Energetics 1

Question 1

What is enthalpy change?

Answer 1

The amount of heat energy taken in or given out during any change in a system provided the pressure is constant

Question 2

What is the symbol for enthalpy?

Answer 2

H

Question 3

What happens when an enthalpy change occurs?

Answer 3

• Energy is transferred between system and surroundings
• System is chemicals
• Surrounding is everything outside chemicals

Question 4

What is the enthalpy change in an exothermic reaction?

Answer 4

Negative

Question 5

Why is the enthalpy change negative for an exothermic reaction?

Answer 5

• Energy transferred from system to surroundings
• Products have less energy than reactants

Question 6

What is the enthalpy change in an endothermic reaction?

Answer 6

Positive

Question 7

Why is the enthalpy change positive for an endothermic reaction?

Answer 7

• Energy transferred from surroundings to system
• Requires an input of heat energy
• Products have more energy than reactants

Question 8

What are the standard conditions?

Answer 8

298 K
100 kPa pressure
Solutions at 1 moldm^-3

Question 9

What is the standard enthalpy change of formation of a compound?

Answer 9

The enthalpy change when 1 mol of a compound is formed form its constituent elements in standard states under standard conditions

(in equation, mol of product is always 1)

Question 10

What is the enthalpy change of formation of an element in its standard state?

Answer 10

0 kJmol^-1

Question 11

What is the standard enthalpy of combustion of a substance?

Answer 11

The enthalpy change when 1 mol of a substance is burned completely in oxygen in standard states under standard conditions

(in equation, substance being combusted is always 1)

Question 12

Will incomplete combustion be less or more exothermic than complete?

Answer 12

Less

Question 13

What is enthalpy change of a reaction?

Answer 13

Enthalpy change when mols of reactants in the balanced equation equal the products and react together

Question 14

What is the enthalpy of neutralisation?

Answer 14

Enthalpy change when solutions of acid and alkali react to form 1 mol of water in standard states and under standard conditions

Question 15

What is the equation for energy in calorimetry?

Answer 15

Q = mcΔT

m = mass of water or not the actual substance you’re calculating enthalpy change for
c = specific heat capacity
ΔT = change in temperature

Question 16

What is the equation for enthalpy change?

Answer 16

Q/mol

Question 17

In a calorimety question, if the temperature increases, what do you need to add to your answer and why?

Answer 17

You make your answer negative as temp has increased so it is an exothermic reaction where ΔH is negative

Question 18

What is Hess’s law?

Answer 18

The total enthalpy change for a reaction is independent of the route by which the chemical change takes place

Question 19

What is the sum for the formation of enthalpy change of an element (Hess’s law)?

Answer 19

Sum of ΔHf products - Sum of ΔHf reactants

Question 20

What is the sum for the formation of enthalpy change of combustion (Hess’s law)?

Answer 20

Sum of ΔHc reactants - Sum of ΔHc products

Question 21

What is bond enthalpy?

Answer 21

The enthalpy change to break one mole of covalent bonds in the gas phase, averaged over different molecules

Question 22

What are the limitations of calculating enthalpy change using bond enthalpies?

Answer 22

The bond enthalpies are only averages and are not accurate