8: Energetics 1 Flashcards
Define enthalpy change (∆H)
Heat energy change in a reaction at constant pressure
kJ/mol
What is ∆Hθ?
Enthalpy change under standard conditions
Also elements were in their standard states
What are standard states?
Reacted in physical states under standard conditions
What are standard conditions?
100 kPa of pressure
298K
What is an exothermic reaction?
Reactions which give out heat energy
∆H is negative
Temp often increases
What is an endothermic reaction?
Reactions that absorbs heat energy
∆H is positive
Temp often decreases
What is an enthalpy level diagram?
Diagram which shows relative energies of reactants and products of a reaction
Does NOT show Ea or progress of reaction
What is on the axis of an enthalpy level diagram?
y axis - enthalpy
no x axis
What value is shown in an enthalpy level diagram?
∆H
This is the difference between the enthalpies of the two substances
How does enthalpy relate to stability?
The less the enthalpy, the more stable
How does the enthalpy of the products of an exothermic reaction compare to the reactants?
Products have a lower enthalpy than the reactants
How does the enthalpy of the products of an endothermic reaction compare to the reactants?
Products have a higher enthalpy than the reactants
What is a reaction profile diagram?
Diagram which shows how the enthalpy changes throughout the reaction
What is shown in a reaction profile diagram?
Curve showing the enthalpy throughout the reaction
Activation energy
∆H
What is the abbreviation for the activation energy?
Ea
What are the relative positions of the products and reactants in a reaction profile diagram?
Products are higher than the reactants
What on the axis in a reaction profile diagram?
x axis - progress of reaction
y axis - enthalpy (kJ/mol)
How can enthalpy of a system be measured?
Cannot measure directly
This is irrelevant, enthalpy change is what matters
How can you find enthalpy changes?
Experimentally or using data books
What is to note about the enthalpy changes found in data books?
They are usually done under standard conditions
Why is it important enthalpy changes are measured in standard conditions?
Changes in enthalpy are affected by temperature and pressure
What is the standard enthalpy change of reaction ∆rHθ?
Enthalpy change when the reaction occurs at the molar quantities shown in the chemical equation
Under standard conditions
What is the standard enthalpy change of formation ∆fHθ?
Enthalpy change when 1 mole of a compound is formed from its elements in standard states
Under standard conditions
What is the standard enthalpy of combustion ∆cHθ?
Enthalpy change when 1 mole of a substance is completely burned in oxygen
Under standard conditions
What is the standard enthalpy change of neutralisation ∆neutHθ?
Enthalpy change when an acid and alkali react together to form 1 mole of water
Under standard conditions
What is required when calculating enthalpy changes in experiments?
Number of moles of the reactants
Change in temperature of the reaction
When is the heat change the same as the enthalpy change?
Under constant pressure
How can you experimentally measure temperature when you have solids and liquids?
Mix them and measure the temperature change using a thermometer
What is the experimental procedure for determining enthalpy change when mixing two liquids?
Add set volume of liquid to a polysterene cup and put lid on with a hole for a thermometer
Measure temp after 30s to stabilise
Add known amount of solid/liquid
Measure temp every 30s until reached max or min temp and has been returning to normal for a few minutes
Draw graph of temperature against time
Extrapolate line from where reaction is returning, back to original temp when added
Read of temp when reaction starts, and calculate change in temp
Then use to find enthalpy test
What is done when the enthalpy of reactions involving gases is being experimentally calculated?
Calorimeter used to calcualte how much heat is given out by reaction
Measures the temperature change of water around it
Which reactions are usually involving gases for enthalpy calculations?
Combustion
What is the set up for a calorimeter?
Spirit burner with known volume of fuel
Underneath a container with a known volume of water present
Container should be insulated and stirrer may be present
Thermometer should be placed in the water
How does combustion calorimtetry mean enthalpy can be measured?
Fuel burning heats the water
Calculate heat absorbed by water
USe to calculate enthalpy changes
What is the equation for calculating the heat energy lost/gained?
q = mc∆T
q - heat lost/gained (J)
m - mass of water in calorimeter or solution in container (grams)
c - specific heat capacity of water (4.18 J/g/K)
∆T - change in temperature of water (K)
What is the specific heat capacity of water?
4.18 J g-1 K-1
What is the specific heat capacity of water?
Amount of energy required to raise the temperature of 1 gram of water by 1 K
Why do you not have to convert between ºC and K for ∆T?
The change is the same
What can be done after calculating the heat energy gained/lost from q=mc∆T?
Find the moles of fuels which caused this change
Divide heat energy (q) by the moles
Value calculated is the enthalpy change of combustion
What is Hess’s Law?
Total enthalpy change of a reaction is always the same, no matter which route is taken
When is Hess’s law especially important?
Calculating enthalpy changes which cannot be measured directly by an experiment
What is the ∆fHθ of elements?
Always 0
Is breaking bonds endo or exothermic?
Endothermic as energy required to break bonds
∆H is +ve
Is forming bonds endo or exothermic?
Exothermic as energy is released
∆H is -ve
What is the ∆H of a reaction based on in terms of bonds?
Overall affect of energy required to break bonds and the energy released when new ones form
What is bond enthalpy?
The energy required to break 1 mole of a type of bond in a molecule in the gas phase
What keeps molecules together in ionic and covalent bonding?
Ionic - ions are electrostatically attracted to each other
Covalent - nuclei are attracted to shared pair of electrons
Are bond enthalpies always the same?
No
Range of values depending which compound they are found in
What is the mean bond enthalpy?
Energy needed to break 1 mole of bonds in the gas phase, averaged over many different compounds
What values related to bond enthalpies are found in data books?
Mean bond enthalpies
What is the ∆H of a reaction in terms of bonds?
Enthalpy change of reaction = Sum of bond enthalpies of reactants - Sum of bond enthalpies of products
What occurs to the enthalpy change of combustion when you blow out the flame of the water?
Blows away unburnt fuel
More evaporation so mass loss greater than true amount that has undergone combustion
Calculated value for enthalpy of combistion will be less negative than true value
