Question | Answer |
Endothermic | The system GAINS energy from the Surroundings (energy is taken in). Delta H is positive. |
Exothermic | The system LOSES energy to the surroundings (energy is given out), Delta H is Negative. |
When Bonds break Energy is: | Taken into the system (Endothermic). |
When Bonds Form energy is: | Given out from the system (Exothermic). |
Enthalpy Change: | The measure of heat energy change (Delta H) under constant pressure. |
Standard Enthalpy of Combustion: | The Energy released by one mole of a substance when it burns completely in Oxygen. |
Delta H is positive: | Endothermic Reaction (heat taken into the system). |
Delta H Is negative: | Exothermic Reaction (Heat given off from system). |
In an Enthalpy Level Diagram, in terms of stability, what are the products, if they are at a lower level of Enthalpy? | They are more stable, Energy has been given out from the system as stronger bonds have been formed. (exothermic Reaction) |
In an Enthalpy Level Diagram, in terms of stability, what are the products, if they are at a higher level of Enthalpy? | They are less stable, energy was added to the system in order to break bonds. (Endothermic Reaction). |
Standard enthalpy of Combustion: | The change in Enthalpy when one mole of a substance is completely burnt in Oxygen under standard conditions, with the reactants and products in their standard states. |
Standard Enthalpy of Formation: | Is the Enthalpy change when one mole of a substance is formed from its basic elements under standard conditions, with the reactants and products in their standard states. |
Specific Heat Capacity: | The energy required to raise the temperature of 1g of a substance by one degree K/C. Measured in Joules per Gram per Degree. |
Change in Heat (q) = | Mass (m) x Specific Heat Capacity (c) x Change in Temp (Delta-T). Measured in Degrees C/K. |
Temperature: | The average Kinetic Energy of the particles in a System. It is independent of how much of a substance there is. |
Heat: | The Total energy of a given amount of a Substance. |
q = | m x c x (delta)T |
A Calorimeter: | An instrument used to measure the heat changes which accompany chemical changes. E.g. Burning a fuel. |
Hess's Law: | Change in Enthalpy of a Reaction = Change in Enthalpy of Formation of Products - Change in Enthalpy of Formation of Reactants. |
Enthalpy of Elements: | The Enthalpies of Elements in their standard states is taken to be 0. |
Mean Bond Enthalpy: | The average energy required to break a covalent bond for a species (eg C-H). As bond enthalpies are usually slightly different in different molecules. |
Bond Dissociation Enthalpy: | The enthalpy change required to break a covalent bond in all species in a gaseous state. |