Reactivity 1.2 Flashcards
What sort of process is bond breaking?
Endothermic
What sort of process is bond forming?
Exothermic
In terms of bond breaking and forming, describe when a reaction would be exothermic.
The energy released from forming new bonds is greater than the energy needed to break existing bonds
In terms of bond breaking and forming, describe when a reaction would be endothermic.
The energy needed to break existing bonds is greater than the energy released from forming new bonds
To use average bond enthalpies to determine an enthalpy change of reaction, which state must all reactants and products be
Gas
Describe how to calculate the enthalpy of reaction using bond enthalpy values
The sum of the energies needed to break bonds in the reactants minus the energy released when the new bond are formed in the products
Define bond enthalpy
The enthalpy change needed to break one mole of bonds in a gaseous molecule averaged over similar compounds
Explain why experimentally determined enthalpy of reaction values are always different from theoretical enthalpy of reaction values calculated using bond enthalpy data
Bond enthalpy values in the data booklet are averages of that bond in many different molecules, whilst the experimental data will be determined using a specific molecules.
Explain why the bond enthalpy values in the data booklet are averages
They are an average value of the bond enthalpy for the same bond in different environments or molecules
Determine the enthalpy required to break all the bonds in 1.00 mol of nitrogen molecules
(+) 945 kJ mol⁻
Determine the enthalpy released when forming all the bonds in 1.00 mol of nitrogen molecules
(-) 945 kJ mol⁻
Determine the enthalpy required to break all the bonds in 1.00 mol of oxygen molecules
(+) 498 kJ mol⁻
Determine the enthalpy released when forming all the bonds in 1.00 mol of oxygen molecules
(-) 498 kJ mol⁻
Determine the enthalpy required to break all the bonds in 1.00 mol of carbon dioxide molecules
(+) 1608 kJ mol⁻
Determine the enthalpy released when forming all the bonds in 1.00 mol of carbon dioxide molecules
(-) 1608 kJ mol⁻
Determine the enthalpy required to break all the bonds in 1.00 mol of methane molecules
(+) 1656 kJ mol⁻
Determine the enthalpy released when forming all the bonds in 1.00 mol of methane molecules
(-) 1656 kJ mol⁻
Determine the enthalpy required to break all the bonds in 1.00 mol of propene molecules
(+) 3444 kJ mol⁻
Determine the enthalpy released when forming all the bonds in 1.00 mol of propene molecules
(-) 3444 kJ mol⁻
Using section 12 of the data booklet, determine the total enthalpy required to break all the bonds in the following reactants
(+) 1898 kJ mol⁻
Using section 12 of the data booklet, determine the total enthalpy required to break all the bonds in the following reactants
(+) 2582 kJ mol⁻
Using section 12 of the data booklet, determine the total enthalpy required to break all the bonds in the following reactants
(+) 6494 kJ mol⁻
Using section 12 of the data booklet, determine the total enthalpy released to form all of the bonds in the following products
(-) 1997 kJ mol⁻
Using section 12 of the data booklet, determine the total enthalpy released to form all of the bonds in the following products
(-) 2385 kJ mol⁻
Using section 12 of the data booklet, determine the total enthalpy released to form all of the bonds in the following products
(-) 8528 kJ mol⁻
Using section 12 of the data booklet, calculate the overall enthalpy change of the following chemical reaction
- 99 kJ mol⁻ - MUST INCLUDE THE NEGATIVE SIGN
Using section 12 of the data booklet, calculate the overall enthalpy change of the following chemical reaction
(+) 197 kJ mol⁻ - MUST INCLUDE THE POSITIVE SIGN
Using section 12 of the data booklet, calculate the overall enthalpy change of the following chemical reaction
- 2034 kJ mol⁻ - MUST INCLUDE THE NEGATIVE SIGN
A forward reaction has an enthalpy change of +244 kJ mol⁻, what is the enthalpy change for the reverse reaction?
-244 kJ mol⁻ - MUST INCLUDE THE NEGATIVE SIGN
What is the first thing you should do when calculating the total bond enthalpy for a molecule
Draw out the molecule including all of the bonds
If a substance in a chemical reaction has a coefficient greater than 1, how would you calculate the total bond enthalpy for the substance
Total bond enthalpy for one molecule of the substance multiplied by the coefficient
When counting the bonds in a molecule, how can you ensure you do not forget to count a bond
Tick off each bond on the molecule drawing once it has been counted
Describe the relationship between bond enthalpy and bond length
The shorter the bond length, the higher the bond enthalpy
Describe the relationship between bond enthalpy and bond polarity
The higher the bond polarity, the higher the bond enthalpy
Define Hess’s Law
The overall enthalpy change for a chemical reaction is the same independent of the route taken as long as the initial and final conditions are the same.
Construct an equation to use Hess’s law to determine ΔH
ΔH = ΔH1 + ΔH2 + ΔH3
Construct an equation to use Hess’s law to determine ΔH1
ΔH1 = AH - ΔH3 - ΔH2
Construct an equation to use Hess’s law to determine ΔH2
ΔH2 = -ΔH1 + ΔH - ΔH3
Construct an equation to use Hess’s law to determine ΔH3
ΔH3 = -ΔH2 -ΔH1 + ΔH
Construct an equation to use Hess’s law to determine ΔH2
ΔH2 = ΔH1 - ΔH3
Construct an equation to use Hess’s law to determine ΔH5
ΔH5 = -ΔH4 + ΔH1 - ΔH6
When using Hess’s law describe what you must do to the enthalpy change if you are moving in the opposite direction to the reaction arrow
Change the sign OR Change from + to - and vice versa
Define the standard enthalpy of formation
The enthalpy change when 1 mole of a substance is formed from its constituent elements in the standard states under standard conditions
State the standard enthalpy of formation for any element
0
State the standard enthalpy of formation for hydrogen gas
0
Describe how to use standard enthalpy of formation data to calculate the standard enthalpy of reaction
The sum of the standard enthalpies of formation of the products minus the sum of the standard enthalpies of formation of the reactants.
Construct a balanced symbol equation to represent the standard enthalpy of formation of 1.00 of water
H₂ (g) + 1/2O₂ (g) –> H₂O (l)
Construct a balanced symbol equation to represent the standard enthalpy of formation of 1.00 of benzene
3H₂ (g) + 6C (s) –> C₆H₆ (l)
Construct a balanced symbol equation to represent the standard enthalpy of formation of 1.00 of ethanol
2C (s) + 1/2O₂ (g) + 3H₂ (g) –> C₂H₅OH (l)
When constructing a balanced symbol equation to represent the standard enthalpy of formation, what should the coefficient of the product be?
1
When constructing a balanced symbol equation to represent the standard enthalpy of formation, how should you represent 1 H atom as a reactant?
1/2 H₂ or 0.5H₂
