Energetics

Question 1

Define enthalpy change

Answer 1

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

Question 2

What happens to the reactants and products during a reaction?

Answer 2

Reactants- bonds are broken
Products- new bonds are made

Question 3

Why do we measure enthalpy change?

Answer 3

Because enthalpy itself cannot be measured

Question 4

What is the shape for enthalpy

Answer 4

Triangle

Question 5

What is activation energy?

Answer 5

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

Question 6

What are the types of reactions involved with enthalpy

Answer 6

Exothermic and endothermic reactions

Question 7

Draw the two enthalpy diagrams

Answer 7

Check physical flashcards to see if you’re right

Question 8

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

Answer 8

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

Question 9

Define standard enthalpy of formation

Answer 9

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

Question 10

Define standard enthalpy of combustion

Answer 10

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

Question 11

What is the standard enthalpy of combustion and why

Answer 11

0kJ mol because oxygen cannot combust with itself

Question 12

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

Answer 12

• Incomplete combustion
• Heat loss

Question 13

How can enthalpy of combustion be measured?

Answer 13

Using a calorimeter

Question 14

What is formula for heat energy (Q)

Answer 14

Q= mass x specific heat capacity x change in Temperature

Question 15

What is the formula for enthalpy change

Answer 15

Heat capacity/ mol

Question 16

Draw the graphs of endothermic and exothermic reactions

Answer 16

Check physical flashcards to check if you’re right

Question 17

Define mean bond enthalpy

Answer 17

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

Question 18

What is the simple equation of calculating enthalpy

Answer 18

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

Question 19

What direction does the arrow go for enthalpy of formation

Answer 19

Towards the products and reactants

Question 20

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

Answer 20

Away from the the reactants and products

Question 21

Describe the required practical

Answer 21

• 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

Question 22

what is the general formula for change in enthalpy

Answer 22

total energy absorbed-total energy released

Question 23

what is the formula for enthalpy of formation

Answer 23

change in f= products - reactants

Question 24

What is the formula for enthalpy of combustion

Answer 24

reactants-products

Question 25

Give the equation for the enthalpy of combustion of propanol

Answer 25

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

Question 26

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

Answer 26

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

Question 27

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

Answer 27

Because it’s harder to judge

Question 28

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

Answer 28

Reaction would be too slow

Question 29

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)

Answer 29

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

Question 30

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

Answer 30

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

Question 31

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

Answer 31

• 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

Question 32

Give examples of exothermic and endothermic reactions

Answer 32

Exothermic- oxidation reactions Endothermic-thermal decomposition of calcium carbonate

Question 33

What are the characteristics of breaking and making bonds

Answer 33

- 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

Question 34

Why aren't mean bond enthalpies exact

Answer 34

- 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

Question 35

Define Hess's law and give its purpose

Answer 35

- 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

Question 36

What is the enthalpy of formation of elements

Answer 36

zero

Question 37

What value is used to calculate enthalpy of formation

Answer 37

Enthalpy of formation of products and reactants that are compounds

Question 38

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

Answer 38

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

Question 39

How many significant figures must you convert your answers to?

Answer 39

3 significant figures

Question 40

How many significant figures is the enthalpy change given

Answer 40

3 significant figures

Question 41

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

Answer 41

• Attracts chloride ion more strongly/ more ionic

Question 42

When calculating in energetic what must your answer be

Answer 42

negative for combustion and enthalpy change reactions

Question 43

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

Answer 43

- Less accurate due to greater heat loss - More accurate due to smaller % uncertainty in temperature change

Question 44

What should you always calculate in terms of temperature readings?

Answer 44

How many readings are taken before you calculate the uncertainty

Question 45

Explain the meaning of the term mean bond enthalpy

Answer 45

• Enthalpy to break a bond • Averaged over different molecules

Question 46

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

Answer 46

Mean bond enthalpies are not exact

Question 47

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

Answer 47

- 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