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Physical chemistry > Energetics > Flashcards

Energetics Flashcards

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1
Q

Define enthalpy change

A

It is the heat energy transferred in a reaction at constant pressure in their standard states under standard conditions KJ mol-1

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2
Q

What happens to the reactants and products during a reaction?

A

Reactants- bonds are broken
Products- new bonds are made

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3
Q

Why do we measure enthalpy change?

A

Because enthalpy itself cannot be measured

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4
Q

What is the shape for enthalpy

A

Triangle

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5
Q

What is activation energy?

A

Minimum energy that a particle needs for a successful reaction to occur

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6
Q

What are the types of reactions involved with enthalpy

A

Exothermic and endothermic reactions

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7
Q

Draw the two enthalpy diagrams

A

Check physical flashcards to see if you’re right

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8
Q

Give the standard conditions values for pressure, temperature and concentration for all solutions

A

Pressure= 100kPa
Temperature= 298k
All solutions= 1.0mol dm-3

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9
Q

Define standard enthalpy of formation

A

Enthalpy involved when one mole of a compound is formed from its elements in their standard states and under standard conditions

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10
Q

Define standard enthalpy of combustion

A

Enthalpy involved (change) when 1 mole of a substance is completely burnt in excess of oxygen under standard temperatures and pressure

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11
Q

What is the standard enthalpy of combustion and why

A

0kJ mol because oxygen cannot combust with itself

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12
Q

Why aren’t values you get after calculation the same as in the question

A

• Incomplete combustion
• Heat loss

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13
Q

How can enthalpy of combustion be measured?

A

Using a calorimeter

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14
Q

What is formula for heat energy (Q)

A

Q= mass x specific heat capacity x change in Temperature

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15
Q

What is the formula for enthalpy change

A

Heat capacity/ mol

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16
Q

Draw the graphs of endothermic and exothermic reactions

A

Check physical flashcards to check if you’re right

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17
Q

Define mean bond enthalpy

A

The average bond enthalpy term is the average amount of energy per mole needed to break a specific type of bond, measured over a wide variety of different molecules

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18
Q

What is the simple equation of calculating enthalpy

A

Enthalpy change = sum of bonds broken - sum of bonds made

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19
Q

What direction does the arrow go for enthalpy of formation

A

Towards the products and reactants

20
Q

What direction do the arrows go when calculations enthalpy is of combustion

A

Away from the the reactants and products

21
Q

Describe the required practical

A

• Measure 50cm3 of water using a measuring cylinder
• Place into an insulated container (polystyrene cup)
• Record the initial temperature for at least 4 minutes
• Add a known mass
• Measure and record the temperature every minute for another 7mins
• Plot temperature vs time on a graph
• Extrapolate the line of best fit for the initial temperature and after the solid was added, use this to find the temperature rise
• Use Q= mct to find Q in J (/1000)
• divide my no of moles to work change in H

22
Q

what is the general formula for change in enthalpy

A

total energy absorbed-total energy released

23
Q

what is the formula for enthalpy of formation

A

change in f= products - reactants

24
Q

What is the formula for enthalpy of combustion

A

reactants-products

25
Q

Give the equation for the enthalpy of combustion of propanol

A

C3H8(g) + 5O2(g) = 3O2(g) + 4H2O(l)

26
Q

Give the equation for the enthalpy of formation for methylbenzene, C7H8

A

7C(s) + 4H2(g) = C7H8(l)

27
Q

Suggest why the student records the times to the nearest second and not to the nearest 0.01s

A

Because it’s harder to judge

28
Q

Suggest why the student chose not to carry out experiments at temperatures in the the range 1-10C

A

Reaction would be too slow

29
Q

Suggest by considering the products of this reaction why small amounts of reactants are used in this reaction

• Na2S2O3(aq) + 2HCl(aq) -> 2NaCl(aq) + SO2(g) + S(s) + H2O(l)

A

SO2 is a toxic gas/ limit the amount of SO2 formed

30
Q

Explain why the value given for the O=O bond enthalpy is not a mean value?

A

Oxygen is the only substance in the reaction that has O=O bond

31
Q

Suggest one reason other than the use of mean bond enthalpies, why a value for the enthalpy of combustion of a liquid alkane is different from the value obtained using the expression in question 4.2

A

• Alkane is not gaseous
• Equation relates to gaseous alkanes
• It takes energy to convert it to gas
• That water/alkane/substances are gaseous in calculations using bond enthalpies

32
Q

Give examples of exothermic and endothermic reactions

A

Exothermic- oxidation reactions
Endothermic-thermal decomposition of calcium carbonate

33
Q

What are the characteristics of breaking and making bonds

A
  • reactant bonds are broken and product bonds are formed
  • You need energy to break bonds, so breaking is endothermic, stronger bonds need more energy to break
  • Stronger bonds release more energy when they form
  • The enthalpy change for a reaction is the overall effect of these two changes. If you need more energy to break bonds than is released when bonds are made, the enthalpy is positive
34
Q

Why aren’t mean bond enthalpies exact

A
  • Bond enthalpy is the energy required to break bonds
  • Every time you broke the same bond, it would not require the same energy
  • The energy needed to break a specific type of bond depends on the environment
  • Mean bond enthalpies are the average energy needed to break a certain type of bond
  • Breaking bonds is always endothermic so mean bond enthalpies are always positive
  • So they are slightly less accurate than enthalpy change values calculated using Hess’s law
35
Q

Define Hess’s law and give its purpose

A
  • The enthalpy change at constant pressure in a reaction is independent of the route/path taken
  • Handy for working out enthalpy changes you can’t find directly by doing an experiment
36
Q

What is the enthalpy of formation of elements

A

zero

37
Q

What value is used to calculate enthalpy of formation

A

Enthalpy of formation of products and reactants that are compounds

38
Q

Suggest why the students value for enthalpy of combustion of methanol is different from the data book

A

• Experiment not done under standard conditions
• Evaporation of methanol
• Heat up calorimeter

39
Q

How many significant figures must you convert your answers to?

A

3 significant figures

40
Q

How many significant figures is the enthalpy change given

A

3 significant figures

41
Q

What are the advantages of magnesium being a smaller ion than calcium ion

A

• Attracts chloride ion more strongly/ more ionic

42
Q

When calculating in energetic what must your answer be

A

negative for combustion and enthalpy change reactions

43
Q

In an experiment, the water was heated for 5 minutes. The student thought that the experiment could be improved by heating the water for 10 minutes.

Explain whether the accuracy in the student’s calculated value for 🔼cH may or may not be improved by heating for longer

A
  • Less accurate due to greater heat loss
  • More accurate due to smaller % uncertainty in temperature change
44
Q

What should you always calculate in terms of temperature readings?

A

How many readings are taken before you calculate the uncertainty

45
Q

Explain the meaning of the term mean bond enthalpy

A

• Enthalpy to break a bond
• Averaged over different molecules

46
Q

Explain why the value in part b for the mean bond enthalpies is different from that obtained in part cii

A

Mean bond enthalpies are not exact

47
Q

When do we use Hess’s law to calculate enthalpy in contrast with products-reactants/reactants-products

A
  • We use Hess’s law when the product cannot be measured directly i.e. one product is given in the question
  • If all the products are given in the question, you can use products/reactants/reactants-products to calculate the enthalpy change

Physical chemistry (10 decks)

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