chapter 9

Question 1

describe the conservation of energy

Answer 1

• energy cannot be created or destroyed
• when a reaction involving enthalpy changes takes place, heat energy is transfered between the system and the surroundings

Question 2

what is enthalpy

Answer 2

• the measure of heat energy present in a chemical system

Question 3

what is enthalpy change

Answer 3

• the amount of energy released or taken in during a chemical reaction

Question 4

what is an endothermic reaction

Answer 4

• a reaction which takes in energy from the surroundings
• it has a positive enthalpy value

Question 5

what is an exothermic reaction

Answer 5

• a reaction which releases energy into the surroundings
• has a negative enthalpy value

Question 6

what is activation energy

Answer 6

• the minimum energy required for a reaction to take place

Question 7

what are standard conditions

Answer 7

• 100 kPa
• 298 K
• standard states

Question 8

what is the standard enthalpy change of a reaction

Answer 8

• the enthalpy change associated with a stated equation in the molar quantities shown in the chemical equation under standard conditions

Question 9

what is the standard enthalpy change of formation

Answer 9

• the enthalpy change associated with the formation of 1 mole of a compound from its elements under standard conditions

Question 10

what is the standard enthalpy change of combustion

Answer 10

• the enthalpy changes associated with the complete combustion of 1 mole of a substance under standard conditions

Question 11

what is the standard enthalpy change of neutralisation

Answer 11

• the enthalpy change associated with the formation of 1 mole of water from a neutralisation reaction between an acid and a base, under standard conditions

Question 12

describe an experiment to find the enthalpy change

Answer 12

• calorimeter experiment
• measure a volume of water into the calorimeter
• weigh the spirit burner containing the fuel with the lid on
• measure the initial temperature of the water
• place the spirit burner under the calorimeter
• light the spirit burner and heat the water, stirring with the thermometer
• use the lid of the spirit burner to put out the flame
• record the highest temperature reached by the water
• weigh the spirit burner with the lid on and calculate the change in mass

Question 13

what are the 4 biggest flaws in a calorimetry experiment

Answer 13

• heat loss to the surroundings
• incomplete combustion of the fuel
• evaporation of the fuel
• not performed under standard conditions

Question 14

what are cooling curves used for

Answer 14

• can be used to increase the accuracy of the heat energy value as it corrects for heat loss during the experiment

Question 15

what is average bond enthalpy

Answer 15

• the energy required to break one mole of a specified type of bond in a gaseous molecule
• in chapter 5.3 its a measurement of the strength of a covalent bond, the larger the bond enthalpy value, the stronger the bond

Question 16

what are the limitations of average bond enthalpies

Answer 16

• text book values often use the actual bond enthalpy values, whereas we use the average bond enthalpy values
• the experiment is not performed under standard conditions

Question 17

what makes a reaction endothermic

Answer 17

• more energy is required to break bonds than released when making bonds

Question 18

what makes a reaction exothermic

Answer 18

• more energy is released when making bonds than required to break the bonds

Question 19

what is the calculation for calculating enthalpy changes

with enthalpies of reactants and products

Answer 19

• the sum of the enthalpies in the reactants - the sum of bond enthalpies in products

Question 20

what is hess’ law

Answer 20

• if a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route