Chapter 9-Enthalpy Flashcards
What is the heat energy reactions give out/take in called?
Enthalpy, H
How do we calculate enthalpy?
By looking at temperature changes of the surroundings-we can’t directly measure enthalpy
What are the 2 parts to measuring enthalpy?
The system and the surroundings
What are enthalpy changes calculated under?
Standard conditions denoted by the symbol ΔH⦵
What are the 3 standard conditions?
298K (5 more than room temp)
1atm
100kPa
What is ΔH?
H products - H reactants
What is ΔH measured in?
kJ (kilojoules)
When are reactions exothermic?
- H products < H reactants
- ΔH is -ve
- Heat loss from system to surroundings
When are reactions endothermic?
- H products > H reactants
- ΔH is +ve
- Heat gain from surroundings to system
What is the standard enthalpy change of reaction symbol and definition?
- ΔᵣH⦵
- Energy change associated with a given reaction, under standard conditions
What is the standard enthalpy change of formation symbol and definition?
- ΔfH⦵
- Energy change when one mole of a compound is formed from its constituent elements in their standard state, under standard conditions
What is the standard enthalpy change of combustion symbol and definition?
- Δ꜀H⦵
- Energy change when one mole of a substance is completely combusted under standard conditions
What are the 2 types of calorimetry?
Direct and indirect calorimetry
How does direct calorimetry work?
- By mixing zinc and excess copper sulfate solution in a polystyrene cup you can calculate the temp change with respect to the mass of zinc reacting
- You can plot the temperature on a graph and extrapolate the cooling curve to predict the max possible temperature at T=0 where the reactants were added
How does indirect calorimetry work?
- By using a spirit alcohol to combust an alcohol, you can calculate the heat energy transferred to a copper cup full of water for a certain mass of alcohol burnt
- Why in indirect calorimetry innacurate?
- Heat lost to the surroundings
- Non standard conditions
- Incomplete combustion
- Other random errors
What is the formula for calorimetry?
q = mcΔT
What do the letters mean in the formula for calorimetry?
- q = Heat exchange with surroundings (J)
- m = mass of water (g)
- c = specific heat capacity of water (4.18 Jg⁻¹K⁻¹)
- ΔT = temperature change
What is needed to be done to q before finding the ΔH of a substance?
It needs to be divided by 1000 to be in kJ
What is the formula for ΔH of a substance?
- q/n
n= no. of moles
What does bond breaking and making do with energy?
- Bond breaking takes in energy so is endothermic
- Bond making releases energy so is exothermic
What is the average bond enthalpy?
The mean energy needed for one mole of a given type of bond in the gaseous state, to undergo homolytic fission
What is ΔH in terms of bond enthalpies?
Σ [bond enthalpies of reactants]-Σ[bond enthalpies of products]
What is Hess’s law?
If a reaction can take place by more than one route and the initial and final conditions are the same, then the total enthalpy change for the reaction is independent of the route the reaction takes.
