topic 8 -Energetics Flashcards
What is the definition of enthalpy change?
Enthalpy change,(Delta H) is the heat energy change in a reaction at constant pressure.
Unites kJ/mol
What happens in an exothermic reaction?
Exothermic reaction give out heat energy. Delta H is negative. In exothermic reactions the temperature often goes up.
What happens in an endothermic reaction?
Endothermic reactions absorb heat energy. Delta H is positive. In the endothermic reaction the temperature often falls.
What does an enthalpy level diagram for and exothermic reaction show?
In an exothermic reaction the reactants release energy to the surroundings so the products have less enthalpy than the reactants.
What happens in an enthalpy level diagram for an endothermic reaction?
In an endothermic reaction the reactants take in energy from the surroundings, so the products have more enthalpy than the reactants.
What is meant by the standard enthalpy change of a reactions?
It is the enthalpy change when the reaction occurs in the molar quantities shown in the chemical equations under standard conditions.
What is meant by the standard enthalpy change of combustion?
It is the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions in standard states.
What is meant by the standard enthalpy change of formation?
It is the enthalpy change when 1 mole of a compound is formed from its elements in standard states under standard conditions.
What is meant by the standard enthalpy change of neutralisation?
It is the enthalpy change measured at standard conditions when 1 mole of water is produced by the neutralisation of an acid with an alkali.
How do you use a calorimeter to find the enthalpy change of combustion?
You can use a calorimeter to find how much heat is given out by a reaction by measuring the temperature change of some water.
1. You burn the liquid as it burns it heats the water. You can then find the heat absorbed using Q=mcT
State Hess’s law?
The total enthalpy change of a reaction is independent of the route taken by the products to form the reactants.
Why can’t you find the enthalpy change of the thermal decomposition of calcium carbonate by measuring temperature change?
It’s an endothermic reaction, so you’d expect the temperature to fall. But you need to hear it up for the reaction to take place.
What is bond enthaply?
-is the enthalpy change when one mole of a bond in the gaseous state is broken.
Mean bond enthalpy?
- is the enthalpy change when one mole of a bond, averaged out over many different molecules, is broken
give two properties of propane that make it suitable for use as a fuel?
- releases thermal energy when it burns and ignites easily.
