Enthalpy changes Flashcards
What is the definition of enthalpy change?
Heat energy change measured at constant pressure
What happens when bonds are formed?
Energy is released
Process is exothermic
Δ H is negative
What happens when bonds are broken?
Energy is absorbed
Process is endothermic
ΔH is positive
What is the definition of an exothermic reaction?
More energy is released in bond forming than is absorbed in bond breaking, resulting in an overall negative ΔH
What is the definition of an endothermic reaction?
More energy is absorbed in bond breaking than is released in bond forming, resulting in an overall positive ΔH
What is the definition of activation energy of a reaction?
Minimum energy required for a reaction to occur
What is the symbol for enthalpy change?
ΔH
What is the symbol for activation energy?
Ea
What are some examples of exothermic reactions?
Combustion
Test tube heating up after adding a metal to an acid
What are some examples of endothermic reactions?
Photosynthesis
Decomposition of carbonates
Cool packs
What is the standard condition for temperature?
298k
What is the standard condition for pressure?
100kPa
What is the standard condition for solution concentration?
1 moldm-3
What is the definition for the standard enthalpy of formation?
The formation of 1 mole of a compound from its elements
What is the definition for the standard enthalpy of combustion?
The complete combustion of 1 mole of a substance
What is the definition for the standard enthalpy of neutralisation?
The formation of 1 mole of water from neutralisation
What is the definition for the standard enthalpy of reaction?
The enthalpy change associated with a stated equation
What is the definition for the average bond enthalpy?
The breaking of 1 mole of bonds in gaseous molecules
Give of an example of the standard enthalpy of formation
C (s, graphite) +2H2 (g) —> CH4 (g)
Give of an example of the standard enthalpy of combustion
CH4 (g) + 2O2 (g) —> CO2 (g) + 2H2O (l)
Give of an example of the standard enthalpy of neutralisation
HCl (aq) + NaOH (aq) —> NaCl (aq) + H2O (l)
Give of an example of the standard enthalpy of reaction
N2 (g) + 3H2 (g) —> 2NH3 (g)
C2H6O (l) —> C2H4 (g) + H2O (l)
Give of an example of the average bond enthalpy
1/3 NH3 (g) —> 1/3 N (g) + H (g)
What equation is used to calculate the standard enthalpies of reactions?
q = mcΔT
Define the enthalpy of neutralisation
The energy change when 1 mol of water is created by the neutralisation of an acid with an alkali under standard conditions
Define the enthalpy of combustion
The enthalpy change when 1 mole of a substance in its standard state
reacts completely with oxygen under standard conditions
Why might the ΔcH value be less exothermic than the expected?
Heat loss to surroundings
Incomplete combustion
None standard conditions used
What would happen if a student repeated the enthalpy of combustion experiment with a lid on the beaker and with the same mass of fuel burned?
ΔT would be greater; because less heat is lost to surroundings
q would increase
ΔH would become more exothermic/negative
What would happen if the student repeated the enthalpy of combustion experiment with a supply of gaseous oxygen feeding the flame?
ΔT would increase; because the fuel burns with more complete combustion
q would increase
ΔH would become more exothermic/negative
What would happen if the student repeated the enthalpy of combustion experiment with a fresh beaker of water that had an initial temperature of 15° C?
ΔT would remain similar because n(fuel) and v(H2O) remain the same
q would stay the same
ΔH would stay the same
What would happen if the student reads the measuring cylinder incorrectly and added more than X grams of water, without realising their mistake and then heated using the same mass of fuel?
ΔT would decrease; more water is being heated by the same mass of
fuel
q would decrease
ΔH would become less exothermic/negative
What would happen if the student repeated the enthalpy of combustion with a beaker which had a layer of soot on the bottom surface of the glass?
ΔT would decrease because heat is absorbed by the soot
q would decrease
ΔH would become less exothermic/negative
Explain the method to calculating ΔrH for a solid and a solution using a cooling curve
Pipette 100cm3 of 0.5moldm-3 CuSO4 (aq) into a polystyrene cup
Weigh out an excess of magnesium
Start a stop-clock and take the temperature of the solution every 30s until the solutions temperature stays constant
Add the magnesium to the solution and stir the mixture
Record the temperature every 30 sec from 30 second after the addition of magnesium until the temperature has fallen for several minutes
Plot a graph of temperature against time
