3.2.1 Enthalpy changes Flashcards
including measuring enthalpy changes spec. ref. = 3.2.1, 2.1.3 and Hess's Law and enthalpy cycle questions spec. ref. = 3.2.1
what happens in an exothermic reaction
energy is released causing the temperature of the surroundings to increase
describe the enthalpy change profile of an exothermic reaction
Energy is released
SO products are lower in energy than reactants
SO enthalpy change (𝚫H) is negative
What happens in an endothermic reaction
Energy is absorbed causing the temperature of the surroundings to decrease
Describe the enthalpy change profile of an endothermic reaction
Energy is absorbed
SO products are higher in energy than reactants
SO enthalpy change (𝚫H) is positive
desribe the enthalpy change (𝚫H) arrow on an enthalpy profile
The arrow goes from the reactants to the products it will be up for endothermic (+ve) reactions and pointing down for exothermic reactions (-ve)
what is activation energy (Ea)
Activation energy is the minimum amount of energy required to start a reaction (by breaking bonds)
Describe the activation energy (Ea) arrow on an enthalpy profile
The arrow always goes up. it goes from the same level as the reactants to the top of the arch (therefore needing to be in the middle NOT on the reactant line)
What 2 equations are used to calculate 𝚫H
and in what order
Q = mc𝚫T
then 𝚫H = Q/n
energy = mass of water x specific heat capacity x the change in temperature
enthalpy change = energy divided by number of moles
what are the units for Q = mc𝚫T
Joules = grams x (J/g/℃) x ℃
what
what are the units for 𝚫H = Q/n
(KJ/mol) = joules x number of moles
what is the equation for 𝚫H using the enthalpy of the products and reactants
𝚫H = H(products) - H(reactants)
what is the law of the conservation of energy
energy cannot be created or destroyed
what is this symbol for 𝚫H⦵
the ⦵ should be smaller and in the top right (superscript)
standard enthalpy change
takes place under standard conditions
what are standard conditions
- components in their standard states
- standard conditions:
298K
1atm (101kpa)
1moldm⁻³
what is 𝚫H⦵r for
the r would normally be in the bottom right (subscript)
standard enthaply change of reaction
define standard enthalpy change of reaction
enthalpy change that accompanies a reaction in the molar quantities shown
what is 𝚫H⦵c for
the c would normally be in the bottom right (subscript)
standard enthaply change of combustion
define standard enthalpy change of combustion
enthalpy change when 1 mol of a substance completely reacts with O₂
what is 𝚫H⦵f for
the f would normally be in the bottom right (subscript)
standard enthalpy change of formation
define standard enthalpy change of formation
enthalpy change when 1 mol of a substance is formed from its elements in their standard states
what is 𝚫H⦵neut
the neut would normally be in subscript
standard enthalpy change of neutralisation
define standard enthalpy change of neutralisation
enthalpy change when an acid and a base react to form 1 mol of water
state Hess’s Law
The enthalpy change of any 2 route is the same as long as the initial and final states are the same
what does Hess’s Law do
Hess’s law provides a way of measuring 𝚫H indirectly