Enthalpy Changes

Question 1

What does system mean in a chemical reaction?

Answer 1

The atoms and bonds involved in the chemical reaction

Question 2

Explain the law of conservation

Answer 2

The amount of energy in an isolated system remains the same. Energy cannot be destroyed or created, it can only be transferred form one form to another

Question 3

What energy change is breaking bonds associated with?

Answer 3

Energy is taken in to break bonds so endothermic

Question 4

What energy change is making bonds associated with?

Answer 4

Energy is released when making bonds so exothermic

Question 5

What is an endothermic reactions?

Answer 5

A reaction with an overall +ve delta H
The Enthalpy of products is greater than the Enthalpy of reactants
So temperature of surroundings is lowered as energy is taken in

Question 6

What is an exothermic reaction?

Answer 6

A reaction with an overall negative Enthalpy change -ve delta H
Enthalpy of products is less than the Enthalpy of reactants
Heat is released to surroundings

Question 7

Explain an energy level diagram for an exothermic reaction

Answer 7

Products have a lower energy than reactants
Activation energy is from reactants to top of curve
Delta H is from reactants to products

Question 8

Explain an endothermic reaction?

Answer 8

Reactants is lower than products
Activation energy is from reactants to top of curve
Delta h is from reactants to products

Question 9

What does activation energy mean?

Answer 9

The minimum energy required for a reaction to take place

Question 10

Which way does the arrow point for activation energy in an energy profile diagram?

Answer 10

Upwards

Question 11

What are standard conditions

Answer 11

100 KPa

298 K

Question 12

What does in standard state mean?

Answer 12

The state an element or compound exists at in standard conditions 100 kPa and 298 K

Question 13

Define Enthalpy change of formation

Answer 13

The energy change that takes place when 1 mole of a compound is formed under its constituent elements in their standard state under standard conditions

Question 14

Define Enthalpy change of combustion

Answer 14

The energy when 1 mole of a substance is completely combusted in oxygen in their standard states and under standard conditions 100 kPa and 298 K

Question 15

Define Enthalpy change of neutralisation

Answer 15

The energy change when 1 mole of water is formed on a neutralisation reaction in standard states under standard conditions 100kPa and 298K

Question 16

What does Enthalpy change of a reaction mean?

Answer 16

The energy change associated with a given reaction

Question 17

How can you calculate the Enthalpy change form experimental data ?

Answer 17

Q= mc delta T
M is the mass of water / substance
C is specific heat capacity of water 4.18
Delta T is change in temp initial - max/min

Question 18

What are the advantages of using a bomb calorimeter?

Answer 18

Minimises heat loss

Pure oxygen used ensures complete combustion

Question 19

Why might experimental methods for Enthalpy determination not be accurate?

Answer 19

Heat is lost to the surroundings
Not in standard conditions .
Reaction may not go to completion

Question 20

What does average bond Enthalpy mean?

Answer 20

The mean energy required to break 1 mole of bonds in gaseous molecules

Question 21

Why will using bond enthalpies be as accurate as using standard Enthalpy of combustion/formation ?

Answer 21

Bond enthalpies are a mean for the same bond across different molecules whereas standard Enthalpy of combustion and formation apply just to that molecule, therefore they are more accurate.

Question 22

How to calculate Enthalpy change using average bond enthalpies?

Answer 22

Delta H= Sum of bind Enthalpy of reaction— sum of bond Enthalpy of products