Energetics Flashcards
What is enthalpy
Enthalpy is defined as a thermodynamic property (specify state)
of a system linked to internal energy.
> It is represented by the capital letter H.
The enthalpy change is the heat energy change at constant pressure.
..
The enthalpy relates to
the energy of the bonds broken and made during a chemical reaction
• Some of the important points about enthalpy are listed below.
•
• Reactants - the enthalpy of reactants in a chemical reaction is given as H1
»_space;relates to energy of bonds in reactants.
• Products - the enthalpy of products in a chemical reaction is given as H2.
»_space;relates to energy of bonds in products.
•
• ΔH = the change in enthalpy.
• ΔH = enthalpy of the products - enthalpy of the reactants.
How is enthalpy change impacted by direction of reversible reaction
For an exothermic reaction ΔH is negative;
for an endothermic reaction ΔH is positive.
• Enthalpy changes can be measured and calculated but enthalpy cannot.
ΔH is measured under stated conditions.
> eg. AH29g is the enthalpy change
at a temp 298K and pressure of 100 kPa.
..
Some chemical reactions are reversible.
- For a reversible reaction the ΔH value for reversible reaction
- has same numerical value as the forward reaction
- but the sign (+/-) is changed.
This means that for exothermic reaction in the forward direction
- ΔH is negative,
but in the reverse direction the ΔH value has same value
- but it ΔH is positive.
What are reaction profiles
- The y axis is enthalpy but not ΔH.
- x axis is progress of reaction, reaction coordinate or extent of reaction.
1) Two horizontal lines are drawn and labelled with names/formulae of reactants and products.
2)They represent enthalpy of reactants (on the left) and enthalpy of products (on the right).
3) In an endothermic reaction, product line is at a higher enthalpy value than reactant line
> as reaction has absorbed energy.
4) in exothermic, product line is at a lower enthalpy value than reactant line
> as reaction has released energy.
5)The diff between these lines is labelled AH (change in enthalpy).
6)In endothermic reaction this has a + value.
In exothermic reaction this has a - value.
7)All values are in kJ mol-1.
If actual reactants and products are known, lines should be labelled with their names or formulae.
> instead if just reactants and products
DIAGRAM 1
What are Reaction pathways
The reaction pathway is shown as a line from reactants to products
on an enthalpy level diagram.
> represents route in terms of enthalpy from reactants to products.
-
- Reaction pathways require an input of energy to break bonds in the reactants
> before new bonds can form in products.
-
-This amt of energy is the max height of the pathway above enthalpy level of reactants. »_space;This is called the activation energy.
..
For an exothermic reaction, a typical labelled enthalpy level diagram
showing the reaction pathway wd look..
DIAGRAM 2
For endothermic…
DIAGRAM 3
What are Standard enthalpy values
Standard enthalpy values are the ΔH values
for enthalpy changes of specific reactions measured under standard conditions.
• Standard conditions are represented by the symbol ⦵.
> This symbol is used after ΔH to indicate that an enthalpy change occurs under standard conditions.
•
• Standard conditions are 100 kPa pressure and a stated temperature.
• There are three basic enthalpy changes.
• State symbols should always be included in chem equations to represent enthalpy changes.
What is the first enthalpy change - standard enthalpy of reaction
Standard enthalpy of reaction (ΔrHθ)
This is the enthalpy change
when substances react under standard conditions in quantities
given by the equation for the reaction.
For example:
CaO(s) + H2O(l) → Ca(OH)2 (s)
ΔrH°= -63.7kJmol-1
This means that when
-1 mole of calcium oxide
- reacts with 1 mole of water
- to form 1 mole of calcium hydroxide,
-> 63.7 kJ of heat would be released.
Whats the second enthalpy change - standard enthalpy of formation
Standard enthalpy of formation (ΔfHθ)
> This is the enthalpy change
when 1 mole of a compound is formed from its component elements
>with all reactants and products in standard states under standard conditions
For example:
Ca(s) + C(s) + 1.5O2(g) → CaCO3(s)
ΔfHθ= -1128.8 kJ mol-1.
»_space;This enthalpy change is per mole of calcium carbonate formed
Another example:
2C(s) + 2H2(g) → C2H4(g)
ΔfHθ= +52.5 kJ mol-1.
»_space;This enthalpy change is per mole of ethene formed.
..
The enthalpy of formation of an element is zero
- if the element is in its standard state.
—The enthalpy of formation of oxygen, O2(g), is zero.
Whats the third enthalpy change - standard enthalpy of combustion
Standard enthalpy of combustion (ΔcHθ)
> This is the enthalpy change when
- 1 mole of a substance is burned completely in excess oxygen
- with all reactants and products in their standard states under standard conditions.
For example:
CH4 (g) + 2O2(g) → CO2(g) + 2H2O(l)
- ΔcHθ for this reaction (per mole of methane burned) = -890 kJ mol-1
The enthalpy change for the reaction:
2CH4(g) + 4O2(g → 2CO2(g) + 4H2O(l)
- ΔrHθ = 2 x ΔcHθ = - 1780 kJ
Note that as with all standard combustion reactions and enthalpy values,
- the combustion must be per mole of substance burned
What are some important points on standard enthalpy of reaction, formation and combustion
• the standard enthalpy values of formation and combustion are kept to per mole of what they refer.
• If two moles of a fuel are combusted then the standard enthalpy of reaction
- is the standard enthalpy of combustion multiplied by 2.
• If four moles of a compound are formed from its elements in standard states,
-then standard enthalpy of formation value must be
-multiplied by 4 io get the standard enthalpy value for this reaction.
• The standard enthalpy of combustion of carbon (C(s) + O2(g) → CO2(g))
- has same value as standard enthalpy of formation of carbon dioxide.
- It is the same reaction and all the reactants and products are in standard states in both equations.
This also applies to the standard enthalpy of formation of water and the standard enthalpy of combustion of hydrogen (H2(g) + 0.5O2(g) → H2O(l)).
What are Experimental determination of enthalpy changes ⭐️
During a chemical reaction the enthalpy change in the reaction
- causes a change in temp of the surroundings.
- For many reactions, this change in temp can be measured using thermometer/temp probe.
-
- The energy released/absorbed from reaction
- can increase/decrease temp of a sample of water or solution in wch the reaction occurs.
..
Temperature change (ΔT) may be converted to energy change (q) using the expression:
q=mcΔT
q = change in heat energy in J
m = mass in g of the substance to wch the temp change occurs
> (usually water (combustion)/a solution)
c = specific heat capacity (energy required to raise temp of 1g of a substance by 1 °C)
ΔT = temperature change in °C or kelvin (K).
Calculating enthalpy change in a solution ⭐️
q = mcAT can be used to calculate enthalpy change
per mole of a substance wch dissolves in water to form a solution.
When an acid reacts with an alkali, a neutralisation reaction occurs.
> The enthalpy change of neutralisation reaction can be calculated per mole of water formed in reaction.
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These two types of calculations will be discussed in greater depth below.
- Calculating enthalpy of solution
- Some substances dissolve in water exothermically, others endothermically.
-
- eg. enthalpy of solution of sulfuric acid is very exothermic
- so dilution shd always be performed by adding acid to water, not water to the acid.
How is calculating enthalpy of neutralisation different
The enthalpy change that occurs during a neutralisation reaction when
> is a specific type of enthalpy change in solution.
-
- could be a strong or weak acid.
- enthalpy of neutralisation of a weak acid with strong base
- is less than a strong acid with a strong base
as some energy is used in dissociating acid fully
sodium hydroxide solution and ethanoic acid is -56.1 kJ mol-
sodium hydroxide and hydrochloric acid is -57.9 kJ mol-1 ; its more exothermic
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The enthalpy of neutralisation of a weak base with strong acid
is similar to weak acid with a strong base
•ammonia and hydrochloric acid is
-53.4 kJ mol-1).
»_space; a weak acid with a weak base is less exothermic (ammonia and ethanoic acid is -50.4 kJ mol-1)
SO The energy difference depends on how weak the acid or base actually is.
REQUIRED PRACTICAL
Measuring and evaluating the enthalpy change for the neutralisation of sodium hydroxide and hydrochloric acid
Sodium hydroxide can be neutralised by adding hydrochloric acid.
> This reaction is exothermic.
A student carried out an experiment to determine enthalpy of neutralisation for this reaction.
• Transter 25.0 cm of 1moldm-3 hydrochloric acid to a plastic cup.
• Record temp of the hydrochloric acid to 1dp.
• Transfer 25.0 cm3 of sodium hydroxide to a second clean, dry plastic cup
• and place plastic cup inside a beaker. Stir the sodium hydroxide with a therm and record the temp to 1 dp.
• Every minute for a further 3 minutes stir solution, measure the temp and record.
• At the fourth minute add 25.0 cm3 of hydrochloric acid from plastic cup. Stir mixture but dont record the temp
• Continue to stir mixture and measure the temp at fifth minute, and then every subsequent minute for a further 5 minutes.
Record each temperature in a table. Measure temp of hydrochloric acid at the start.
Whats Hess’s Law
The principle of conservation of energy states that
- energy cannot be created or destroyed,
- only changed from one form into another.
-
Hess’s Law states that
the enthalpy change for a chemical reaction is
>independent of route taken and depends only on initial and final states
Hess’s Law can be used to calculate enthalpy changes for chemical reactions
>from the enthalpy changes of other reactions.
> useful as some reactions cannot be carried out in reality,
»_space; but a theoretical enthalpy change can be determined for these reactions.
What are Bond enthalpies
The mean bond enthalpy is a measure of the energy required
> to break one mole of a covalent bond measured in gaseous state in kJ mol-1
>
> (the ‘per mole’ is per mole of the covalent bond) averaged across many compounds containing the bond.
For example, the mean bond enthalpy of a C-H bond is 412 kJ mol-1
- Bond breaking is endothermic
and bond making is exothermic.
-
- Breaking one mole of C-H bonds requires 412 kJ of energy so positive as endothermic.
• Making one mole of C-H bonds releases 412 kJ of energy so negative value as
exothermic.
What are General points for bond enthalpies
Mean bond enthalpies relate to the strength of a covalent bond.
> A higher bond enthalpy value means a stronger covalent bond.
• Generally the shorter the covalent bond, the stronger the bond.
• Triple covalent bonds are generally shorter than double wch are shorter than single.
•
•The C-C is the longest of the three carbon-carbon covalent bonds and weakest.
•The C=C is the shortest of the three bonds and strongest.
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The bond length and bond strength (expressed often as the mean bond enthalpy)
> are important in organic chemistry and the chemistry of the halogens.
> bond length increases down group as atomic radius of atoms involved in covalent bond increases.
> The strength of covalent bond decreases down group from Cl2 to I2.
>
> bond enthalpy of the F-F bond is lower as I2 as bond is very short
> are repulsions between lone pairs of electrons on F atoms
> lowers energy to break F-F bond.
The bond length and bond enthalpies for the C-X (halo) bonds
will be considered in halogenoalkanes.
Calculations involving mean bond enthalpies ⭐️
These involve calculations of enthalpy changes using mean bond enthalpies.
- enthalpy change for a reaction can be calculated ..
-
- The reactions are in the gas phase
so only covalent bonds are involved.
- In liquid or solid phase, intermolecular forces would also be involved.
ΔH = sum of mean bond enthalpies of bonds broken (reactants) - sum of mean
bond enthalpies of bonds made (products)
