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A Level - Chemistry > 8: Energetics 1 > Flashcards

8: Energetics 1 Flashcards

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1
Q

Define enthalpy change (∆H)

A

Heat energy change in a reaction at constant pressure

kJ/mol

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2
Q

What is ∆Hθ?

A

Enthalpy change under standard conditions

Also elements were in their standard states

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3
Q

What are standard states?

A

Reacted in physical states under standard conditions

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4
Q

What are standard conditions?

A

100 kPa of pressure

298K

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5
Q

What is an exothermic reaction?

A

Reactions which give out heat energy

∆H is negative
Temp often increases

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6
Q

What is an endothermic reaction?

A

Reactions that absorbs heat energy

∆H is positive

Temp often decreases

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7
Q

What is an enthalpy level diagram?

A

Diagram which shows relative energies of reactants and products of a reaction

Does NOT show Ea or progress of reaction

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8
Q

What is on the axis of an enthalpy level diagram?

A

y axis - enthalpy

no x axis

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9
Q

What value is shown in an enthalpy level diagram?

A

∆H

This is the difference between the enthalpies of the two substances

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10
Q

How does enthalpy relate to stability?

A

The less the enthalpy, the more stable

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11
Q

How does the enthalpy of the products of an exothermic reaction compare to the reactants?

A

Products have a lower enthalpy than the reactants

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12
Q

How does the enthalpy of the products of an endothermic reaction compare to the reactants?

A

Products have a higher enthalpy than the reactants

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13
Q

What is a reaction profile diagram?

A

Diagram which shows how the enthalpy changes throughout the reaction

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14
Q

What is shown in a reaction profile diagram?

A

Curve showing the enthalpy throughout the reaction

Activation energy

∆H

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15
Q

What is the abbreviation for the activation energy?

A

Ea

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16
Q

What are the relative positions of the products and reactants in a reaction profile diagram?

A

Products are higher than the reactants

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17
Q

What on the axis in a reaction profile diagram?

A

x axis - progress of reaction

y axis - enthalpy (kJ/mol)

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18
Q

How can enthalpy of a system be measured?

A

Cannot measure directly

This is irrelevant, enthalpy change is what matters

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19
Q

How can you find enthalpy changes?

A

Experimentally or using data books

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20
Q

What is to note about the enthalpy changes found in data books?

A

They are usually done under standard conditions

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21
Q

Why is it important enthalpy changes are measured in standard conditions?

A

Changes in enthalpy are affected by temperature and pressure

22
Q

What is the standard enthalpy change of reaction ∆rHθ​?

A

Enthalpy change when the reaction occurs at the molar quantities shown in the chemical equation

Under standard conditions

23
Q

What is the standard enthalpy change of formation ∆fHθ?

A

Enthalpy change when 1 mole of a compound is formed from its elements in standard states

Under standard conditions

24
Q

What is the standard enthalpy of combustion ∆cHθ?

A

Enthalpy change when 1 mole of a substance is completely burned in oxygen

Under standard conditions

25
Q

What is the standard enthalpy change of neutralisation ∆neutHθ?

A

Enthalpy change when an acid and alkali react together to form 1 mole of water

Under standard conditions

26
Q

What is required when calculating enthalpy changes in experiments?

A

Number of moles of the reactants

Change in temperature of the reaction

27
Q

When is the heat change the same as the enthalpy change?

A

Under constant pressure

28
Q

How can you experimentally measure temperature when you have solids and liquids?

A

Mix them and measure the temperature change using a thermometer

29
Q

What is the experimental procedure for determining enthalpy change when mixing two liquids?

A

Add set volume of liquid to a polysterene cup and put lid on with a hole for a thermometer

Measure temp after 30s to stabilise

Add known amount of solid/liquid

Measure temp every 30s until reached max or min temp and has been returning to normal for a few minutes

Draw graph of temperature against time

Extrapolate line from where reaction is returning, back to original temp when added

Read of temp when reaction starts, and calculate change in temp

Then use to find enthalpy test

30
Q

What is done when the enthalpy of reactions involving gases is being experimentally calculated?

A

Calorimeter used to calcualte how much heat is given out by reaction

Measures the temperature change of water around it

31
Q

Which reactions are usually involving gases for enthalpy calculations?

A

Combustion

32
Q

What is the set up for a calorimeter?

A

Spirit burner with known volume of fuel

Underneath a container with a known volume of water present

Container should be insulated and stirrer may be present

Thermometer should be placed in the water

33
Q

How does combustion calorimtetry mean enthalpy can be measured?

A

Fuel burning heats the water

Calculate heat absorbed by water

USe to calculate enthalpy changes

34
Q

What is the equation for calculating the heat energy lost/gained?

A

q = mc∆T

q - heat lost/gained (J)

m - mass of water in calorimeter or solution in container (grams)

c - specific heat capacity of water (4.18 J/g/K)

∆T - change in temperature of water (K)

35
Q

What is the specific heat capacity of water?

A

4.18 J g-1 K-1

36
Q

What is the specific heat capacity of water?

A

Amount of energy required to raise the temperature of 1 gram of water by 1 K

37
Q

Why do you not have to convert between ºC and K for ∆T?

A

The change is the same

38
Q

What can be done after calculating the heat energy gained/lost from q=mc∆T?

A

Find the moles of fuels which caused this change

Divide heat energy (q) by the moles

Value calculated is the enthalpy change of combustion

39
Q

What is Hess’s Law?

A

Total enthalpy change of a reaction is always the same, no matter which route is taken

40
Q

When is Hess’s law especially important?

A

Calculating enthalpy changes which cannot be measured directly by an experiment

41
Q

What is the ∆fHθ of elements?

A

Always 0

42
Q

Is breaking bonds endo or exothermic?

A

Endothermic as energy required to break bonds

∆H is +ve

43
Q

Is forming bonds endo or exothermic?

A

Exothermic as energy is released

∆H is -ve

44
Q

What is the ∆H of a reaction based on in terms of bonds?

A

Overall affect of energy required to break bonds and the energy released when new ones form

45
Q

What is bond enthalpy?

A

The energy required to break 1 mole of a type of bond in a molecule in the gas phase

46
Q

What keeps molecules together in ionic and covalent bonding?

A

Ionic - ions are electrostatically attracted to each other

Covalent - nuclei are attracted to shared pair of electrons

47
Q

Are bond enthalpies always the same?

A

No

Range of values depending which compound they are found in

48
Q

What is the mean bond enthalpy?

A

Energy needed to break 1 mole of bonds in the gas phase, averaged over many different compounds

49
Q

What values related to bond enthalpies are found in data books?

A

Mean bond enthalpies

50
Q

What is the ∆H of a reaction in terms of bonds?

A

Enthalpy change of reaction = Sum of bond enthalpies of reactants - Sum of bond enthalpies of products

51
Q

What occurs to the enthalpy change of combustion when you blow out the flame of the water?

A

Blows away unburnt fuel

More evaporation so mass loss greater than true amount that has undergone combustion

Calculated value for enthalpy of combistion will be less negative than true value

A Level - Chemistry (26 decks)

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