energetics Flashcards
what is bond energy
During a reaction, enthalpy changes take place because bonds are being broken and formed
Energy (in the form of heat) is needed to overcome attractive forces between atoms
Energy is released from the reaction to the surroundings (in the form of heat) when new bonds are formed
what are exothermic reactions
Bond forming
If more energy is released when new bonds are formed than energy is required to break bonds
heat is given out to the surroundings
and system gets colder
what are endothermic reactions
Bond breaking
surroundings get colder as energy taken in and system gets hotter
what are enthaply profile diagrams
An enthalpy profile diagram is a diagram that shows the energies of the reactants, the transition state(s) and the products of the reaction as the reaction proceeds ( called ‘extent of the reaction’ below)
The transition state is a stage during the reaction at which chemical bonds are partially broken and formed
The transition state is very unstable – a molecule in the transition state cannot be isolated and is higher in energy than the reactants and products
The activation energy (Ea) is the energy needed to reach the transition state
what is activation energy
We can define the activation energy as ‘the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction’
what is the enthalpy profile for an exothermic reaction
In an exothermic reaction, the reactants are higher in energy than the products
The reactants are therefore closer in energy to the transition state
This means that exothermic reactions have a lower activation energy compared to endothermic reactions
enthalpy change is negative
what is the enthalpy profile for an endothermic reaction
In an endothermic reaction, the reactants are lower in energy than the products
The reactants are therefore further away in energy to the transition state
This means that endothermic reactions have a higher activation energy compared to exothermic reactions
enthalpy change is positive
what is enthalpy
The total chemical energy inside a substance is called the enthalpy (or heat content)
When chemical reactions take place, changes in chemical energy take place and therefore the enthalpy changes
An enthalpy change is represented by the symbol ΔH (Δ= change; H = enthalpy)
An enthalpy change can be positive or negative
what is enthalpy change
a heat energy change that is measured at a constant pressure
total energy of surroundings remain constant
what are the values standard enthalpy changes
To be able to compare the changes in enthalpy between reactions, all thermodynamic measurements are carried out under standard conditions
These standard conditions are:
A pressure of 100 kPa
A temperature of 298 K (25 oC)
Each substance involved in the reaction is in its standard physical state (solid, liquid or gas)
To show that a reaction has been carried out under standard conditions
ΔH= the standard enthalpy change
what is calorimetry
process of measuring a energy change experimentally
Calorimetry is the measurement enthalpy changes in chemical reactions
A simple calorimeter can be made from a polystyrene drinking cup, a vacuum flask or metal can
what is specific heat capactiy
The energy needed to increase the temperature of 1 g of a substance by 1 oC is called the specific heat capacity (c) of the liquid
The specific heat capacity of water is 4.18 J g-1 oC-1
The energy transferred as heat can be calculated by:
what equation is used to find the heat energy give out
q= m c deltaT
q=heat given out
m= mass of water
delta T = temp rise in kelvin
c = specific het capacity
how to find enthalpy in terms of J or kJmol
e change per mole = q/moles
what is apparatus for reactions taking place in a solution- experiment
-insulated beaker and temp changed measured
-fast so max temp can be reached before it starts to cool
-specific heat capacity must be taken as 4.18 and usually use mass of water
-and stir
what is required for flame calorimeters
for combustion of substances
fuel is in bottle with a wick and it is burned so the heat is passed to the water which it heats
sph is used but in more accurate ones the heat capacity of flame can be determined
what are some main sources of error for calorimetry experiments
-heat capacity of metals not included in calcs
-heat lost to surroundings - place lid on cup to improve
-reaction could be incomplete or slow
-density of solution is taken to be the same as the water
are combustion reactions endothermic or exothermic
exothermic so negative value
are neutralisation reactions exo or endo
exo
what is Hess’s law
overall enthalpy changeat constant pressure for a reaction is independent of the route the reaction takes
what is Hess’s law built upon
the law of thermodynamics- energy cannot be created or destroyed only transferred bwt stores/ change form
- in a closed system- so energy present is always constant
why is Hess’s law better than calorimetry
-reduce energy dissipation to surroundings
-reduce human error
what can Hess’ law be used to find
the standard enthalpy change of a reaction from known standard enthalpy changes
what is the theory of Hess’ law
Calculating ΔHr from ΔHf using Hess’s Law energy cycles
The products can be directly formed from the elements = ΔH2
OR
The products can be indirectly formed from the elements = ΔH1 + ΔHr
The enthalpy change from elements to products (direct route) is equal to the enthalpy change of elements forming reactants and then products (indirect route)
Equation
ΔH2 = ΔH1 + ΔHr
Therefore,
ΔHr = ΔH2 – ΔH1
an example calcuation-enthaply
Calculating the enthalpy change of reaction
Calculate the ΔHr for the following reaction:
2NaHCO3 (s) → Na2CO3 (s) + CO2 (g) + H2O (I)
Step 1: Write the balanced equation at the top
Step 2: Draw the cycle with the elements at the bottom
Step 3: Draw in all arrows, making sure they go in the correct directions. Write the standard enthalpy of formations
Step 4: Apply Hess’s Law
Calculating average bond energies using Hess’s cycles
Bond energies cannot be found directly so enthalpy cycles are used to find the average bond energy
This can be done using enthalpy changes of atomisation and combustion or formation
The enthalpy change of atomisation (ΔHatꝋ ) is the enthalpy change when one mole of gaseous atoms is formed from its elements under standard conditions.
Eg. ΔHatꝋ [H2] relates to the equation:
½ H2(g) → H(g
what are standard enthalpy changes of formation
enthalpy change when one mole of a substance is formed from its constituent elements with all reactants and products in standard states under standard conditions
what are the standard states and standard condition
solid, liqud, gas
100 kPa and 25’C or 298K
how to calculate overall enthalpy change of formation
(sum of enthalpy change of formation of products)-(sum of enthalpy change of formation of reactants)
what is the standard enthalpy change of formation for all elements
zero
what is the standard enthalpy change of formation for all molecules
given from a databook form previous experiments
what is the standard enthalpy change of combustion
when 1 mole undergoes complete combustion under standard conditions
what is the formula of enthalpy of combustion
sum of enthalpy of reactants - sum of enthalpy of products
what do you have to make sure of when doing enthalpy calcs eg which equation do u use
look at the values given in the question and use the formula for that regardless of what u are asked to find
what are bond enthalpies
when a covalent bond is broken
are bond enthalpies endo or exo
endo as you are breaking bonds which means u need to put energy back in to break
what state must molecules be in for bond enthalpy
gas - must start and finish in gas state
what is the enthalpy change for a diatomic molecule
A-B(g) = A(g) + B(g)
what properties do bond enthalpies depend on
atomic size
nuclear charge
shielding by inner electrons
bond length
what is the definition for the enthalpy change of bond dissociation (bond energy)
enthalpy change when one mole of bonds of the same type are broken in gaseous molecules under standard conditions
what is the enthalpy change of bond dissociation of molecules that are the same
it will be the same eg hydrogen is always +436
why do datebooks provide mean bond enthalpies
the exact environment of the bonds will vary due to the atoms the bond has around them
what is the formula for bond dissociation
sum of enthalpy of bonds broken - sum of enthalpy of bonds made
break - make / react-products
what is helpful do to for bond enthalpy calcs
draw out the molecules and cross of individual bonds when done
what are the 2 types of calorimetry experiments
Enthalpy changes of reactions in solution
Enthalpy changes of combustion
what is the method for finding the enthalpy changes of reactions in solution rp2
Weigh out between 3.90 g and 4.10 g of anhydrous copper(II) sulfate in a dry weighing bottle. The precise mass
should be recorded.
2. Using a volumetric pipette, place 25 cm3 of deionised water into a polystyrene cup and record its temperature at the
beginning (t=0), start the timer and then record the temperature again every minute, stirring the liquid continuously.
3. At the fourth minute, add the powdered anhydrous copper(II) sulfate rapidly to the water in the polystyrene cup and
continue to stir, but do not record the temperature.
4. Reweigh the empty weighing bottle
5. At the fifth minute and for every minute up to 15 minutes, stir and record the temperature of the solution in the
polystyrene cup.
6. Plot a graph of temperature (on the y-axis) against time. Draw two separate best fit lines; one, which joins the points
before the addition, and one, which joins the points after the addition, extrapolating both lines to the fourth minute.
7. Use your graph to determine the temperature change at the fourth minute, which theoretically should have
occurred immediately on addition of the solid.
8. Using q= m x cp x T calculate energy change
= 20 x 4.18 x T
9. Calculate Hsolution by dividing q by number of moles of anhydrous copper(II) sulfate in mass added
The above method is then repeated using hydrated copper sulfate. The two Hsolution can then be used to calculate
the H for the enthalpy change of forming a hydrated salt as in the example above
to find the hydrated copper sulphate use same method but with 6.20g - 6.30g of CuSO4 and 24 cm3 of water
what are the errors in rp2
- energy transfer from surroundings (usually loss)
- approximation in specific heat capacity of solution. The method assumes all
solutions have the heat capacity of water. - neglecting the specific heat capacity of the calorimeter- we ignore any energy
absorbed by the apparatus. - reaction or dissolving may be incomplete or slow.
- Density of solution is taken to be the same as water.
how do u counteract temp results rp2
If the reaction is slow then the exact temperature rise can be difficult to obtain as cooling occurs simultaneously with the reaction
To counteract this we take readings at regular time intervals and extrapolate the temperature curve/line back to the time the reactants were added together.
We also take the temperature of the reactants for a few minutes before they are added together to get a better average temperature. If the two reactants are solutions then the temperature of both solutions need to be measured before addition and an average temperature is used.
how to process results rp2
draw graph and extrapolate back to 4th min
then connect inital temp line
and then find temp on y x=axis
then take this value away from intial to find temp change
method for Measuring Enthalpies of Combustion using Flame Calorimetry
fill copper can with water
record initial temp of water
measure and record mass of empty spirit burner and them fill with fuel and record mass
light wick and stir for set time or until set temp rise eg 50
record temp of water
record mass of fuel burner and fuel
subtract to find mass of fuel burnt
then calc enthalpy change
It is important that you record the starting temperature, and the final temperature in order to complete the calculations
calculate the moles, which will be used to convert Q to an enthalpy change in your calculations
what are the units of calorimetry
kJ mol -1
method of acid-alkali calorimetry calculation
-add vol of HCl to polysturene cup
-add vol of NaOH see temp that it was raised
-q =m x c x delta t
-use the combined mass of both liquids - add two volumes together as g cm-1 = vol
-then find mols
delta H = q/mol
what mass of substance can u use if not given a mass of molecule
the mass of water for q = mc delta t
