Section 5 - Physical Chemistry P2

Question 1

What are reaction profiles?

Answer 1

diagrams the show the relative energies of the reactants and products in a reaction, and how the energy changes over the course of a reaction

Question 2

What does an energy profile of an exothermic reaction look like?

Answer 2

• draw a graph of reaction by energy
• the products have a lower energy line than the reactants (-ΔH [energy released] is the distance between the reactant and product line)
• between the two lines there is a hill (the activation energy, it’s the distance from the reactants to the peak of the hill)

Question 3

What is the activation energy?

Answer 3

The activation energy is the minimum amount of energy the reactants need to collide with each other and react.

Question 4

What does an energy profile of an endothermic reaction look like?

Answer 4

• draw a graph of reaction by energy
• the products have a higher energy line than the reactants (+ΔH [energy absorbed] is the distance between the reactant and product line)
• between the two lines there is a hill (the activation energy, it’s the distance from the reactants to the peak of the hill)

Question 5

What is needed for bonds to break?

Answer 5

Energy must be supplied for bonds to break

Question 6

For an endothermic process what happens to bonds?

Answer 6

Energy must be supplied to break existing bonds

-so bond breaking is an endothermic process

Question 7

For an exothermic process what happens to bonds?

Answer 7

Energy is released when new bonds are formed

-so bond making is an exothermic process

Question 8

What happens to the energy during an endothermic reaction?

Answer 8

the energy used to break bonds is greater than the energy released by forming them

Question 9

What happens to the energy during an exothermic reaction?

Answer 9

the energy released by forming bonds is greater than the energy used to break them

Question 10

What is the general equation for working out enthalpy change?

Answer 10

enthalpy change (ΔH) = 
total energy absorbed to make the bonds - total energy released in making bonds

Question 11

How do you work out enthalpy change?

Answer 11

• work out the energy require for the bonds on the reactant side
• work out the energy required for the bonds on the product side
• reactant energy - product energy = enthalpy change

Question 12

What type of graph can you show activation energy on?

Answer 12

Reaction profile

-activation energy is the difference between the energy of the reactants and the highest point on the curve

Question 13

What is the effect of a catalyst on a reaction profile?

Answer 13

• the highest point on the curve for a reaction with a catalyst will be lower than without a catalyst
• so the distance from the reactant line to the highest point of the curve will be less

Question 14

What conditions does a reaction have to take place within for equilibrium to be reached?

Answer 14

In a closed system (none of the products or reactants can escape)
-then a state of equilibrium will always be reached

Question 15

Define Equilibrium:

Answer 15

Equilibrium means that the concentrations of reactants and products will reach a certain balance and stay there

Question 16

What is Dynamic Equilibrium?

Answer 16

The forward and backward reactions are still taking place but the overall effect is nil.

• the forward and backward reactions cancel each other out
• the reactions are occurring at exactly the same rate

Question 17

What does the position of equilibrium depend on?

Answer 17

position of equilibrium - the relative amounts of products and reactants
-depends on the temperature and pressure of the reacting mixture

Question 18

Why would you change the position of equilibrium?

Answer 18

-altering the temperature or pressure can move the position of equilibrium to give more product and less reactant

Question 19

What effect does temperature have on the position of equilibrium?

Answer 19

All reactions are exothermic in one direction and endothermic in the other

• if you raise the temperature, the endothermic reaction will increase to use up the extra heat
• if you reduce the temperature, the exothermic reaction will increase to give out more heat

Question 20

What effect does pressure have on the position of equilibrium?

Answer 20

Most gaseous reactions have more moles of gas on one side than another

• if you raise the pressure it will encourage the reaction which produces fewer moles of gas
• if you lower the pressure it will encourage the reaction which produces more moles of gas

Question 21

What effect does a catalyst have on the position of equilibrium?

Answer 21

HAS NO EFFECT ON THE POSITION OF EQUILIBRIUM

-it speeds up the forward and backward reactions by the same amounts