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Chemistry A Level AQA > 3.1.4- Energetics (PAPER 1+2) > Flashcards

3.1.4- Energetics (PAPER 1+2) Flashcards

1
Q

What is enthalpy? What are its units?

A

A measure of the amount of energy that is stored in a system- this cannot be measure directly.

Measured in KJmol-1

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2
Q

What letter is enthalpy represented by and what is enthalpy change represented by?

A

Enthalpy= H
Enthalpy change= ΔH

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3
Q

What happens due to the enthalpy of the reactants and products not being the same?

A

Energy is either given out (exothermic) or taken in (endothermic) during the reaction. This is the enthalpy change.

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4
Q

Overall enthalpy change equation?

A

energy to make bonds + energy to break bonds

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5
Q

Definition of enthalpy change?

A

The change in heat energy of a substance at constant pressure and temperature.

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6
Q

What is the system, the surroundings and the universe regarding the conservation of energy?

A

The system- the reactants and products.

The surroundings- the equipment and the immediate surrounding space.

The universe- both the system and surroundings.

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7
Q

What is a positive enthalpy change? Draw the energy level diagram.

A

+ 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- Ae is from reactants to peak of products.

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8
Q

What is a negative enthalpy change? Draw the energy level diagram.

A
  • 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.

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9
Q

When is a reaction exothermic?

A

When the energy released from the bonds being made (products) is greater than the energy taken in from bonds being broken (reactants).

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10
Q

When is a reaction endothermic?

A

When the energy to break the bonds (reactants) is greater than the energy released when the bonds are made (products).

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11
Q

What is activation energy - Ea?

A

The minimum energy required for a reaction to take place.

Certain energy needs to be achieved to break the bonds of the reaction.

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12
Q

What is a standard enthalpy change?

A

If the enthalpy change is measured under standard conditions of temperature and pressure.

ΔH°

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13
Q

What are the standard conditions?

A

Standard pressure= 100 kPa
Temperature= 298K
Standard concentration= 1moldm-3

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14
Q

What is the standard enthalpy of formation?

A

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!!!

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15
Q

Write the equation for enthalpy of formation of CH4 and enthalpy of formation of NH3.

A

C(s) + 2H2 (g)—–> CH4(g)

1/2 N2(g) + 3/2 H2 (g) —–> NH3 (g)

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16
Q

What is standard enthalpy of combustion?

A

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.

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17
Q

Write the equation for enthalpy of combustion of C2H6 and enthalpy of combustion of C6H14.

A

C2H6 (g) + 7/2 O2(g) ——-> 2CO2 (g) + 3H2O (l)

C6H14(g) + 19/2 O2(g) ——–> 6CO2(g) + 7H2O (l)

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18
Q

What is standard enthalpy of atomisation?

A

The enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state, under standard conditions.

19
Q

What is standard enthalpy of neutralisation?

A

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.

20
Q

Write the equation for enthalpy of neutralisation of H2SO4 (aq) and NaOH (aq)

A

1/2 H2SO4 + NaOH (aq) —–> 1/2 Na2SO4 (aq) + H2O (l)

21
Q

Describe a method to find enthalpy change of combustion of a fuel?

A

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

22
Q

Why is the value for enthalpy of combustion lower than in data books?

A

-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.

23
Q

How to improve simple calorimetry experiment?

A

-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.

24
Q

How to calculate enthalpy change?

A

q=mcΔt

energy change (J)= mass (g) x SHC x temperature change (K)

25
Q

How to find ΔH solution?

A

This can be measured using insulated plastic cups as a calorimeter.

26
Q

Describe a method to find enthalpy change of neutralisation?

A

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)

27
Q

Describe a method to find enthalpy change for a displacement reaction?

A

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

28
Q

Write the equation for the displacement reaction between copper sulphate solution and zinc?

A

CuSO4 (aq) + Zn (s)——> ZnSO4 (aq) + Cu(s)

29
Q

What is the definition of Hess’ law?

A

The overall enthalpy change for a reaction is independent of the route the reaction takes at constant pressure.

30
Q

Draw the formation cycle using Hess’ law

A

Reactants—–> Products

         elements

elements have 2 arrows facing up towards reactants and products

31
Q

Draw the combustion cycle using Hess’ law

A

Reactants——> Products

combustion products (CO2 + H2O)

combustion products have 2 arrows going down from reactants and products

32
Q

Calculate the enthalpy change using formation enthalpies of SO2 + 2H2S——> 3S + 2H20

SO2= -297, H2S= -20.6, H20= -286

A

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

33
Q

Calculate the enthalpy change using combustion enthalpies of C+H2 + O2—————>C2H5OH

C2H5OH= -1367, C= -394, H2= -286

A

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

34
Q

What is the equation for enthalpy change of formation?

A

(sum of ΔfH products) - (sum of ΔfH reactants)

products-reactants

35
Q

What is the equation for enthalpy change of combustion?

A

(sum of ΔcH reactants) - (sum of ΔcH products)

reactants-products

36
Q

What is the definition of mean bond enthalpy?

A

The average (mean) bond dissociation enthalpy for a particular bond in a range of different compounds.

37
Q

Why may experimental enthalpy values differ from calculated mean bond enthalpies?

A

The mean bond energies are averages and not compound-specific values.

38
Q

What is the equation for ΔH involving bond energies?
ALL REACTANTS AND PRODUCTS MUST BE GASES

A

(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

39
Q

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

A

break-make
2640-1852= -698 kJmol-1

40
Q

Definition of enthalpy change as applied to a chemical reaction?

A

The amount of heat energy transferred during a chemical reaction at constant pressure.

41
Q

If it asks you to find enthalpy of combustion using standard enthalpies of formation what should you use?

A

products- reactants as you are using the enthalpies of formation

42
Q

What is total heat capacity in q=mc ∆t?

A

Total heat capacity is both m and c, the mass and specific heat capacity combined.

43
Q

If it asks you to find enthalpy of formation using combustion data what should you use?

A

reactants- products as you have combustion data

Chemistry A Level AQA (11 decks)

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