Module 3 Section 2 Flashcards
Define enthalpy change
Enthalpy change (delta H) is the heat energy transferred in a reaction at constant pressure
Units are kJmol^-1
For what kind of reaction is the enthalpy change NEGATIVE
Exothermic
For what kind of reaction is the enthalpy change POSITIVE?
Endothermic reaction
Look up enthalpy profile diagrams
Textbook
Define standard enthalpy change of reaction, delta Hr
Enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions.
Define the standard enthalpy change of formation, delta Hf
Enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions.
Define standard enthalpy change of combustion
The enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions
What is bond breaking?
Endothermic (delta H is positive)
You need to take in energy to break bonds
What is bond making
Exothermic (delta H is negative)
Energy is released when bonds are formed
Why are bond enthalpies an average?
A certain bond has different bond enthalpies depending on where it’s placed in a molecule, so they use an average.
How do you find enthalpy changes in a lab
You can find the enthalpy of combustion of a flammable liquid
You do this by burning a known mass of water and working out the temperature change
You then use the moles of your fuel used and the energy transferred to the water (using mc(deltaT))
Look up Hess’s Law stuff
Textbook
What is collision theory?
A reaction between two particles won’t occur unless:
- they collide in the correct orientation
- they collide with a minimum amount of kinetic energy
What is the minimum amount of kinetic energy particles need to react called?
Activation energy
Look at Boltzmann distribution stuff
Textbook
How do catalysts help the rate of reaction?
Provides an alternative reaction pathway with a lower activation energy
It is chemically unchanged at the end of the reaction
What is a heterogeneous catalyst
One which is in a different phase/state to the reactants
Reaction happens on the surface of the catalyst
So increasing the surface area of the catalysts means more reactants can react at the same time
What is a homogeneous catalyst
They’re in the same physical state as the reactants
Usually an aq catalyst between 2 aq solutions
Works by forming an intermediate species. Reactants combine with catalyst to make it
Intermediate then reacts to form the products and reform the catalyst
What ways can you measure the rate of reaction?
- change in mass
- volume of gas given off
- colour changes
- changes in pressure
- changes in conductivity
What happens to the concentrations of reactants and products when dynamic equilibrium is reached?
They stay constant
What does it mean for the equilibrium of a reaction to have moved to the LEFT?
Means more of the stuff on the left (reactants) will be made
What does it mean for the equilibrium of a reaction to be on the RIGHT?
Means more of the stuff on the right (products) is being made
What is Le Chatelier’s Principle?
“If there’s a change in concentration, pressure or temperature, the equilibrium will move to counteract that change
What happens to equilibrium position when you increase the temperature of a reaction?
It will shift in the endothermic direction, to absorb the heat
The opposite occurs for the opposite situation
What happens to equilibrium position when you increase the pressure of a reaction?
Equilibrium shifts to the side where there are fewest moles of gas, thus decreasing the pressure
If you decreased the pressure, it would shift to where there are more moles of gas
What do catalysts do to equilibrium?
They increase the rate of both the forward and reverse reactions by the same amount
Look up equilibrium constant, Kc
Textbook
What is the general formula for calculating the equilibrium constant?
Concentration of products divided by the concentrations of reactants
If in the word equation there is more than 1 mole of something, you put the concentration of that compound to the power of that ratio
E.g. if it were 2H2O, you’d do [H2O]^2
How do you use the the equilibrium constant, Kc?
You can use it to estimate the position of equilibrium
The larger Kc is, the further to the right equilibrium is, and the more products there are than reactants
The lower Kc is, the further to the left equilibrium is, and the more reactants there are than products
