4 - Energetics Flashcards
Define ‘enthalpy change’
The heat energy change measured under conditions of constant pressure
What units is enthalpy change (ΔH) measured in?
kJ mol-1
What are the standard conditions?
- 298K (25°C)
- 100kPa
- Solutions at 1 moldm-1
Reactions can be ____thermic or _____thermic
Exothermic or endothermic
What happens in an exothermic reaction?
Energy is transferred from the chemicals to the surroundings
- Enthalpy change is negative
In exothermic reactions, do the products have more or less energy than the reactants?
Less energy
What are some examples of exothermic reactions?
- Combustion
- Neutralisation
What happens in an endothermic reaction?
Energy is transferred from the surroundings to the chemicals
- enthalpy change is positive
In endothermic reactions, do the products have more or less energy than the reactants?
More energy
What are some examples of endothermic reactions?
- Thermal decomposition
Define standard enthalpy of formation
The enthalpy change when 1 mole of a substance is formed from its elements under standard conditions, all reactants and products being in their standard states
Formation example - magnesium chloride
Mg (s) + Cl2 (g) → MgCl2 (s)
Define standard enthalpy of combustion
The enthalpy change when 1 mole of substance is combusted completely in oxygen under standard conditions, all reactants and products being in their standard states
Combustion example - methane
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)
What does incomplete combustion produce?
- Soot (carbon)
- Carbon monoxide
- Water
Calorimetry
What equation do we use to measure the enthalpy change in solution?
Energy change = mass of solution x heat capacity x temperature change
q = m x c x ΔT
Calorimetry
What are the units of q = m x c x ΔT?
q = J
m = grams
c = J g-1K-1
ΔT = K
Calorimetry
What equation do we use to calculate the enthaply change per mole?
ΔH (per mole) = q / number of moles
What is Hess’s Law?
That the total enthalpy change for a reaction is independent of the route by which the chemical change takes place
What is meant by ‘mean bond enthalpy’?
The average enthalpy change when 1 mole of bonds in gaseous molecules are broken
How do we calculate the enthalpy change for reactions in the gaseous phase?
ΔH = ∑ bond energies broken - ∑ bond energies made
Why do values from mean bond enthalpy calculations differ from those determined by Hess’s law?
Mean bond enthalpy calculations use mean values while Hess’s law uses enthalpy data from individual compounds so therefore values calculated from Hess’s law will be more accurate
Bond breaking __________ energy
requires
Bond making ___________ energy
Releases
