Energetics I Flashcards
What does the first law of thermodynamics states?
That energy can neither be created nor destroyed however one form can be conversed into another. ( Law of conservation)
What is the SI unit of energy?
J - joules
What form is energy changes of chemical reactions typically expressed in?
Kilojoules. (1KJ = 1000J)
What is the formula to express amount of heat transferred and temperature changed?
Heat = m x c x 🔺T (T= Celsius/Kelvin)
What is specific heat capacity?
The heat required to increase the temperature of 1g of the substance by 1*c.
What is the specific capacity of water?
4.18 Jg^-1*c^-1
What happens in an exothermic reaction?
Chemical energy is converted into heat energy and the temperature of the system rises. (Result in immediate temp increase)
What happens in an endothermic reaction?
Heat energy is converted into chemical energy and the temperature of the system falls (Decrease in temp mixture)
What is enthalpy change ?
Is the heat energy change measured at constant pressure
Within energetics all the chemical reactions are carried out in a container what does this mean?
That the reactions takes place at constant pressure (1atm or 100kPa)
If a reaction produces heat it is ….
Exothermic
If in a reaction a mixture cools ….
It is endothermic
What is a change in the enthalpy of chemicals caused by?
The release or absorption of heat energy by a system at constant pressure.
Can absolute enthalpy value be found if not then what?
No only value that can be measured is the difference in enthalpy when reaction/physical change takes place
Is the enthalpy of product different from enthalpy of reactant in a chemical change?
Yes
ΔH° is given by the formula …
H product - H reactant
In an exothermic reaction , if enthalpy is converted into heat energy is ΔH (+/-)
Negative - H products is less than H reactant. ( insert energy level diagram)
In an endothermic reaction, if heat energy is converted into enthalpy is ΔH (+/-)
Positive - H product is greater than H reactant ( insert energy level diagram)
What is most enthalpy changes quoted as?
Standard Enthalpy Changes
What are the standard condition in which heat produced measured in?
A pressure of 100KPa
Standard temperature (25*c = 298k)
All solutions at a concentration of 1 moldm^-3
Physical state of each substance in the change
What is the symbol for standard conditions?
Subscript - circle with dash. Hence ΔH°
In a exothermic reaction the final enthalpy of the system is ____ than its initial enthalpy
Less
In a endothermic reaction the final enthalpy of the system is ______ than its initial enthalpy
More
Illustrate an enthalpy level diagram for exothermic reactions
H (reactants) > H (products)
Illustrate an enthalpy level diagram for endothermic reactions
H (reactants) < H (products)
What is Hess’s Law?
The enthalpy change for any reaction is independent of the route taken from reactants to products.
Show the Hess’s Law cycle equation?
(Insert diagram)
ΔH1 = ΔH2 + ΔH3.
ΔH3 = ΔH1 - ΔH2
When can’t you measure the enthalpy change directly?
If the molar quantities result in a mixture of unburnt molecules.
How do you counteract the issue when enthalpy change can not be directly measured?
By using substance in excess - oxygen.
Define standard enthalpy of formation.
Enthalpy change when 1 mol of a substance is formed from its element in their standard states at 100kPa pressure + at a stated temperature (298k).
The units are KJmol^-1
What does standard state mean?
The form in which the element is normally found under the quoted conditions of pressure and temperature .
Define standard enthalpy of reaction.
The enthalpy change when the number of moles of the substance in the equation as written react under standard conditions of 100kPa pressure + a stated temperature (298k).
The units are KJmol^-1
2Na (s) + Cl2 (g) ⇒ 2NaCl (s)
The value of ΔHr is -822KJ.
Whereas for
Na(s) + 0.5Cl2(g) ⇒ NaCl (s)
The value of ΔH is -411KJ.
State observation.
The second reaction is half the value for the first reaction.
This is because half the amounts of sodium and chlorine are reacting.
How can the enthalpy of reaction be calculated from standard enthalpy of formation data ?
From the standard enthalpy of formation using Hess’s Law cycle.
What is the formula that enables the enthalpy of a reaction to be calculated ?
ΔHr = ΣΔHf (products) - ΣΔHf (reactants)
To determine enthalpy of reaction in an experiment what is required?
If a reaction takes place at a reasonable rate at room temperature, heat change can be measured using an expanded polystyrene cup as a calorimeter.
What is the advantages of using a polystyrene cup as calorimeter?
It is a thermal insulator hence not only does it minimise heat loss to the surroundings, it absorbs little heat itself.
Which reactants are utilised when measuring the heat change in a polystyrene cup?
Zinc reacting with copper (II) sulfate solution.
Zn (s) + CuSO4 (aq) ⇒ ZnSO4 (aq) + Cu (s)
What is the procedure to measure the enthalpy of reaction?
- Measured volume of CuSO4 (aq) of known concentration is pipetted into an expanded polystyrene cup held in a glass beaker
- Some powdered Zinc metal is weighed out, so metal is in excess
- Temperature of CuSO4 (Aq) measured every 30s for 2 minutes.
At 2.5min powdered metal tipped in. - Solution is stirred and the temperature recorded every 30s until a maximum temp reached, then for further 2 min.
What are the 3 quantities that have it be evaluated before enthalpy of the reaction can be calculated?
AMOUNT (moles) of CuSO4 that reacts - Moles of CuSO4 = CONC X VOL (dm^3)
Mass of solution heated - assumed to be numerically same as volume cm^3
Temperature Rise -
graph temp-time ΔT via extrapolation
What are some assumptions that can be made from this experiment?
Density + specific heat capacity of the solution is the same as pure water (1gcm-3)
⇒ if 50cm^3 of CuSO4 had been used, the mass in the expression heat 50g
Amt of heat absorbed by polystyrene cup, the stirrer or thermometer is neglible
What are some possible errors in this experiment?
Reaction is slow so some heat is lost to the surroundings. Extrapolating the graph, rather than calculating ΔT as T(max) - T(start) allows for most of this heat loss.
⇒ makes experimentally determined value less exothermically than correct Value.
Some heat will be absorbed by the iron metal and thermometer.
⇒ makes experimentally determined value less exothermically than correct Value.
What is a similar method but without measuring the temperature over a period of time?
Using rapid reactions that takes place in solution. E.g. precipitation of the insoluble salt barium sulfate
Outline the method of rapid reactions for determining the value of ΔHr⦵.
Measure 25cm^3 of 1.0dm^3 barium chloride solution into a polystyrene cup and measure its temperature.
Measure out 25cm^3 of 1.1dm^3 sulfuric acid solution into a beaker and measure its temperature
Average the two temperatures
Pour the contents of the beaker into the polystyrene cup, stir the mixture with the thermometer and measure the temperature reached.
How would you calculate ΔHr⦵ using the rapid reactions experiment.
Mass of solution (50.0g)
Specific heat capacity of solution (4.18JGg^-1C^-1)
ΔT = final - mean
Heat = mxcxΔT
Amount of Ba 2+ ions reacted = 1.0 x (25/1000) = 0.0250mol
ΔHr⦵ = +- heat / amount of Ba 2+ reacted
Why is there no need to plot a temperature time curve when using rapid reactions?
The reaction is rapid and the heat loss to the surrounding is negligible.
Define standard enthalpy of combustion
Enthalpy change when 1 mole of the substance is burnt in excess oxygen under standard conditions of 100kPa pressure + stated temperature (298k).
What is the symbol for standard enthalpy of combustion ?
ΔH°c
When writing an equation for ΔH°c what must you specify ?
Important to specify one mole of the substance of the substances is being burned
What is for ethanol equation when 1 mole of ethanol reacts?
C2H5OH (l) + 3O2 (g) ⇒ 2CO2 (g) + 3H2O (l)
What must the equation for ΔH°c have ?
Ther must only be 1 mole of the substance on the left hand side in the ΔH°c eqn
enthalpy of combustion is measured using….
using a series of alcohols in a spirit burner
To find the ΔH°c of a liquid what do you need to know?
Known mass liquid burnt
Heat produced used to warm up known volume of water.
Outline the experimental procedure to find ΔH°c of liquid
Weighted spirit burner w/ liquid
Known volume of water added to the copper calorimeter.
Temp of water in calorimeter measured every 4 min
Burner lit after 4.5 min
Temp of water measured every min
When temp reaches 20C + room temp , the flame in the spirit burner is extinguished + burner immediately reweighed.
Temp readings stopped 5 min after the temp has reached max value
What are the 3 quantities that have to evaluated before ΔH°c is calculated?
Mass of water heated - density 1gcm-3 ⇒ so I’d 100cm^3 of water , 100g mass
Temp rise - lots of heat lost w/in experiment ⇒ ΔT value accurate extrapolation w/ graph
Amount in moles of ethanol burnt = mass used / molar mass
What are the stages of calculating enthalpy change of combustion
- Calculate heat change Q transferred to the water
- Calculate the amount n of alcohol burned
- Use the eqn Q/n
Why is the experimentally found value less exothermic than accepted data?
Experiment takes too long ⇒ not all heat lost to surrounding is compensated via extrapolation
Some of the heat released heats air and not the water
Beaker absorbs some heat produced
Some ethanol may not completely burn to CO2 +H2O (Incomplete⇒ black soot deposited bottom of beaker)
Conditions are to standard. Water vapour produced
Evaluate sources of error + assumptions ΔH°c
Some of the heat energy produced in burning is transferred to the air and not the water
Some alcohol may not burn completely to form carbon dioxide and water
Conditions not standard
Can ΔH°f be calculated directly?
No but can be using Hess’s Law + ΔH°c Data
Give the formulates that can be derived from Hess’s law using ΔH°c Data.
ΔH°f + ΔH°c of compound = Σ of ΔH°c of elements
ΔH°f = ΣΔH°c Of elements - ΔH°c of compounds
Define standard enthalpy of neutralisation
The enthalpy change when 1 mole of water is produced by the neutralisation f a solution of an acid by excess base under standard conditions.
For hydrochloric acid the ΔH°neut is the enthalpy change represented by….
HCl (aq) + NaOH (aq) ⇒ NaCl (aq) + H2O (l)
For sulfuric acid the ΔH°neut is the enthalpy change represented by….
0.5 H2SO4 (aq) + NaOH (aq) ⇒ 0.5 Na2SO4 (aq) + H20 (l)
What type of acid is completely ionised in solution?
ALL STRONG ACIDS
What is the ionic equation representing the ΔH°neut of any strong acid by a strong base?
H+ (aq) + OH- (aq) ⇒ H2O (l) ΔH°= -57.9KJmol^-1
What is the main experimental method to find ΔH°neut Of an acid?
Using acid that should have a concentration of 1.00 mol dm-3. The alkali used to neutralise must be in slight excess so that all the acids react.
Outline the procedure to find ΔH°neut of an acid?
Using a pipette, measure out 25cm^3 of the 1.00moldm-3 acid solution into an expanded polystyrene cup
Measure temperature of acid
Measure temperature of the alkali - common ⇒ sodium hydroxide of conc 1.1moldm-3
Calculate mean of two temperatures
Measure out 25cm^3of the alkali using a pippete+ add to acid solution
Stir the mixture with the thermometer + measure maximum temperature reached
What is an alternative method to find ΔH°neut?
Using measuring cylinders to measure out the acid and the alkali. Weight the polystyrene cup before and after audition of solutions
What is a source of error that could occur when finding the ΔH°neut?
Experimentally derived value for ΔH°neut is slightly low because of the heat absorbed by the polystyrene cup + thermometer
Define the term bond enthalpy
The enthalpy change when the bond in a gaseous molecule is broken.
What is the enthalpy change for the H-H bond enthalpy
H2 (g) ⇒. 2H (g). ΔH= +436 KJmol-1
What would be expected when the bond enthalpy worked out for a particular compound
The value obtained is slightly different from the average value.
Bond breaking is _____________. + bond making is ______________
Bond breaking is endothermic + bond making is exothermic
Steps to calculating ΔH°r using mean bond enthalpies
List all the broken bonds. Write down the energy required (positive no) to break each bond. Sum of all = total energy required
List the bonds made. Write down the energy released (negative no) to make each bond.Sum of all = total energy released
Add the two totals to give ΔHr°
Which bond breaks / made in the reaction?
Break all the bonds - Reactant.
Make all the bonds - Products
Explain the limitation of calculating ΔH°r using mean bond enthalpies
The bond enthalpy in one compound is not quite the same as the mean bond enthalpy.
Bond enthalpies are measured in gaseous state but in some reactions they can be (l)
How can bond that is likely broken in a chemical reaction be indicated by bond enthalpy?
The weaker bond is predicted to break.
What else can bond enthalpies be used to predict?
Relative rates of reaction
