Question | Answer |
What is enthalpy? | The heat released or absorbed in a reaction at constant pressure. |
What is the enthalpy value? | ΔH = ∑H products - ∑H reactants |
What are the certain standard conditions for determining enthalpy? | 1) Set temperature of 25°C
2) 1 mol of substance involved
3) Constant pressure of one atmosphere
4) Heat is measured in kJ
5) Solutions are 1M |
What is an exothermic reaction? | A reaction wherein energy is released. The enthalpy value is negative and there is a temperature rise in the surroundings. |
What is an endothermic reaction? | A reaction wherein energy is absorbed. The enthalpy value is positive and there is a temperature decrease in the surroundings. |
What is the standard enthalpy of reaction?
ΔH° | The heat evolved or absorbed when the molar quantity of the reactants have completely reacted under standard conditions. |
What is the standard enthalpy of formation?
ΔH°f | The heat evolved or absorbed when 1 mole of a compound is formed from its constituent elements under standard conditions. |
What is the standard enthalpy of combustion?
ΔH°c | The change when 1 mole of substance is completely burnt in excess oxygen under standard conditions. |
What is the standard enthalpy of neutralisation?
ΔH°neut | The heat evolved when 1 mole of hydrated hydrogen ions (H+)from an acid is neutralised by 1 mole of hydrated hydroxide ions (OH-) from an alkali. The ΔH°neut is always -57kJ. |
What is the standard enthalpy of solution?
ΔH°sol | The heat evolved or absorbed when 1 mole of solute is dissolved in sufficient solvent so that further dilution causes no further temperature change. |
What is the formula for energy/heat released? | Energy/heat released = 4.18 x ΔT x V
*Where ΔT is the initial - final temperature, V is the volume in mL and the energy/heat is given in joules (J) |
What is the formula for enthalpy? | ΔH = 4.18 x ΔT x V/1000 x n |
What assumptions are made in calculating the enthalpy value? | 1) it takes 4.18J to raise 1.0g of water to 1°C (specific heat capacity of water).
2) The density of water is 1g/mL
3) No is heat is lost or gained from the surroundings.
4) Any solution has the same heat capacity and density as water. |
What is an exothermic reaction in nature? | Respiration |
What is an endothermic reaction in nature? | Photosynthesis |