3.2.1 Enthalpy changes Flashcards
What are exothermic reactions (+examples)?
Reactions where energy is given out, bond making. The enthalpy change is negative.
1. Combustion of fuel: CH4 (g) + 2O2 (g) -> CO2 (g) +2H2O (l)
2. Oxidation of glucose: C6H12O6 (aq) + 6O2 (g) -> 6CO2 (g) + 6H2O (l) (+energy)
What are endothermic reactions (+examples)?
Reactions where energy is absorbed, bond breaking. The enthalpy change is positive.
1. Thermal decomposition of CaCO3: CaCO(3) (s) -> CaO (s) + CO2 (g)
2. Photosynthesis in leaves using energy from sunlight: 6CO2 (g) + 6H2O (l) -sunlight-> C6H12O6 (aq) + 6O2 (g)
construction of enthalpy profile diagrams to show the difference in the enthalpy of reactants compared with products
What is the activation energy?
The minimum energy input needed for the reaction to happen.
What are standard conditions?
Pressure of 100 kPa and a temperature of 298 K. It is represented by a surscript theta.
What is the standard enthalpy change of reaction?
ΔHr: The enthalpy change that occurs between molar quantities of reactants, as shown in their chemical equations, in their standard state, under standard conditions. (Definition not required)
What is the standard enthalpy change of combustion?
ΔHc: The enthalpy change when one mole of an element or compound reacts by complete combustion with excess oxygen under standard conditions.
What is the standard enthalpy change of formation?
ΔHf: The enthalpy change when one mole of a compound is formed from it’s elements under standard conditions with both compound and elements being in their standard states.
What is the standard enthalpy change of neutralisation?
ΔHr: The enthalpy change when one mole of water is formed in a neutralisation reaction.
(around -50kJ/mol)
Equation for enthalpy change:
ΔH= mcΔT/mol
What is the average bond enthalpy?
The average bond enthalpy term is the average amount of energy needed to break one moles of bonds homolytically in gaseous molecules, measured over a wide variety of different compounds.
(Definition not required)
What is Hess’ Law?
Hess’ law states that if a reaction can take place by more than one route, with the initial and final conditions being the same, the total enthalpy change is the same.
Why might we need to use Hess’ law?
The reaction could be too slow.
The activation energy could be very high.
There could be side reactions which means direct measurements can’t be taken.
Why might experimental values of enthalpy change be different from the actual values? (5)
- What you’re heating could evaporate, which means energy is being used to break the bonds.
- Water could be a product.
- Heat could be lost to the surroundings.
- Incomplete combustion is a different reaction with a different enthalpy change.
- Heat can be added due to the kinetic energy of stirring.
What is the specific heat capacity of water?
4.18
