3.1.4- Energetics (PAPER 1+2) Flashcards
What is enthalpy? What are its units?
A measure of the amount of energy that is stored in a system- this cannot be measure directly.
Measured in KJmol-1
What letter is enthalpy represented by and what is enthalpy change represented by?
Enthalpy= H
Enthalpy change= ΔH
What happens due to the enthalpy of the reactants and products not being the same?
Energy is either given out (exothermic) or taken in (endothermic) during the reaction. This is the enthalpy change.
Overall enthalpy change equation?
energy to make bonds + energy to break bonds
Definition of enthalpy change?
The change in heat energy of a substance at constant pressure and temperature.
What is the system, the surroundings and the universe regarding the conservation of energy?
The system- the reactants and products.
The surroundings- the equipment and the immediate surrounding space.
The universe- both the system and surroundings.
What is a positive enthalpy change? Draw the energy level diagram.
+ enhalpy change value= endothermic.
Energy is transferred from the surroundings to the system (energy taken in)
The temperature decreases.
Diagram- products have higher enthalpy than reactants- Ea is from reactants to peak of products.
What is a negative enthalpy change? Draw the energy level diagram.
- enthalpy change= exothermic.
Energy is transferred from the system to the surroundings (energy given out)
Temperature increases.
Diagram- products have lower enthalpy than reactants- Ae is from peak of reactants to bottom of reactants.
Where are bonds broken/ made?
Bonds broken in reactants- energy taken in.
Bonds made in products- energy released.
When is a reaction exothermic?
When the energy released from the bonds being made (products) is greater than the energy taken in from bonds being broken (reactants).
When is a reaction endothermic?
When the energy to break the bonds (reactants) is greater than the energy released when the bonds are made (products).
What is activation energy - Ea?
The minimum energy required for a reaction to take place.
Certain energy needs to be achieved to break the bonds of the reaction.
What is a standard enthalpy change?
If the enthalpy change is measured under standard conditions of temperature and pressure.
ΔH°
What are the standard conditions?
Standard pressure= 100 kPa
Temperature= 298K
Standard concentration= 1moldm-3
What is the standard enthalpy of formation?
The enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions.
MUST BE ONE MOLE AT THE END OF THE EQUATION!!!
Write the equation for enthalpy of formation of CH4 and enthalpy of formation of NH3.
C(s) + 2H2 (g)—–> CH4(g)
1/2 N2(g) + 3/2 H2 (g) —–> NH3 (g)
What is standard enthalpy of combustion?
The enthalpy change when 1 mole of substance is completely burnt in excess oxygen under standard conditions, all reactants and products in their standard states.
Write the equation for enthalpy of combustion of C2H6 and enthalpy of combustion of C6H14.
C2H6 (g) + 7/2 O2(g) ——-> 2CO2 (g) + 3H2O (l)
C6H14(g) + 19/2 O2(g) ——–> 6CO2(g) + 7H2O (l)
What is standard enthalpy of atomisation?
The enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state, under standard conditions.
What is standard enthalpy of neutralisation?
The enthalpy change when 1 mole of water is formed when solutions of an acid and an alkali react together under standard conditions.
This is always - as heat is released.
Write the equation for enthalpy of neutralisation of H2SO4 (aq) and NaOH (aq)
1/2 H2SO4 + NaOH (aq) —–> 1/2 Na2SO4 (aq) + H2O (l)
Describe a method to find enthalpy change of combustion of a fuel?
1- Measure 100cm3/100g of water into a calorimeter.
2- Weigh a spirit burner containing the liquid/fuel to be burnt.
3- Measure initial temp. of the water using a thermometer.
4- Use spirit burner to heat the water.
5- Stop heating after reasonable temp. change.
6- Reweigh spirit burner then use weigh by difference.
7-Calculate temp change of reaction and mass of fuel burnt (difference between spirit burner masses)
8-Use q=mcΔt, calculate moles of fuel by mass/mr
9- Change J into kJ and then divide by moles
Why is the value for enthalpy of combustion lower than in data books?
-Heat losses to the surroundings from the spirit burner, wick and calorimeter.
-Incomplete combustion of the fuel.
-Reaction did not occur in standard conditions.
-Loss of water by evaporation.
How to improve simple calorimetry experiment?
-Use lid on calorimeter to reduce heat loss to the surroundings.
-Minimise the distance between the flame and the calorimeter.
-Insulate calorimeter and spirit burner to reduce heat loss.
-Burn in a supply of pure oxygen to reduce incomplete combustion.
How to calculate enthalpy change?
q=mcΔt
energy change (J)= mass (g) x SHC x temperature change (K)
How to find ΔH solution?
This can be measured using insulated plastic cups as a calorimeter.
Describe a method to find enthalpy change of neutralisation?
1- Place polystyrene cup in glass beaker for support.
2- Rinse measuring cylinder with 1moldm-3 HCl then measure 25cm3 and transfer into the cup.
3-Stir acid with thermometer and measure temp.
4- Rinse second measuring cylinder with 1moldm-3 NaOH (aq), measure 25cm3.
5- Add NaOH to acid, stir, record highest temp. reached. Highest temp-lowest temp.
6- Add the solution volumes together= mass (50cm3), sub into q=mcΔt, divide by 1000
7- If moles are the same, divide by the moles.
8- If moles are different, use the limiting reagent: moles= mass/mr or volume (dm3) x conc. (moldm-3)
Describe a method to find enthalpy change for a displacement reaction?
1- Weigh out roughly 2g zinc (in excess)
2- Pipette 25cm3 0.500moldm-3 copper sulphate solution into dry polystyrene cup in a beaker.
3- Place lid on cup, put under tripod and put thermometer through hole. Stir + record temp every minute.
4- At 4 minutes, add zinc and continue stirring and recording temp. until 15mins.
5- Extrapolate graph of temperature against time back to 4 minutes to find maximum rise.
6- If you do not know which reactant is the limiting one, calculate moles for both.
kJ/smallest moles
Write the equation for the displacement reaction between copper sulphate solution and zinc?
CuSO4 (aq) + Zn (s)——> ZnSO4 (aq) + Cu(s)
What is the definition of Hess’ law?
The overall enthalpy change for a reaction is independent of the route the reaction takes at constant pressure.
Draw the formation cycle using Hess’ law
Reactants—–> Products
elements
elements have 2 arrows facing up towards reactants and products
Draw the combustion cycle using Hess’ law
Reactants——> Products
combustion products (CO2 + H2O)
combustion products have 2 arrows going down from reactants and products
Calculate the enthalpy change using formation enthalpies of SO2 + 2H2S——> 3S + 2H20
SO2= -297, H2S= -20.6, H20= -286
can use products-reactants
-572–338.2= -233.8
OR hess’ cycle
CHANGE LEFT ARROW FROM GOING UP TO GOING DOWN
sign of -338.2 changes to +338.2
-572+338.2 =-233.8
Calculate the enthalpy change using combustion enthalpies of C+H2 + O2—————>C2H5OH
C2H5OH= -1367, C= -394, H2= -286
balance equation
2C+ 3H2+ 1/2 O2—–> C2H5OH
can use reactants-products
-1646–1367= -279
OR hess’ cycle
CHANGE RIGHT ARROW FROM GOING DOWN TO GOING UP
sign of -1367 changes to +1367
-1646 + 1367= -279
What is the equation for enthalpy change of formation?
(sum of ΔfH products) - (sum of ΔfH reactants)
products-reactants
What is the equation for enthalpy change of combustion?
(sum of ΔcH reactants) - (sum of ΔcH products)
reactants-products
What is the definition of mean bond enthalpy?
The average (mean) bond dissociation enthalpy for a particular bond in a range of different compounds.
Why may experimental enthalpy values differ from calculated mean bond enthalpies?
The mean bond energies are averages and not compound-specific values.
What is the equation for ΔH involving bond energies?
ALL REACTANTS AND PRODUCTS MUST BE GASES
(sum of ΔH for breaking bonds) - (sum of ΔH for making bonds)
bond breaking= endothermic (+) occurs in reactants
bond making = exothermic (-) occurs in products
REACTANT BONDS- PRODUCT BONDS
Calculate enthalpy of combustion for CH4 +2O2 —–> CO2 +2H20
O=O is 496
C=O is 743
C-H is 412
O-H is 463
break-make
2640-1852= -698 kJmol-1
Definition of enthalpy change as applied to a chemical reaction?
The amount of heat energy transferred during a chemical reaction at constant pressure.
If it asks you to find enthalpy of combustion using standard enthalpies of formation what should you use?
products- reactants as you are using the enthalpies of formation
What is total heat capacity in q=mc ∆t?
Total heat capacity is both m and c, the mass and specific heat capacity combined.
If it asks you to find enthalpy of formation using combustion data what should you use?
reactants- products as you have combustion data
