Physical 4: Energetics Flashcards
What is an endothermic reaction?
A reaction where energy is taken in
The products have more energy than the reactants
So enthalpy change is positive
E.g. thermal decomposition
What is an exothermic reaction?
A reaction where energy is given out
The reactants have more energy than the products
So enthalpy change is negative
E.g. Combustion or neutralisation
What is enthalpy change?
Heat change at constant pressure
What are standard conditions?
100kPa
298K
Define standard enthalpy of combustion.
The enthalpy change when one mole of a substance is burned completely in oxygen
In standard conditions
With all reactants and products in their standard states
Define standard enthalpy of formation.
The enthalpy change when one mole of a substance is formed from its constituent elements in standard conditions with all reactants and products in standard states
What is the difference between heat and temperature?
Heat is a measure of the total energy of all particles present in a given amount of a substance dependent on how many particles are present
Temperature is a measure of the average kinetic energy of particles in a system independent of how many particles are present
What is specific heat capacity?
The amount of energy needed to raise the temperature of 1g of a substance by 1K
What are the units for specific heat capacity?
Joules per gram per Kelvin
Jg-1K-1
What are the equations for calorimetry?
q = mcΔT
ΔH = q/moles in reaction
Where q = enthalpy change
m = mass of substance being heated
c = specific heat capacity
ΔT = change in temperature
A simple calorimeter was used to measure the enthalpy change of combustion of methanol.
CH3OH(l) + 3/2O2(g) —> CO2(g) + 2H2O(l)
0.32g(0.01moles) of methanol were burnt and the temperature of 200g of water rose by 4.0K Calculate the enthalpy of combustion.
Q = mcΔT
Q = mass of substance being heated x specific heat capacity x change in temp
Q = 200 x 4.18 x 4
Q = 3360J
ΔH = q/moles
ΔH = 3360/0.01
ΔH = 336,0000J
= -336kJmol-1 (negative because combustion is exothermic)
50cm3 of 1moldm3 hydrochloric acid and 50cm3 of 1moldm3 NaOH were mixed in an expanded polystyrene beaker. The temperature rose by 6.6K
The total volume of the mixture is 100cm3. Calculate the enthalpy of neutralisation.
Mass of solution is 100g because density of water and dilute solutions is 1gcm3
Q = mcΔT
Q = mass of total solution x specific heat capacity x change in temperature
Q = 100 x 4.18 x 6.6
Q = 2772J
Number of moles = concentration x volume / 1000
N = 1 x 50 / 1000
N = 0.05moles
ΔH = q/moles
ΔH = 2772/0.05
ΔH = 55440J = 55.44kJ
ΔH = -55.44kJmol-1 (negative because neutralisation reactions are exothermic)
How is a flame calorimeter better than a simple calorimeter?
Flame calorimeters have been improved to reduce heat loss:
- the spiral chimney is made of copper so has a high thermal conductivity and ensures uniform temperature distribution
- the flame is enclosed
- the fuel burns in pure oxygen rather than in air, makes incomplete combustion less likely
What material is usually used for a calorimeter?
Expanded polystyrene beakers
They are good insulators
Have low heat capacity so absorb very little heat
0.50g of zinc was added to 25cm3 of 0.20moldm3 copper sulphate solution. The temperature rose by 10K.
Calculate the enthalpy change.
CuSO4 + Zn —> ZnSO4 + Cu
Mass of copper sulphate solution = 25g
Q = mcΔT
Q = 25 x 4.18 x 10
Q = 1050J
Moles of zinc = 0.50/65.4 = 0.0076moles
Number of moles in CuSO4 solution = c x v / 1000
= 0.2 x 25 / 1000
= 0.005 moles
This means that zinc was in excess, so 0.005 moles of CuSO4 and zinc react, leaving 0.0026 moles of zinc behind
ΔH = q/moles
ΔH = 1050/0.005 = -210kJmol (negative because exothermic)
How would you use a thermochemical cycle to work out the enthalpy of formation?
How would you use a thermochemical cycle to work out the enthalpy of combustion?
How do you find enthalpy of formation from enthalpy of combustion?
