Energetics Flashcards
Definition of exothermic
Reaction in which the enthalpy of the products is less than the enthalpy of the reactants
Endothermic definition
A reaction in which the enthalpy of products is greater than the enthalpy of reactants
Write the equation for the enthalpy change of a reaction
(Enthalpy of products) - (enthalpy of reactants)
Draw the graph of the enthalpy change for an exothermic process
See notes
Draw the graph of the enthalpy change of an endothermic reaction
See notes
Describe the characteristics of an exothermic reaction
Enthalpy change is negative
Products have a smaller enthalpy than the reactants
Temp of surroundings rises due to energy given out to surroundings
Describe the characteristics of an endothermic reaction
Enthalpy change is positive
Energy is absorbed from surroundings
Temperature of surroundings falls
Products have a higher enthalpy than reactants
What does the value of H depend on
- The amounts of reactant used
- The physical states of reactants/products
- The temperature and pressure at which the reaction is carried out
Name the “standard conditions”
298K (20C) and 100kPa
Definition of standard enthalpy change
The change in heat energy at a constant pressure; measured at standard conditions
Definition of the standard enthalpy of a reaction
The enthalpy change when the number of moles of substances are as written in the equation
Definition of the standard enthalpy of formation
The enthalpy change when one mole of a compound is formed from its elements under standard conditions
Definition for the standard enthalpy of combustion
The enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions
What is the enthalpy of formation of an element
It is always 0
Definition of the standard enthalpy of neutralisation
The enthalpy change when one mole of water is produced in a neutralisation reaction under standard conditions
Draw the diagram for the enthalpy of combustion
See diagram on page 5
Write the equations used to find the standard enthalpy of combustion
See page 5
What do the letters stand for in the equation q=mc 🔼T
q = enthalpy change (J) m = mass of water/solution (g) c = specific heat capacity (JK^-1g^-1) 🔼T = change in temperature (K)
What is meant by “specific heat capacity”?
It is the energy required to raise the temperature of 1g of substance by 1K
What are the units for standard enthalpy changes?
KJmol^-1
The method for finding the enthalpy of neutralisation
- Measure 50cm^3 of HCl (1M) into a polystyrene cup
- Record the temp
- Add 50cm^3 of NaOH (1M)
- Attach the lid and stir
- Record max temp
- Use the two equations
The law of conservation of energy
Energy cannot be created or destroyed but it can change from one form to another
Hess’s Law
The enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same
Definition for average bond enthalpy
The energy required to break one mole of a given bond averaged over many compounds
Write the method for calculating the enthalpy of a reaction using bond energies
- Write a balanced equation using structural formulae
- List the bonds present in reactants and products
- List the bond energies (bond breaking in reactants is endo) (bond making in product is exo)
Why are values using bond energies different from Hess’s law?
Average bond enthalpies give an estimate of the value as they are not specific to the compounds involved in the reaction
What is an endothermic reaction in terms of bonds, is it positive or negative KJmol^-1
Bond breaking
Positive
What is an exothermic reaction in terms of bonds and is it positive or negative KJmol^-1
Bond making
Negative
