Enthalpy

Question 1

What is enthalpy?

Answer 1

The measure of heat energy within a chemical system

Question 2

How do you calculate enthalpy change?

Answer 2

Enthalpy of products - Enthalpy of reactants

Question 3

Where is energy transferred in an exothermic change?

Answer 3

Energy is transferred from system to the surroundings

Question 4

Where is energy transferred in an endothermic change?

Answer 4

Energy is transferred from the surroundings to the system

Question 5

What are 4 key features of an exothermic reaction?

Answer 5

Enthalpy change is negative
Chemical system loses energy
Surroundings gain energy
Temperature of surroundings increases

Question 6

What are 4 key features of an endothermic reaction?

Answer 6

Enthalpy change is positive
Chemical system gains energy
Surroundings lose energy
Temperature of surroundings decreases

Question 7

What is activation energy?

Answer 7

The minimum energy required for a reaction to take place

Question 8

What are standard conditions?

Answer 8

25 C or 298K
100kPa
1 mol dm^3

Question 9

Define standard enthalpy change of formation

Answer 9

Enthalpy change when one mole of a substance is formed from its constituent elements with all substances in their standard states

Question 10

What is the enthalpy change of formation for all elements?

Answer 10

0

Question 11

Define standard enthalpy change of combustion

Answer 11

Enthalpy change when one mole of a substance undergoes complete combustion in oxygen with all substances in standard states

Question 12

Define standard enthalpy change of neutralisation

Answer 12

Enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions

Question 13

What is 0 degrees C in K?

Answer 13

273K

Question 14

Define specific heat capacity

Answer 14

The energy required to raise the temperature of 1g of a substance by 1K

Question 15

What is the specific heat capacity of water?

Answer 15

4.18

Question 16

What is the equation for calculating heat energy?

Answer 16

Q= mc delta T

Question 17

Give reasons for inaccuracy in heat energy of combustion practical

Answer 17

Heat energy lost to the surroundings
Incomplete combustion
Evaporation of alcohol from the wick
Non standard conditions

Question 18

How would you minimise effect of heat loss or incomplete combustion?

Answer 18

Draught screens

Input of oxygen gas

Question 19

What piece of equipment is used to measure enthalpy change of reaction between two solutions?

Answer 19

Polystyrene cups

Question 20

Define ‘Average Bond Enthalpy’

Answer 20

Energy required to break one mole of a specified type of bond

Question 21

Are bond enthalpies exothermic or endothermic?

Answer 21

Endothermic (always have a positive value)

Question 22

What is an average bond enthalpy calculated from?

Answer 22

Actual bond enthalpies in different environments

Question 23

Bond breaking- exothermic or endothermic?

Answer 23

Endothermic

Question 24

Bond making- exothermic or endothermic?

Answer 24

Exothermic

Question 25

What determines whether a reaction is overall exothermic or endothermic?

Answer 25

The difference between the value for making the bond and value for breaking bonds

Question 26

How is enthalpy change calculated from bond enthalpies?

Answer 26

Bond enthalpies in reactants - Bond enthalpies in products

Question 27

What are the limitations for using average bond enthalpies?

Answer 27

The bond enthalpies are slightly different in each environment, however not a major difference as an average bond enthalpy is used. Need all molecules to be gaseous

Question 28

What does 'Hess' Law' state?

Answer 28

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