Topic 5

Question 1

What is heat?

Answer 1

the transfer of energy between objects of different temperature

According to the second law of thermodynamics, heat will spontaneously flow from an object of higher temperature to an object of lower temperature

Once the two objects reach the same temperature, which is known as thermal equilibrium, no more energy will be transferred.

Question 2

What is temperature

Answer 2

Measure of the average kinetic energy of particles

- absolute temperature in kelvin is directly proportional to the average kinetic energy of particles in a substance

Question 3

What determines which way heat will flow?

Answer 3

Temp differences determine which way heat will flow.

Heat always flows from higher to lower temperature.

Question 4

The higher the enthalpy of a substance…….

Answer 4

The higher the enthalpy of a substance, the less energetically stable it is.

Question 5

system

Answer 5

all the reactants, products and solvents

Question 6

surroundings

Answer 6

includes apparatus for the reaction, thermometers, measuring devices, lab, everything external to the reacting substances

Question 7

calorimeter

Answer 7

is any apparatus used to measure the amount of heat being exchanged with the surroundings

Question 8

Law of conservation of energy

Answer 8

energy can be converted from one form to another and the total amount of energy for a given system will remain constant

Question 9

Exothermic reactions

Answer 9

heat is transferred from the system to the surroundings (-∆H)

• Bond formation is an exothermic process
• products are more energetically stable than reactants
• metal replacement reaction between zinc and copper sulfate

Question 10

Endothermic reactions

Answer 10

chemical reactions that absorb heat from the surroundings (+∆H)

• Bond breaking/ bond disassociation is an endothermic process
• products are less energetically stable than reactants
• ammonium nitrate dissolving in water

Question 11

What is ∆H?

Answer 11

∆H is the change in enthalpy (heat) - the heat transferred by a closed system during a chemical reaction (kJ)

Question 12

What is the the standard enthalpy change of combustion?

Answer 12

the heat evolved upon complete combustion of 1 mol of substance

Question 13

What is the standard enthalpy change of formation?

Answer 13

the energy change upon the formation of 1 mol of a substance from its constituent elements in their standard state.

• can be either positive or negative.
• – negative value means that under standard conditions the compound is energetically more stable than its elements
• – positive value means that it is less energetically stable
• The enthalpy change of formation of an element in its most stable form in the standard state is zero by definition.
• e.g, O2

Question 14

heat of the substance is dependent on?

Answer 14

heat of the substance is dependent on: mass, type of substance, and temperature change

Question 15

what is specific heat capacity?

Answer 15

the amount of energy (J) required to raise the temperature of 1g of a substance by 1 K. It can also be in kJ and for 1kg of a substance - JK-1g-1 or kJK-1kg-1

• Specific heat of water: 4.18 JK-1g-1or 4.18 kJK-1kg-1
• Specific heat of copper: 0.39 JK-1g
• • therefore, copper is a better conductor

Question 16

What is the main systematic error in a calorimetry experiment?

Answer 16

heat loss to the surroundings

Question 17

What is the unit for q?

Answer 17

J Joules

Question 18

what is the unit for m?

Answer 18

g

Question 19

what is c?

Answer 19

specific heat capacity

Question 20

deduce, from an enthalpy level diagram, the relative stabilities of reactants and products, and the sign of the enthalpy change for the reaction.

Answer 20

the most stable state is where all energy has been released. When going to a more stable state, energy will be released, and when going to a less stable state, energy will be gained (from the surroundings).
- On an enthalpy level diagram, higher positions will be less stable (with more internal energy) therefore, if the product is lower, heat is released (more stable, ΔH is -ve) but if it is higher, heat is gained (less stable, ΔH is +ve).

Question 21

What formula is used to calculate the heat energy change when the temperature of a pure substance is changed.

Answer 21

q = mcΔT

Question 22

What is average bond enthalpy?

Answer 22

The energy required to break a bond in standard conditions
- Bond Enthalpy is the energy required when one mole of bonds is broken in the gaseous state (bond enthalpies are valid only in the gaseous state). Bond breaking needs energy and is an endothermic process. Bond making releases energy and it is an exothermic process. Average bond energy is the average energy required to break a mole of the same type of bonds in the gaseous state in a variety of similar compounds.

Question 23

What are the standard conditions?

Answer 23

298K, 100kPa and everything in their standard states

Question 24

Are enthalpy changes in combustion pos or neg & why?

Answer 24

always negative as heat is released during

combustion processes.

Question 25

what is the standard enthalpy of combustion of carbon?

Answer 25

The standard enthalpy of combustion of carbon is the heat released when one mole of solid carbon (graphite) is completely combusted to form carbon dioxide gas under standard conditions

Question 26

What is the relationship between the enthalpy of combustion of the alkanes and the number of carbon atoms?

Answer 26

• a linear relationship between the enthalpy of combustion of the alkanes & the number of carbon atoms bc the compounds in the homologous series have an extra –CH2– group between successive members

Question 27

Hesses Law

Answer 27

if a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same

Question 28

Where can stored chemical energy come from?

Answer 28

• attractions between protons & electrons
• repulsions between atomic nuclei
• repulsions between electrons
• electron movement
• vibrations & rotations around bonds

Question 29

What is the specific heat capacity of water?

Answer 29

4.18Jg−1K−1.

Question 30

the higher something is in specific heat capacity….

Answer 30

the smallest increase in temp it will have

Question 31

State the region of the electromagnetic spectrum that provides the energy required to break molecular oxygen into oxygen atoms in the upper atmosphere.

Answer 31

UV light

Question 32

Law of conservation of energy

Answer 32

energy can’t be created or destroyed. So, if a reaction can happen in two different ways the overall energy change for the first way must be equal to the overall energy change for the second.

Question 33

What happens when heat is transferred to an object?

Answer 33

when heat is transferred to an object, it increases in the average kinetic energy of its particles & therefore causes an increase in its temperature or a change in phase

Question 34

Why is the change in heat enthalpy positive in endothermic reactions?

Answer 34

bc the amount of energy in the bonds which is stored has increased

Question 35

what is standard enthalpy change of reaction (ΔHƟ).

Answer 35

when the enthalpy change is measured under standard conditions

Standard conditions

pressure: 100kPa
temp: 298