas and state symbols in the right places - these do need to be balanced
enthalpy of a chemical system increases before it decreases - what is this called
This increase is called the activation energy
In order for any reaction to take place, we stat by breaking the chemical bonds in the reactant molecules
Breaking chemical bonds requires energy
This represents the Ea of the reaction
What is the activation energy
The activation energy is the minimum energy required for a reaction to take place
describe what is meant by standard conditions
Standard enthalpy changes refer to standard conditions,
ie 100 kPa and a stated temperature (eg ∆H298Ɵ)
we always determine enthalpy changes under standard conditions - referred to as standard enthalpy changes
Standard temperature is 298 K which is 25 degrees Celsius
Standard pressure is 100kPa
Standard concentration is 1 mol dm-3 for chemicals in solution
All chemicals must be in their standard states under standard conditions - a liquid, solid or gas
what is the symbol and unit for standard enthalpy change
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what does the symbol of circle and horizontal line mean - standard conditions
Unit - kJ mol-1
describe different standard enthalpy changes
Standard enthalpy change of combustion
Standard enthalpy change of neutralisation
Standard enthalpy change of formation
What is the standard enthalpy change of combustion
The standard enthalpy change of combustion is the enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions and all of the chemicals are in their standard states
write out the standard enthalpy change of complete combustion of ethane
C2H6 (g) + 3.5O2 –> 2CO2 (g) + 3H2O
Above arrow (∆cHƟ
all chemicals are in their standard states for 298 K and 100kPa pressure
Equation shows we are combusting one mole of ethane
cant double everything
standard enthalpy change of combustion always refers to the combustion of one mole of substance
What is the standard enthalpy change of neutralisation
The enthalpy change when one mole of water is formed in a neutralisation reaction under standard conditions
What is the standard enthalpy change of formation
The standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its elements under standard conditions and all of the chemicals are in their standard states
∆fHƟ
state the standard enthalpy change of formation of butane
4C (s) + 5H2 (g) –> C410 (g)
∆fHƟ over the arrow
reacting 4 moles of carbon with 5 moles of hydrogen
What is the standard enthalpy change of reaction
Standard enthalpy change of reaction is the enthalpy change when a reaction takes place in the molar quantities shown by the balanced chemical equation under standard conditions
All of the chemicals must be in their standard states
Describe how to determine the standard enthalpy change of combustion
the fuel will be combusted and use the thermal energy released to heat a known mass of water
By measuring the temperature change of the water, we can determine the standard enthalpy change of combustion
Ethanol in a spirit burner
Above this, we have a metal can containing a fixed mass of water
Metal can called a calorimeter
A temperature is used to measure the temp of the water
First use a balance to measure the starting mass of the spirit burner and the fuel
Use a thermometer to measure the start temperature of the water
Next remove the cap from the spirit burner and immediately light the wick
It is very important that we dont leave the burner uncapped if it is not lit
That is because fuel will evaporate and make our final results less accurate
The thermal energy released now causes the temperature of the water to increase
Stir the water using the thermometer to make sure that the thermal energy is distributed
After several minutes, extinguish the flame by replacing the cap on the burner and use a thermometer to read the temperature of the water
Then use the balance to measure the final mass of the spirit burner and the fuel
By subtracting the final mass from the starting mass, we can calculate the mass of fuel that combusted
mass of water = x g
temperature change of water = + x degrees Celsius
mass of fuel that combusted = 3.75 g
now calculate standard enthalpy change of combustion
- calculate the thermal energy that transferred into the water
use equation:
q = m c ΔT
q - energy change of water in Joules
m - mass of water in the calorimeter in grams
c - specific heat capacity of water - 4.18 J g-1 K-1
ΔT is the temperature change of the water in degrees celcius
convert into kJ by dividing the value of q by 1000
n = m/Mr - for moles of fuel that was combusted
∆cHƟ = q/1000 / n
Because reaction is exothermic - put a negative sign
Explain why the value we determine may be different from
The published value of standard enthalpy of combustion of a fuel is much higher than what was calculated in the experiment why?
Firstly, if we leave the unlit spirit burner uncapped then the fuel will evaporate
This will make it appear that we burned more fuel than we actually did
So because of this, our results will show the reaction to be less exothermic than it actually was
mass is higher - so moles is higher so ∆cHƟ is lower
A lot of the heat energy released by the fuel does not pass into the water
Some of the heat energy is transferred to the metal calorimeter, and a lot of the heat energy is transferred to the air
It is also possible that now all of the fuel underwent complete combustion
If incomplete combustion occurs then this releases less thermal energy than complete combustion
Lastly, our experiment may not have been carried out under standard conditions
State Hess’s Law
Hess’s law states that if a reaction can be carried out by two different pathways, then the total enthalpy change for these two pathways will be the same providing that the starting and final conditions are the same for both pathways
The enthalpy change for a chemical reaction reaction is independent of it routes taken from reactants to products
State the definition of average bond enthalpy
Average bond enthalpy is the energy needed to break one mole of a specific bond in a molecule in the gaseous state
