Topic 5 : Energetics/Thermochemistry

Question 1

Energy

Answer 1

1. Energy is a measure of the ability to do work. It is a form of energy
2. There are many different types of energy and heat is only one of them
3. During chemical reactions energy flows in and out of the reaction vessels
4. Inside the reaction vessel is known as the system
5. Outside the reaction vessel is known as the surroundings

Question 2

3 types of system

Answer 2

1. Open system
2. Closed system
3. Isolated

Question 3

Isolated systems

Answer 3

Isolated systems are very rare; most chemical reactions are open systems

Question 4

Law of conservation of energy

Answer 4

Although energy can be exchanged between open and closed systems and the surroundings, the total energy of the process cannot change

Question 5

Enthalpy

Answer 5

1. The total chemical energy inside a substance is called the enthalpy (or heat content)
2. When chemical reactions take place, changes in chemical energy take place and therefore the enthalpy changes
3. An enthalpy change is represented by the symbol ΔH (Δ= change; H = enthalpy)
4. An enthalpy change can be positive or negative

Question 6

Exothermic reactions

Answer 6

1. A reaction is exothermic when the products have less enthalpy than the reactants (reactants are higher in energy than the products)
2. Heat energy is given off by the system to the surroundings in terms of the temperature of the surroundings increases or the temperature of the system decreases
3. There is an enthalpy decrease during the reaction so ΔH is negative
4. Exothermic reactions are thermodynamically possible (because the enthalpy of the reactants is higher than that of the products)
5. However, if the rate is too slow, the reaction may not occur. In this case the reaction is kinetically controlled

Question 7

Endothermic reactions

Answer 7

1. A reaction is endothermic when the products have more enthalpy than the reactants
2. Heat energy is absorbed by the system from the surroundings so the temperature of the surroundings decreases or/and the temperature of the system increases
3. There is an enthalpy increase during the reaction so ΔH is positive

Question 8

Standard conditions explanation

Answer 8

To compare the changes in enthalpy between reactions, all thermodynamic measurements are made under standard conditions

Question 9

Standard condition details

Answer 9

1. A pressure of 100 kPa
2. A concentration of 1 mol dm-3 for all solutions
3. Each substance involved in the reaction is in its standard state (solid, gas or liquid)
4. Temperature is not part of the definition of standard state, but a temperature of 298 K (25 oC) is usually given as the specified temperature

Question 10

Standard enthalpies for reactions

Answer 10

1. the enthalpy change when reactants in the stoichiometric equation react to give the products under standard conditions
2. Can be either endothermic or exothermic

Question 11

Standard enthalpies for formation

Answer 11

1. the enthalpy change when one mole of a compound is formed from its elements under standard condition
2. can be endothermic or exothermic

Question 12

standard enthalpies for combustion

Answer 12

1. the enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions
2. exothermic

Question 13

standard enthalpies for neutralisation

Answer 13

1. the enthalpy change when one mole of water is formed by reacting an acid and alkali under standard conditions
2. exothermic

Question 14

Calorimetry

Answer 14

1. Calorimetry is a technique used to measure changes in enthalpy of chemical reactions
2. A calorimeter can be made up of a polystyrene drinking cup, a vacuum flask or metal can

Question 15

Specific Heat capacity

Answer 15

1. The energy needed to raise the temperature of 1 g of a substance by 1 K is called the specific heat capacity (c) of the liquid
2. The specific heat capacity of water is 4.18 J g-1 K-1

Question 16

energy transferred a heat formula

Answer 16

q = mc delta t

q = heat transferred (J)
m = mass 
c = specific heat capacity
t = temperature (in kelvins)

Question 17

Types of calorimetry experiments

Answer 17

1. Enthalpy changes of reactions in solution
2. Enthalpy changes of combustion

Question 18

Enthalpy changes for reactions in solution

Answer 18

1. The principle of these calorimetry experiments is to carry out the reaction with an excess of one reagent and measure the temperature change over the course of a few minutes

For the purposes of calculations certain assumptions are made:

1. That the specific heat capacity of the solution is the same as pure water, i.e. 4.18 J g-1 K-1
2. That the density of the solution is the same as pure water, i.e. 1 g cm-3
3. The specific heat capacity of the container is ignored
4. The reaction is complete
5. There are negligible heat losses

Question 19

Temperature correction graphs

Answer 19

1. For reactions which are not instantaneous there may be a delay before the maximum temperature is reached
2. During that delay the substances themselves may be losing heat to the surroundings, so that the true maximum temperature is never actually reached
3. To overcome this problem we can use graphical analysis to determine the maximum enthalpy change

Question 20

Steps to make a temperature correction graph

Answer 20

1. Take a temperature reading before adding the reactants for a few minutes to get a steady value
2. Add the second reactant and continue recording the temperature and time
3. Plot the graph and extrapolate the cooling part of the graph until you intersect the time at which the second reactant was added

Question 21

Enthalpy of Combustion Experiments

Answer 21

1. The principle here is to use the heat released by a combustion reaction to increase the heat content of water
2. A typical simple calorimeter is used to measure the temperature changes to the water
3. Not all the heat produced by the combustion reaction is transferred to the water: Some heat is lost to the surroundings and some heat is absorbed by the calorimeter
4. To minimise the heat losses the copper calorimeter should not be placed too far above the flame and a lid placed over the calorimeter
5. Shielding can be used to reduce draughts

Question 22

Main sources of error of in an enthalpy of combustion calorimetry experiment

Answer 22

1. Heat losses
2. Incomplete combustion

Question 23

Hess’s Law

Answer 23

1. Hess’s law can be used to calculate the standard enthalpy change of a reaction from known standard enthalpy changes
2. Hess’s law states that: “The total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same.”
3. This means that whether the reaction takes place in one or two steps, the total enthalpy change of the reaction will still be the same
4. You can use cycles or equations to solve Hess’s Law

Question 24

Enthalpy Changes

Answer 24

1. When bonds are broken or made enthalpy changes take place
2. A chemical bond is a force of attraction between two atoms
3. The energy required to break a particular bond is called the bond dissociation enthalpy
4. This is usually just shortened to bond enthalpy or bond energy

Question 25

Breaking and Making bonds

Answer 25

1. Breaking the bond requires the input of energy it is therefore an endothermic process
2. Bond formation is the opposite of bond breaking and so energy is released when bonds are formed
   It is therefore an exothermic process

Question 26

Overall enthalpy changes - exothermic

Answer 26

1. If more energy is released when new bonds are formed than energy is required to break bonds, the reaction is exothermic
2. The products are more stable than the reactants

Question 27

Overall enthalpy changes - endothermic

Answer 27

1. If more energy is required to break bonds than energy is released when new bonds are formed, the reaction is endothermic
2. The products are less stable than the reactants

Question 28

Average bond energy

Answer 28

1. ‘The energy needed to break one mole of bonds in a gaseous molecule averaged over similar compounds’
2. Bond energies are affected by other atoms in the molecule (the environment)
3. Therefore, an average of a number of the same type of bond but in different environments is calculated
4. This bond energy is known as the average bond energy

Question 29

Average bond enthalpy of C-H in methane

Answer 29

1. The average bond enthalpy of C-H is found by taking the bond dissociation enthalpy for the whole molecule and dividing it by the number of C-H bonds
2. The first C-H bond is easier to break than the second as the remaining hydrogens are pulled more closely to the carbon
3. However, since it is impossible to measure the energy of each C-H bond an average is taken
4. This value is also compared with a range of similar compounds to obtain an accepted value for the average bond enthalpy

Question 30

Standard enthalpy of reaction formula

Answer 30

Standard enthalpy of reaction formula = Enthalpy change for bonds broken + Enthalpy change for bonds formed

Question 31

Energy Profiles

Answer 31

1. An energy profile or energy level diagram is a diagram that shows the energies of the reactants, the transition state(s) and the products of the reaction with time
2. The transition state is a stage during the reaction at which chemical bonds are partially broken and formed
3. The transition state is very unstable – it cannot be isolated and is higher in energy than the reactants and products

Question 32

Activation energy

Answer 32

1. ‘the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction’
2. The activation energy (Ea) is the energy needed to reach the transition state
3. exothermic reactions have a lower activation energy compared to endothermic reactions