Energetics 3.1.4 Flashcards
(34 cards)
Exothermic reaction
Release energy to the surroundings
Rise in temp
Internal energy of system decreases
Exothermic reaction
Release energy and heat to the surroundings
Rise in temperature
Internal energy of system decreases
△H<0
Endothermic reactions
Energy transferred from the surroundings to the system
Take in heat from the surroundings
Internal energy of system increases
△H>0
Enthalpy change
The heat energy change at constant pressure
Standard conditions
Pressure -100kPa
Temp -298K (25°C)
Conc. -1.0mol
Enthalpy change of a reaction symbol
△Hr⦵
Enthalpy of combustion
The enthalpy change when 1 mol of a substance is completely burned in oxygen under standard conditions, all reactants and products are in their standard states
e.g. C₂H₆ (g) + 3.5 O₂ → CO₂ + 3H₂O
Enthalpy of combustion symbol
△Hc⦵
Enthalpy of formation
The enthalpy change when 1 mole of a compound is formed from its constituent elements under standard conditions and all reactants and products are in their standard states
e.g H₂ (g) + 1/2 O (g) → H₂O (l)
Enthalpy of formation symbol
ΔHf⦵
The amount of energy in a system depends on
Temp
Mass
Material
What is the difference between heat and temperature
Temperature measures the average kinetic energy of particles in a system
Heat is a form of energy measured in joules
Heat energy (equation)
q=mc△T
q= heat energy (joules)
M= mass of substance who’s temp is being changed (g)
△T= temp change
C= specific heat capacity
What is calorimetry
Determining heat absorbed, evolved or transferred
Enthalpy change of combustion practical method (simple, mark scheme)
- Weigh spirit burner before and after combustion
- Put water in a calorimeter/beaker
- Measure volume of water
- Burn the alcohol to heat the water
- Measure temperature rise in water
Identify sources of error in the practical to find enthalpy of combustion
Heat loss to surroundings
Incomplete combustion
Evaporation of alcohol
Heat absorption by the apparatus
Inadequate stirring
Measuring enthalpy change method (RP2) (not shortened yet)
- Measure mass of anhydrous copper (II) sulfate (3.90-4.10g)
- Put 25cm³ of deionised/distilled water in a polystyrene cup
- RECORD INITIAL TEMP
- Start timer and record the temp every minute for 3 mins
- At minute 4 add copper sulfate (don’t record temp)
- Stir constantly
- At minute 5 continue recording temp every minute
- Record results in a graph and extrapolate lines to find temp change at minute 4
Errors in Measurement of enthalpy change practical
Heat transfer to surroundings
Heat absorbed by apparatus is ignored in calculations
Reaction may be incomplete
Desnisty of solution is assumed to be the same as water
Incomplete combustion (if using a fuel)
How to increase accuracy of measurement of enthalpy change practical
Stir liquid and solution constantly
Add powdered copper sulfate solution rapidly
Use lid on cup to minimise heat loss
Electric temperature sensor and data logger
Enclosed flame
Fuel burning in pure oxygen instead of air
Why may it not always be possible to measure enthalpy change of a reaction directly
High Activation energy
Slow reaction rate/incomplete reaction
More than 1 reaction taking place
Hess’ Law
The total enthalpy change for a chemical reaction is independent of the rate of the route by which the reaction takes place, provided the initial and final, conditions are the same
Hess’ law using enthalpy of formation
A+B=C
