Enthalpy Changes

Question 1

What does system mean in a chemical reaction?

Answer 1

The atoms and bonds involved in a 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 from one form to another

Question 3

What energy change is breaking bonds associated with?

Answer 3

Energy is taken in to break bonds ->

Endothermic reaction

Question 4

What energy change is making bonds associated with?

Answer 4

Energy is released to make bonds
-> exothermic reaction

Question 5

What does activation energy mean?

Answer 5

The minimum energy required for a reaction to take place

Question 6

Which way does the arrow for activation energy point on an enthalpy profile programme?

Answer 6

Always points up

Question 7

What are the standard conditions?

Answer 7

100 KPa

298 K

Question 8

What does “in standard state” mean?

Answer 8

The state an element exists at in standard conditions

Question 9

Describe enthalpy change of formation?

Answer 9

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

Question 10

Define enthalpy change of combustion?

Answer 10

The energy change that takes place when 1 mole of a substance is completely combusted

Question 11

Define the enthalpy change of neutralisation?

Answer 11

The energy change that takes place when 1 mole of water is formed from a neutralisation reaction

Question 12

What does enthalpy change of reaction mean?

Answer 12

The energy change associated with a given reaction

Question 13

How can you calculate enthalpy change from experimental data?

Answer 13

Use the equation Q=mc T

M= mass of the substance being heated

C= specific heat capacity of that substance - (waters SHC= 4.18

T= change in temperature

Question 14

Why might experimental for enthalpy determination not be accurate?

Answer 14

-Heat is lost to the surroundings

• not in standard conditions

-reaction may not go to completion

Question 15

What does average bond enthalpy mean?

Answer 15

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

Question 16

Why will using bond enthalpies not be as accurate as using standard enthalpy of combustion?

Answer 16

Bond enthalpies are a mean for the same bond across different molecules whereas standard enthalpy of combustion and formation apply just to that molecule= more accurate

Question 17

How do you calculate enthalpy change of reaction using average bond enthalpies?

Answer 17

Enthalpy change of reaction= (bond enthalpies of reaction) - (bond enthalpies of products)

Question 18

How do you work out mean bond Enthalpy Of a compound ?

Answer 18

(Bond energies broken)-(bond energies made)

Then minus the enethalpy Change

And divide by the number of moles of the compound being asked about

Question 19

Is an exothermic reaction an example of bonds being broken or formed?

Answer 19

Making as energy is released

Question 20

Is an endothermic reaction an example of bonds being formed or broken?

Answer 20

Bonds are broken when energy is absorbed

Question 21

How do you calculate the Enthalpy of reaction using the Q=MCT equation?

Answer 21

1. Calculate Q using MxCxT
   
   • can use volume for mass
     -divide by 1000 to get KJ
2. Calculate moles of substance
3. Divide Q by moles and then flip the sign