Energetics 1

Question 1

Meaning Enthalpy Change

Answer 1

the heat energy change in a reaction at constant pressure & temperature

Question 2

what are standard conditions

Answer 2

• standard state
• 100 KPa
• 298K Temperature

Question 3

what are exothermic reactions

Answer 3

when heat energy is released from the system to the surround, increasing the temperature of the surroundings. Enthalpy change is negative as energy is released when bonds are formed

Question 4

what are endothermic reactions

Answer 4

when heat energy is absorbed from the surroundings to the system, decreasing the temperature of the surroundings. Enthalpy change is positive, as energy is required when bonds are broken

Question 5

meaning activation energy

Answer 5

the minimum amount of energy needed to begin breaking the reactant bonds, and to start the chemical reaction

Question 6

why are enthalpy changes in the data booklet often under standard conditions

Answer 6

because enthalpy change is affected by pressure & temperature, they need to stay constant

Question 7

Meaning Enthalpy of Formation

Answer 7

enthalpy change when 1 mole of substance is formed from its constituent elements with all substances in their standard states

Question 8

is Enthalpy of Formation exothermic or endothermic

Answer 8

exothermic

Question 9

Meaning Enthalpy of Combustion

Answer 9

enthalpy change when 1 mole of substance undergoes complete combustion in oxygen with all substances in their standard states

Question 10

is Enthalpy of Combustion exothermic or endothermic

Answer 10

exothermic

Question 11

Meaning Enthalpy of Neutralisation

Answer 11

enthalpy change when 1 mole of water is formed in a reaction between an acid & an alkali, under standard conditions

Question 12

is Enthalpy of Formation exothermic or endothermic

Answer 12

exothermic

Question 13

Meaning Enthalpy of Reaction

Answer 13

enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions

Question 14

how to measure enthalpy change of solids/liquids

Answer 14

• mix the reactants together in a polystyrene cup and measure the temperature with a thermometre every 30 seconds until it returns to its initial temperature

Question 15

how to measure enthalpy change of gases (flammable liquid)

Answer 15

• as the fuel burns, water heats up. By using.a calorimeter, work out the enthalpy change

Question 16

why is enthalpy change of own experiments not the same as the results shown in the data booklet

Answer 16

• heat may have escaped through the calorimeter
• fuel might not have combusted completely
• the conditions may not have been standard

Question 17

meaning hess’s law

Answer 17

the enthalpy change accompanying a chemical change is independent of the route by which the chemical change occurs

Question 18

which way do the arrows point in combustion

Answer 18

down

Question 19

which way do the arrows point in formation

Answer 19

up

Question 20

explain what happens when enthalpy change is positive

Answer 20

more energy is needed to break the bonds than form them

Question 21

explain what happens when enthalpy change is negative

Answer 21

less energy is needed to break the bonds than form them

Question 22

meaning bond enthalpy

Answer 22

the amount of energy required to break 1 mole of a type of bond in a molecule in the gas state

Question 23

meaning mean bond enthalpy (values in the data book)

Answer 23

the energy needed to break 1 mole of bonds in the gas state, averaged over many different compounds

Question 24

how to work out enthalpy change of a reaction

Answer 24

bond enthalpies of reactants - bond enthalpies of products

Question 25

meaning Enthalpy of Vaporisation

Answer 25

the energy needed to convert one mole of a liquid to vapour under standard conditions

Question 26

meaning Enthalpy of Atomisation

Answer 26

the enthalpy change when one mole of gaseous atoms is formed from an element under standard conditions