3.1.4 Energetics Flashcards
What is an endothermic reaction?
Reactions that absorb energy. ΔH is positive.
What is an exothermic reaction?
Reactions that give out energy. ΔH is negative.
What is enthalpy change, ΔH?
The heat energy transferred in a reaction at constant pressure. The units of ΔH are kJ mol-!.
What is standard enthanlpy?
Enthalpy under standard conditions:
100kPa
298K
What is the definition of the Standard enthalpy of formation, ΔfH?
the enthalpy change when 1 mole of a substance is formed from its elements in their standard states under standard conditions, e.g. 2C + 3H2 + ½/02 →СНОН (add state symbols)
What is the definition of the Standard enthalpy of combustion, ΔcH?
The enthalpy change when 1 mole of a substance is completely burned in oxygen with all substances in standard states and under standard conditions, e.g. C2H4 + 3O2 → 2CO2 + 2H2O (write state symbols for substances under standard states)
What is Hess’s law?
The total enthalpy change of a reaction is independent of the route taken.
Is breaking and making bonds endothermic or exothermic?
You need energy to break bonds, so bond breaking is endothermic. Stronger bonds take more energy to break.
Energy is released when bonds are formed, so bond making is exothermic.
Stronger bonds release more energy when they form.
What is the effect of different environments on bond enthalpies?
The energy needed to break a bond depends on the environment it’s in. So you can only find the mean bond enthalpy.
What is the standard enthalpy of atomisation?
The enthalpy change when 1 mole of gaseous atoms is produced from its element in its standard state under standard conditions.
What is the standard enthalpy of neutralisation?
The enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions.
What are the sources of inaccuracy in the calorimetry experiment?
Heat loss to surroundings. To reduce effects, use a lid.
Heat loss to equipment
Incomplete reactions
Define mean bond enthalpy.
The amount of energy needed to break a covalent bond averaged over different compounds.
Define bond enthalpy.
The amount of energy needed to make or break a bond.
What is specific heat capacity?
The amount of energy required to raise the temperature of 1g a substance by 1K.
q = mc∆T
