Topic 5 - Energy Changes in Chemistry Flashcards
(25 cards)
What is enthalpy change (ΔH)?
The difference in enthalpy between the reactants and the products.
What is the enthalpy change of solution (ΔH⁰ₛₒₗ)?
The enthalpy change when 1 mole of a substance is completely dissolved in water.
What is the formula for heat change in a reaction?
q = mcΔT, where:
m = mass of water/solution (g), c = specific heat capacity (usually 4.18 Jg⁻¹K⁻¹), ΔT = temperature change (K or °C)
How is enthalpy change (ΔH) calculated from q?
ΔH = −q / n, where n is the number of moles of limiting reagent.
What sign does ΔH have for exothermic reactions?
Negative (−)
What sign does ΔH have for endothermic reactions?
Positive (+)
What is Hess’s Law?
ΔH is independent of the path taken between reactants and products.
How is ΔH calculated using bond energies?
ΔH = Σ(bonds broken) − Σ(bonds formed)
Why is ΔH large for some reactions?
Due to large bond energy differences between bonds broken and formed.
What are standard conditions in chemistry?
298 K (25 °C) and 1.0 atm or 1.0 bar pressure.
What is the standard enthalpy change of reaction (ΔH⁰ᵣ)?
ΔH when reactants are completely converted to products under standard conditions.
What is the enthalpy change of neutralisation (ΔHₙₑᵤₜ)?
ΔH when an acid reacts with a base to form 1 mole of water.
What is the standard enthalpy change of combustion (ΔH⁰𝚌)?
ΔH when 1 mole of substance burns completely in oxygen under standard conditions.
What is the standard enthalpy change of solution (ΔH⁰ₛₒₗ)?
ΔH when 1 mole of substance is completely dissolved in water under standard conditions.
What is the standard enthalpy change of formation (ΔH⁰𝚏)?
ΔH when 1 mole of a compound is formed from its elements in their standard states.
What is the standard enthalpy change of atomisation (ΔH⁰ₐₜ)?
ΔH when 1 mole of atoms is formed from the element in its standard state.
Why is ΔH negative for combustion?
Because heat is released when substances burn in oxygen.
Why is ΔH positive for decomposition reactions?
Because heat is absorbed to break bonds.
What unit is used for enthalpy change?
kJ mol⁻¹
What does a high ΔH indicate about a reaction?
It involves strong bonds being broken or formed.
In calorimetry, what substance’s temperature is usually measured?
Water or the aqueous solution
What is specific heat capacity (c) of water?
4.18 J g⁻¹ K⁻¹
Why is q = mcΔT used in calorimetry?
To calculate heat energy transferred to or from water/solution.
How is ΔH affected if the reaction is not at standard conditions?
It may be different — enthalpy changes depend on temperature and pressure.
