✅8 - Energetics I Flashcards
What is enthalpy change?
The heat energy change in a reaction at constant pressure
What are the units for enthalpy change?
kJmol-1
What are the standard conditions?
Elements in their standard states
100kPa
298K
What are exothermic reactions?
Give out energy, negative enthalpy change
What are endothermic reactions?
Take in energy, positive enthalpy change
What do enthalpy level diagrams show?
Overall relative change in a reaction
What does an enthalpy level diagram look like for an exothermic reaction?
Reactants higher than products
What does an enthalpy level diagram look like for an endothermic reaction?
Products higher than reactants
What do reaction profile diagrams show?
Enthalpy changes during a reaction
What is the activation energy?
The minimum energy needed for the reaction to occur
Why are standard conditions important?
Changes in temperature and pressure can affect enthalpy
What is the standard enthalpy change of reaction?
The enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation under standard conditions
What is the standard enthalpy change of formation?
The enthalpy change when one mole of a product is formed from its constituent elements in their standard states under standard conditions
What is the standard enthalpy change of combustion?
The enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions
What is the standard enthalpy change of neutralisation?
The enthalpy change when an acid and an alkali react to form one mole of water under standard conditions
How would you calculate an enthalpy change?
Mass of solution heated x specific heat capacity of solution x temperature change
What is Hess’ Law?
The total enthalpy change of a reaction is always the same, no matter which route is taken
What is the enthalpy change of formation for elements?
Zero
How can the enthalpy changes of reaction be worked out from enthalpy changes of formation?
Using the formation of the reactants and products, and the enthalpy change of reaction on top
How can enthalpy changes be worked out from enthalpy of combustion?
By using the combustion of both reactants and products and putting the unknown on top
How can indirect enthalpy changes be found?
By doing the required experiments and then using Hess’ Law
Bond breaking is…
…endothermic
Bond making is…
…exothermic
What is bond enthalpy?
The energy needed to break q mole of a type of bond in a molecule in the gas phase
How are mean bond enthalpies calculated?
By taking an average of all the different values fr a bond type
How can enthalpy changes be worked out from bond enthalpies?
Sum of bond enthalpies of reactants - sum of bond enthalpies of proudcts
