Topic 8 - Energetics 1

Question 1

What is an endothermic reaction?

Answer 1

a reaction where energy is absorbed from surroundings
-bonds broken
+ve enthalpy change

Question 2

What is an exothermic reaction?

Answer 2

a reaction where energy is given out to surroundings

• bonds formed
• ve enthalpy change

Question 3

What is an enthalpy change?

Answer 3

the energy change at standard conditions

Question 4

What are standard conditions?

Answer 4

temp 298K (25°C)
pressure 1atm (100kPa)

Question 5

What is standard enthalpy change of formation?

Answer 5

enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions

-usually exothermic

Question 6

What is standard enthalpy change of combustion?

Answer 6

enthalpy change when one mole of a substance (fuel) undergoes complete combustion under standard conditions

-exothermic

Question 7

What is standard enthalpy change of neutralisation?

Answer 7

enthalpy change when one mole of water is formed from the reaction of an acid and alkali under standard conditions

-exothermic

Question 8

Q =

Answer 8

mcΔT

Question 9

What assumptions are made when using Q=mcΔT?

Answer 9

• all energy produced is exchanged with products and water in calorimeter
• 1cm³ of dilute aqueous solution weighs 1g
• 1g of water requires 4.18J to raise the temp by 1°C

Question 10

What does Hess’ law state?

Answer 10

the overall enthalpy change of a reaction is independent of the route taken

Question 11

What is bond enthalpy?

Answer 11

the energy required to break one mole of a covalent bond into gaseous atoms

Question 12

What is the mean bond enthalpy?

Answer 12

the overall energy required to break one mole of a covalent bond into gaseous atoms across all molecules containing that bond