# Topic 8 - Energetics I Flashcards

What are the standard conditions for a reaction?

- 298K
- 100kPa

What is the definition of the Standard Enthalpy change of Formation?

The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions of 100kPa and a specified temperature (normally 298K)

What is the definition of the Standard Enthalpy change of Combustion?

The enthalpy change when one mole of a compound is completely burned in excess oxygen, converted to standard conditions of 100kPa pressure and a specified temperature

What is the definition of the Standard Enthalpy change of Reaction?

The enthalpy change when the chemical equation molar quantities react under standard conditions and in their standard states

What is the definition of the Standard Enthalpy change of Neutralisation?

The enthalpy change when the acid and alkali, in the equation for the reaction, neutralise each other to form one mole of water

What is the definition of specific heat capacity?

The energy required to raise the temperature of one gram of a substance by 1K without a change in state

What is the equation for the energy change of a substance in a Calorimetry experiment?

Q = mc(delta)T

m - mass of water/solution (g)

c - specific heat capacity of water

(delta)T - temperature increase or decrease caused by reaction (K)

What must we do when using the Q = mc(delta)T equation in Calorimetry calculation questions?

Ensure the units are all in the correct form

What is the equation used to calculate enthalpy change?

(delta)H = Q/n

(delta)H - enthalpy change (Kjmol^-1)

Q - energy change (Kj)

n - number of moles of the limiting reagent

What units is Q in when you use the equation Q = mc(delta)T?

Joules, J

What does the n in the equation (delta)H = Q/n stand for?

Number of moles of the limiting reagent (the reagent NOT in excess)

What do you have to do between using the equation Q = mc(delta)T and (delta)H = Q/n?

Convert your value of Q from Joules, J, to Kilojoules, Kj

What must your value of enthalpy change always have?

a sign, + or -, to indicate whether the reaction is exothermic or endothermic

What sign does your enthalpy change value have when the reaction is exothermic?

-ve , energy is released to the enivronment so your products have less energy than your reactants

What sign does your enthalpy change value have when the reaction is endothermic?

+ve, energy is taken in from the environment so your products have more energy than your reactants

What is the definition of the limiting reagent?

The reagent which is not in excess and, therefore, limits how far the reaction can proceed

What are the three approximations that we make when doing calorimetry calculations?

- The mass = mass of solution (the value of m you use in the equation)
- The density of water = density of your solution, 1gdm^3, therefore, the volume of your solution will equal its mass
- The specific heat capacity of water = specific heat capacity of the solution

What does the value of m in the equation Q = mc(delta)T stand for?

The mass of your solution, it DOES NOT include the mass of any solid added

What is the specific heat capacity of water?

4.18Jg^-1K^-1

What is your value of m in the equation equation Q = mc(delta)T when you have two solutions?

The sum of the two solutions

What do you have to do when you are not given which solution/substance is in excess in a Calorimetry calculation question?

You need to figure out the reacting moles of each reactant:

- If the reactants have the same number of moles, you use that number

- If the reactants have different numbers of moles, then you use the lower value (as this is the limiting value)

What are the axis for an energy level diagram?

X-AXIS - Progress of Reaction

Y-AXIS - Energy

What are the axis for a reaction profile?

X-AXIS - Progress of Reaction

Y-AXIS - Energy

What is the difference between an energy level diagram and a reaction profile?

ENERGY LEVEL DIAGRAM

- Shows the energies of the products and the reactants

- Shows the enthalpy change

REACTION PROFILE

- Shows the energies of the products and reactants

- Shows the enthalpy change

- Shows the activation energy (by a curve)