Unit 1 Section 2 Energetics Flashcards
What is enthalpy change?
The heat energy transferred in a reaction at constant pressure.
What are the units for enthalpy change?
KJ /Mol
What does ΔH mean?
That is the enthalpy change.
What does ΔH° mean?
The elements were in their standard states and measurements were made under standard conditions.
What are standard conditions?
100 kPa of pressure
a temperature of 298K
Substances in their standard states.
Solutions with a concentration of 1 mol dm⁻³
Is oxidation exothermic or endothermic ?
Exothermic.
What are exothermic reactions?
They give out energy. ΔH is negative.
What are endothermic reactions?
The reaction absorbs energy. ΔH is positive. The temperature of the surroundings will fall.
Name some endothermic reactions.
Thermal decomposition and the main reactions of photosynthesis.
Name some exothermic reactions.
Oxidation of carbohydrates and combustion of fuel.
What do enthalpy profile diagrams show?
They show how the enthalpy(energy) changes during reactions.
The less enthalpy a substance has the………..it is.
More stable
What is activation energy?
The minimum amount of energy needed to begin breaking reactant bonds and start a chemical reaction.
What is the symbol for activation energy?
E”a”
Why are standard conditions important ?
Enthalpy can be affected by temperature and pressure.
What are the different types of standard enthalpy change?
Change of reaction, formation, combustion, neutralisation and atomisation.
Explain standard enthalpy change of reaction.
The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation under standard conditions and their standard states.
Explain standard enthalpy change of formation?
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.
Explain standard enthalpy change of combustion.
The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions.
Explain standard enthalpy of neutralisation.
The enthalpy change when 1 mole of water is formed from the reaction of an acid with an alkali under standard conditions. E.g.
H+ and OH- = H”2”O
What is Hess’s law?
The total enthalpy change is independent of the route taken.
What does ΔH°f mean?
The formation energy of a molecule.
What is the energy of formation of an element.
0 as the element is formed form the element.