Module 3: Section 2 Flashcards
Define Enthalpy change, and give its symbol and units of measurement.
Heat energy transferred in a reaction at a constant pressure.
Symbol : ΔH
Units : kJmol-1
What is a endothermic and exothermic reaction?
Exothermic: energy is released (bond-making)
Endothermic : energy is absorbed (bond-breaking)
In a enthalpy profile diagram, where does the arrow for
1. Ea
2. ΔH
get drawn?
Ea = From reactants line to the top of the arch ΔH = Between the reactants and products line.
Define standard enthalpy change of reaction.
Enthalpy change associated with a given reaction, under standard conditions.
Define enthalpy of formation.
Enthalpy change when 1 mol of a compound is formed from its constituent elements, under standard conditions.
Define enthalpy of combustion.
Enthalpy change when 1 mol of a compound is completely combusted in oxygen, under standard conditions.
Define enthalpy of neutralisation.
Enthalpy change when 1 mol of water is formed in a neutralisation reaction, under standard conditions.
What are standard conditions?
298K
100kPa
Define average bond enthalpy.
Energy needed to break 1 mol of bonds in the gas phase, averaged over many different compounds.
When calculating enthalpy changes with q=mcΔt what do they stand for?
Give the units of measurements for each.
q= heat lost or gained (J)
m= mass of surroundings (g)
c= specific heat capacity (4.18Jg-1K-1)
Delta t = change in temp of surroundings (K)
What does Hess’s Law State?
Enthalpy change of a chemical reaction is independent of the route taken.
Why can calculating the enthalpy of formation of hydrocarbons be difficult?
A mixture of hydrocarbons will likely be made, not the specific hydrocarbon required.
How do you calculate the enthalpy change of reaction?
ΔH formation of products
-
ΔH formation of reactants
How do you calculate:
- Bond enthalpy
- ΔC
- ΔF
- BERP
- CORP
- FOPR
When will a reaction take place between 2 particles
- They collide in the right direction (angle)
2. They collide with a minimum amount of kinetic energy
What is activation energy (Ea)?
Minimum amount of kinetic energy particles need to break bonds and start the reaction.
Give 5 factors that affect the speed of chemical reactions.
- Temperature
- Concentration
- Surface Area
- Pressure
- Catalyst
With a Boltzmann diagram, explain how temperature and pressure increase rate of reaction.
Temperature: Curve becomes lower and moves to the right. Larger proportion of molecules have an energy greater or equal to the Ea, so more particles can successfully react.
Catalyst: Ea line moves to the left, as the catalyst provides an alternative pathway of lower Ea. More particles have an energy equal to or greater than the Ea, so can react.
What labels are on the axis of a Boltzmann Curve?
x axis = Kinetic energy
y axis = No. of molecules
Define catalyst.
Substance that increases rate of reaction, by providing an alternative pathway of lower Ea, without itself being used up.
Do catalysts take part in reaction?
Yes, but they are remade at the end.
Give an example of a reaction where a catalyst is used.
Haber Process: N2 + 3H2 —-> 2NH3
Catalyst = Fe
Define:
- Heterogenous catalyst
2. Homogenous catalyst
- Catalyst is in a different state to the reactants and products. (Haber Process)
- Catalyst is in the same state to the reactant and products
Give some benefits of using a catalyst.
Lower production costs (reduce temp…)
More product in the same time
Fossil fuel reservoirs are preserved
Less CO2 released
What is the equation for a catalytic converter?
2CO + 2NO —-> 2CO2 + N2
What is the equation for rate of reaction?
Change in conc / time
OR
y / x
Give 3 properties of a dynamic equilibrium
- forward reaction and reverse reaction occur at the same rate
- quantities of reactants and products stops changing
- only occurs in a closed system
What does Le Chateliers Principle state?
When a system is in dynamic equilibrium and is subject to an external change in condition, the position of equilibrium will shift to minimise the change.
Using Le Chateliers Principle, what will happen if you increase the conc of the reactants in a equilibrium reaction?
equilibrium shifts to the right hand side, to decrease the concentration of the reactants,
Using Le Chateliers Principle, what will happen if you increase the pressure of a equilibrium reaction.
Equilibrium will shift to the side with fewer mols, to decrease the pressure and minimise the change.
Using Le Chateliers Principle, what will happen if you increase the temperature of a equilibrium reaction, with: DeltaH = -92 kJmol-1
Equilibrium will try to decrease the temperature, by absorbing the extra energy, by moving in the endothermic direction. (LHS)
What affect does a catalyst have on the position of equilibrium?
Catalysts have no affect on the position of equilibrium, they only reduce the time taken for equilibrium to be established.
Give the reaction for the formation of ethanol, and state the condition needed.
C2H4(g) + H2O(g) —-> C2H5OH(g)
Ethene + steam ——–> Ethanol
Conditions:
60-70 atms
300 degrees celsius
Phosphoric (V) acid catalyst
What is the equilibrium constant for the following reaction:
aA + bB ——-> cC + dD
Kc = [D]^d [C]^c
—————-
[A]^a [B]^b
On an enthalpy profile diagram, what are the axis labels?
X axis = Reaction pathway
Y axis = Enthalpy kJmol^-1
