1.4 Energetics Flashcards
What is enthalpy change?
The amount of heat energy taken in or given out during any change in a system, provided the pressure is constant.
What is the system and surroundings in a chemical reaction?
The system is the chemicals; the surroundings are everything outside the chemicals.
In an exothermic reaction, is energy released or absorbed?
Released to the surroundings.
What is the sign of ∆H in an exothermic reaction?
Negative.
In an endothermic reaction, is energy released or absorbed?
Absorbed from the surroundings.
What is the sign of ∆H in an endothermic reaction?
Positive.
What are standard conditions for enthalpy change?
100 kPa, 298 K (25°C), 1 mol dm⁻³ solutions, all substances in standard states.
What is the standard enthalpy change of formation?
Enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.
What is the symbol for standard enthalpy of formation?
ΔfH.
What is the standard enthalpy change of combustion?
Enthalpy change when 1 mole of a substance is completely combusted in oxygen under standard conditions.
What is the symbol for standard enthalpy of combustion?
ΔcH.
What is the enthalpy of formation of an element in its standard state?
0 kJ mol⁻¹.
What is the formula for calculating energy change in a solution?
Q = m × c × ΔT.
What is the specific heat capacity of water?
4.18 J g⁻¹ K⁻¹.
What is assumed about the density of aqueous solutions?
Density is taken as 1 g cm⁻³ (same as water).
State Hess’s Law.
Total enthalpy change for a reaction is independent of the route taken.
How do you calculate ΔH using enthalpies of formation?
ΔH = Σ ΔfH products – Σ ΔfH reactants.
How do you calculate ΔH using enthalpies of combustion?
ΔH = Σ ΔcH reactants – Σ ΔcH products.
How do you calculate ΔH using bond energies?
ΔH = Σ bond energies broken – Σ bond energies made.
List three sources of error in calorimetric methods.
- Heat loss to surroundings. 2. Incomplete combustion or reaction. 3. Ignoring calorimeter heat capacity.
Why are experimental enthalpies of combustion lower than theoretical ones?
Due to heat losses and incomplete combustion.
What trend is seen in enthalpies of combustion in a homologous series?
They increase by a constant amount as the number of carbon atoms increases.
How do you find enthalpy change from experiment?
Calculate Q = m × c × ΔT, then divide by moles of limiting reagent, convert to kJ mol⁻¹ and assign correct sign.
