3.1.4 Energetics Flashcards
Define Enthalpy
Heat content in a substance
Define Enthalpy Change (ΔH)
Change in heat energy (of a reaction) at a constant pressure
State the units for enthalpy change (ΔH)
kJ mol-1
What is meant by this symbol ⦵?
Means substances were in their standard states and measurement was under standard conditions
Name the standard conditions
- 1 atm (100 kPa)
- 298 K
Exothermic: ΔH is ______
negative
Endothermic: ΔH is ______
positive
Give 2 examples of exothermic reactions
- Neutralisation
- Combustion
Give 2 examples of endothermic reactions
- Thermal decomposition
- Photosynthesis
Sketch a graph to show an exothermic reaction
Sketch a graph to show an endothermic reaction
Why is bond breaking endothermic (ΔH = positive)?
Need energy to break bonds
Why is bond making exothermic (ΔH = negative)?
Energy released when bonds formed
If more energy is needed to break bonds than is released when bonds made, ΔH is _____
positive
(If it’s less = ΔH is negative)
Define standard enthalpy of formation (ΔfH⦵)
Is the enthalpy change when 1 mole of a compound is formed from its constituent elements under standard states & standard condition
Define standard enthalpy of combustion (ΔcH⦵)
Is the enthalpy change when 1 mole of a substance is burnt completely in oxygen, under standards states & standard conditions
Standard enthalpy of formation equation
K2Cr2O7(l)
2K(s) + 2Cr(s) + 7/2O2(g) → K2Cr2O7(l)
Standard enthalpy of combustion equation
CH4(g)
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
How do we use calorimetry to find how much heat is given by reaction?
By measuring temperature change
Describe how you can find the enthalpy of combustion of a flammable liquid using calorimetry
- Burn flammable liquid inside apparatus (e.g. calorimeter)
- Then can work out heat energy that’s been absorbed by water (heat given by fuel as it burns = be absorbed by water)
Why is it hard to get an accurate result when using calorimetry? Name 3 reasons
- Heat is always lost to the surroundings
- Combustion may be incomplete (less energy given out)
- Flammable liquids often volatile = lose some fuel to evaporation
Describe how you can use calorimetry to calculate the enthalpy change for a neutralisation reaction
- Add known volume of acid to insulated container (e.g. polystyrene cup) & measure temperature
- Add known volume of alkali and record temp. of mixture at regular intervals over period of time
- Stir solution to make sure it’s evenly heated
- Find temp change of experiment
- Use it to calculate enthalpy change of reaction
Calorimetry
Describe how you can use a graph to find an accurate temperature change
- During experiment, record temp at regular intervals
- Start a couple mins before reaction
- Plot graph of results
- Draw 2 lines of best fit
- 1 through points before reaction started
- 1 through points after it started
- Extend both lies so they both pass time when reaction started
- Distance between 2 lines at time reaction (before any heat lost) = accurate temp change (ΔT) for reaction
State what equation (& the units) you can use to calculate the enthaply change in calorimetry
State the equation used to calculate heat energy given out per mole
ΔH = q / moles of reaction
Define Hess’s Law
Total enthalpy change of a reaction is independent of the route taken
What is Hess’s law used for?
Working out enthaply changes that you can’t find directly by doing an experiment
When working out enthalpies changes of formation, what data do you need?
ΔfH⦵ for all reactants and products that are compounds
Value of ΔfH⦵ for all elements = ____
0
(∵ element’s being ‘formed’ from element = no change)
Enthalpies Changes of Formation
Enthalpies Changes of Combustion
2C(g) + 3H2(g) + 1/2O2(g) → C2H5OH(I)
What is bond enthalpy?
Energy required to break bonds
Define mean bond enthalpy
Enthalpy heat (energy) change required to break a covalent bond on average across different compounds
Draw the bonds between CO2
Draw the bonds between NH3
Draw the bonds between water
Draw the bonds between oxygen
Draw the bonds between nitrogen
Mean Bond Enthalpies
What formula do you use to calculate enthalpy changes?
Why are enthaply change values calculated from mean bond enthalpies less accurate than using Hess’s Law?
∵ use average values, enthalpy changes calculated using mean bond enthalpies aren’t exact
142 kJ mol-1
Using calorimetry to find the enthalpy of combustion of a flammable liquid
Suggest 3 general improvements you can do to reduce errors due to heat loss
- Put a draught shield around the apparatus
- Put a lid on the cup
- Reduce the distance between the flame and the beaker
- Use a copper calorimeter rather than a pyrex beaker
Explain why the bond enthalpy of a Cl-Cl bond is greater than that of a Br-Br bond (2)
- Bond is shorter or bonding pair closer to nucleus
- (Cl is a smaller atom)
- So attraction (between nucleus and) (to) bond pair is stronger
