Chemistry- Energetics and Enthalpy Flashcards
What is enthalpy?
The total energy contents of reacting materials.
Symbol = H
Give the equation for enthalpy change between the products and reactants.
Enthalpy change = Enthalpy of products - Enthalpy of reactants.
UNITS = KJmol^-1
What is an Endothermic reaction?
Energy level of the products is higher than the reactants so heat is taken in (absorbed) from the surroundings
What is an Exothermic reaction?
Energy level of the products is lower than the reactants so heat is given out (lost) to the surroundings.
Explain enthalpy change by physical states.
Whether the product and reactants are solid liquid or gas, affects the enthalpy change of a reaction.
Define enthalpy change
The term used to describe the energy exchange that takes place with the surroundings under standard conditions.
Explain in terms of bond energy an exothermic reaction.
The energy released on bond formation in the products is greater than that absorbed through breaking bonds in the reactants.
Energy of reactants > Energy of products
Explain in terms of bond energy an endothermic reaction.
The energy needed to be absorbed to break bonds is greater than the energy transferred to the surroundings as bonds are made in the products.
Energy of reactants < Energy of products
What is the equation to calculate the amount of energy transferred during a chemical reaction. (either burning a fuel or chemical reaction in a solution)
Q = MCΔT
Q= Heat energy transferred M= Mass of liquid used C= Specific heat capacity ΔT= The change in temperature of the liquid.
Define Enthalpy change of a reaction.
The enthalpy change when the amounts of reactants shown in the equation react under standard conditions to give the products in their standard states.
Define Enthalpy change of formation.
The enthalpy change when one mole of a substance is formed from its elements in their standard states.
Define Enthalpy change of combustion
The enthalpy change when one mole of the substance is burnt in excess oxygen under standard conditions.
What are the standard conditions for enthalpy change?
Pressure- 100KPa
Temperature- 298K (25 degrees)
The conditions required for all enthalpy calculations. All substances must also be in their normal/ most stable physical state.
What is Hess’s Law?
The enthalpy change of a reaction is independent of the route take. It depends only on the initial and final states of the reaction.
Define mean bond enthalpy
The average energy required to break the bonds over a range of different molecules.
