Topic 5: Energetics/Thermochemistry

Question 1

Heat vs Temperature

Answer 1

Heat is a form of energy that flows from something at a high temperature to something at a lower temperature

Temperature is a measure of the average kinetic energy of particles

Question 2

Define internal energy

Answer 2

Internal energy is the name given the total amount of energy (kinetic and potential) in a sample of a substance.

If a 100g block of iron at 100˚C is placed in contact with a 50g block of iron at 50˚C, heat will flow from the hotter block to the colder one until they are the same temperature. When they are at the same temperature, the 100g block of iron will have higher internal energy simply because there is more of it, but the average kinetic energy of the particles in the two blocks will be the same because they are the same temperature.

Question 3

What is systems and surrounding in terms of energy flow in chemical reactions?

Answer 3

The ‘system’ is a chemical reaction and the ‘surrounding’ is everything else in the universe

Question 4

What can chemical reactions be classified as?

Answer 4

Exothermic or endothermic

Question 5

What happens in an exothermic reaction?

Answer 5

In an exothermic reaction, heat energy is transferred from a system (chemical reaction) to the surrounding – the surrounding gets hotter

Release heat into the environment

Question 6

What happens in an endothermic reaction?

Answer 6

In an endothermic reaction, a system (chemical reaction) takes in heat energy from the surroundings - the surroundings get cooler

To absorb heat form energy

Question 7

What is enthalpy change (∆H) and what does it tell us?

Answer 7

Enthalpy changes to heat energy exchange with the surroundings at constant pressure

It tells us when the system moves from an initial state to some final state

Question 8

What does ∆ mean?

Answer 8

∆ means a change in a quantity

Question 9

What is ∆H of a system?

Answer 9

The ∆H is basically the amount of heat energy taken in/ given out in a chemical reaction. Enthalpy is given the symbol H and enthalpy change is ∆H

Question 10

Which is negative, which is positive ∆H? Exothermic or endothermic?

Answer 10

∆H for an exothermic reaction is negative (Because the enthalpy of the products is less than the reactants).
∆H for an endothermic reaction is positive

Question 11

What does an enthalpy diagram for the combustion of methane look like?

Answer 11

Question 12

What is the stability of these reactions?

Answer 12

The term stability is usually used to describe the relative energies of reactants and products in a chemical reaction. If the products have less energy than the reactants than they are more stable.

Less energy = More stable

In an exothermic reaction the products are at a lower energy (enthalpy) level than the reactants, and we say that the products are more stable than the reactants

Question 13

How can the enthalpy change be measured with other enthalpy?

Answer 13

enthalpy change = enthalpy of products - enthalpy of reactants

Question 14

What does this symbol mean ⦵?

Answer 14

It means ‘under standard condition’

Question 15

What does standard enthalpy change of reaction mean? ΔHr°?

Answer 15

Standard enthalpy change is the change when molar amounts of reactants react together under standard condition to give products with everything else also in standard condition

Question 16

What does the standard enthalpy change of combustion mean? ?ΔH°c? Is it positive of negative?

Answer 16

Standard enthalpy change of combustion is the enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions. It is always negative because combustion is always an exothermic process.

Question 17

What is the pressure at standard condition?

Answer 17

100kPa (1.00 x 10⁵Pa)

Question 18

What does specific heat capacity mean?

Answer 18

Is the energy required to raise the temperature of 1g/kg of substance by 1˚/1K.
The higher the specific heat capacity the more difficult it is to heat up the substances than substances with low specific heat capacity. It also applies when a substance cools.

Question 19

What is specific heat capacity represented by?

Answer 19

(c)

Question 20

What is the formula to calculate heat energy? q? What does each component stand for?

Answer 20

q=mcΔT

q= heat energy
m= mass
c=specific heat capacity
ΔT= change in temperature

Question 21

How do we use the basic technique calorimetry?

Answer 21

The idea is that the heat given out in a combustion reaction is used to heat another substance of no specific heat capacity such as water.

Question 22

What is the specific heat capacity of water?

Answer 22

4.18J g-1˚C-1

Question 23

What are some systematic errors in the experiment using a calorimetry? How can it be improved?

Answer 23

The most major problem is that of heat lost to the surroundings. Of the heat energy released when the ethanol burns, only some of it goes into heating the water – the rest goes into heating the copper can and the surrounding air. There can also be evaporation of the water and alcohol.

The experiment could’ve been improved by determining the specific heat capacity of the can and taking this into account when doing the calculations, insulating the can so that less he is lost through the can to the surrounding, using some sort of draught shield to reduce convection currents around experiment

Question 24

What is incomplete combustion?

Answer 24

When there is not a sufficient supply of oxygen to the flame, some of the ethanol can burn to produce carbon monoxide and soot as well as water. It gives out less heat than complete combustion. Incomplete combustion causes the flame to be yellow/orange rather than blue because of the presence of soot particles, and the soot can also be seen on the bottom of the cooper can.

Question 25

What is the enthalpy change of neutralisation ΔHn? Exo or Endo?

Answer 25

It is the enthalpy change when one mole of water molecules are formed when an acid (H+) reacts with an alkali (OH) under standard condition.

It is always exothermic

Question 26

What is enthalpy change of solution ΔHsol? Exo or Endo?

Answer 26

It is the enthalpy change when one more solute is dissolved in excess solvent to form a solution of infinite dilution under standard condition.

May be exothermic or endothermic

Question 27

What does infinite dilution mean?

Answer 27

any further dilution of the solution produces no further enthalpy change

i.e the solute particles are assumed not to interact with each other in the solution

Question 28

Why do we use Hess’s Law?

Answer 28

Using Hess’s Law we can work out unknown enthalpy changes from ones that are known

Question 29

What does Hess’s Law tell us?

Answer 29

It says that if you convert reactants A into products B, the overall enthalpy change will be exactly the same whether you do it in one step or two steps or however many steps

Question 30

What will the enthalpy level diagram look like?

Answer 30

Question 31

What is standard enthalpy change of formation? ΔH˚f?

Answer 31

It’s the enthalpy change when one mole of a substance is formed from its elements in their standard states under standard conditions

Question 32

Can a standard enthalpy change of formation be exothermic or endothermic?

Answer 32

It can be exo or endo depending on the substance

Question 33

What does standard state refer to?

Answer 33

to the pure substance at 100kPa and a specified temperature (assume 298K unless another temperature is specified)

Question 34

What is the enthalpy change of formation for elements in its standard state?

Answer 34

0 zero

Question 35

What is the formula of enthalpy change of reaction? ΔH˚r?

Answer 35

ΔH˚r = the sum of ΔHf (products) - the sum of ΔHf (reactants)

Question 36

What do we need to know in order to work out and will be changes for reactions in the gas phase?

Answer 36

It can be worked out if we know the amount of energy required/released when bonds are broken/made

Question 37

What is bond enthalpy/bond energy? When is bond enthalpies used?

Answer 37

Bond enthalpy is the enthalpy change when one mole of covalent bonds, in a gaseous molecule, is broken under standard condition.
They are used for reactions occurring in the gaseous state

Question 38

Which process is endothermic and which process is exothermic? What is it in terms of enthalpy change? Bond breaking or bond making?

Answer 38

Bond breaking requires energy (Endothermic): ΔH positive
Bond making releases energy (Exothermic): ΔH negative

Energy must be supplied to break apart two atoms in a molecule against the attractive force holding them together, therefore the process of breaking bonds must be endothermic.

Energy is released when two atoms come together to form a bond, therefore the process of making bonds is exothermic

Question 39

What is bond enthalpies defined in terms of?

Answer 39

Bond enthalpies are defined in terms of breaking bonds and therefore, have positive values

Question 40

What type of values are given in a bond energies table?

Answer 40

Average values
Because C2H6 has a slightly different value than C6H6
Due to this bond energy calculations may include inaccuracies

Question 41

What is average bond enthalpy?

Answer 41

Average bond enthalpy is the average amount of energy required to break one mole of covalent bonds in gaseous molecules under standard conditions.

‘Average’ refers to the fact that the bond enthalpy is different in different molecules and therefore the value quotes is the average amount of energy to break a particular bond in a range of molecules.

Question 42

Why are values for enthalpy changes of reaction obtained using bond enthalpy less reliable than those calculated using experimental data such as the enthalpy change of combustion or formation?

Answer 42

Because enthalpy changes of formation are specific to the particular substances, whereas bond enthalpies are average values

Question 43

What phase must it be to use bond enthalpy?

Answer 43

It is important to note that bond energies can only be used to decide whether a reaction in the gas phase is exothermic or endothermic. For reactions involving solids and liquids, other interactions, such as intermolecular forces, must also be taken into account

Question 44

In an exothermic and endo reaction are the products or reactants more stable?

Answer 44

In an exothermic reaction, the products are at a lower energy than the reactants and are therefore said to be more stable than the reactants.

For endothermic reaction, the products are at higher energy and are less stable than the reactants

Question 45

When can bond energies only be used in?

Answer 45

to decide whether a reaction in the gas phase is exothermic or endothermic

reactions invovling solids or liquids, other interactions like intermolecular forces must also be taken into account

Question 46

What can a potential energy diagram (profile) be used to show?

Answer 46

to show the energy changes occuring during a chemical reaction

Question 47

What is the role of ozone and oxygen in the atmosphere?

Answer 47

protect the surface of the Earth from the damaging effects of ultraviolet radiation

UV radiation is absorbed by the molecues of O2 and/or O3 a they undergo dissociation - breaking the bond between oxygen atoms

Question 48

Does O2 or O3 absorbed higher energy UV radiation?

Answer 48

O2 molecules

Question 49

why does O2 molecules absorb higher energy UV radiation then O3?

Answer 49

Because the ball between the oxygen atom and O2 is stronger than that between the oxygen atoms in O3

The UV radiation required to break the bond in O2 (double bond) is in the UV-C part of the spectrum and must have a wavelength shorter than 242nm to provide sufficient energy to break the bond

The bond in ozone is between a single bond and double bond and the absorptio of lower energy, UV-B, radiation will break the bond