Module 3 Section 2

Question 1

Define enthalpy change

Answer 1

Enthalpy change (delta H) is the heat energy transferred in a reaction at constant pressure

Units are kJmol^-1

Question 2

For what kind of reaction is the enthalpy change NEGATIVE

Answer 2

Exothermic

Question 3

For what kind of reaction is the enthalpy change POSITIVE?

Answer 3

Endothermic reaction

Question 4

Look up enthalpy profile diagrams

Answer 4

Textbook

Question 5

Define standard enthalpy change of reaction, delta Hr

Answer 5

Enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions.

Question 6

Define the standard enthalpy change of formation, delta Hf

Answer 6

Enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions.

Question 7

Define standard enthalpy change of combustion

Answer 7

The enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions

Question 8

What is bond breaking?

Answer 8

Endothermic (delta H is positive)

You need to take in energy to break bonds

Question 9

What is bond making

Answer 9

Exothermic (delta H is negative)

Energy is released when bonds are formed

Question 10

Why are bond enthalpies an average?

Answer 10

A certain bond has different bond enthalpies depending on where it’s placed in a molecule, so they use an average.

Question 11

How do you find enthalpy changes in a lab

Answer 11

You can find the enthalpy of combustion of a flammable liquid

You do this by burning a known mass of water and working out the temperature change

You then use the moles of your fuel used and the energy transferred to the water (using mc(deltaT))

Question 12

Look up Hess’s Law stuff

Answer 12

Textbook

Question 13

What is collision theory?

Answer 13

A reaction between two particles won’t occur unless:

• they collide in the correct orientation
• they collide with a minimum amount of kinetic energy

Question 14

What is the minimum amount of kinetic energy particles need to react called?

Answer 14

Activation energy

Question 15

Look at Boltzmann distribution stuff

Answer 15

Textbook

Question 16

How do catalysts help the rate of reaction?

Answer 16

Provides an alternative reaction pathway with a lower activation energy

It is chemically unchanged at the end of the reaction

Question 17

What is a heterogeneous catalyst

Answer 17

One which is in a different phase/state to the reactants

Reaction happens on the surface of the catalyst

So increasing the surface area of the catalysts means more reactants can react at the same time

Question 18

What is a homogeneous catalyst

Answer 18

They’re in the same physical state as the reactants

Usually an aq catalyst between 2 aq solutions

Works by forming an intermediate species. Reactants combine with catalyst to make it

Intermediate then reacts to form the products and reform the catalyst

Question 19

What ways can you measure the rate of reaction?

Answer 19

• change in mass
• volume of gas given off
• colour changes
• changes in pressure
• changes in conductivity

Question 20

What happens to the concentrations of reactants and products when dynamic equilibrium is reached?

Answer 20

They stay constant

Question 21

What does it mean for the equilibrium of a reaction to have moved to the LEFT?

Answer 21

Means more of the stuff on the left (reactants) will be made

Question 22

What does it mean for the equilibrium of a reaction to be on the RIGHT?

Answer 22

Means more of the stuff on the right (products) is being made

Question 23

What is Le Chatelier’s Principle?

Answer 23

“If there’s a change in concentration, pressure or temperature, the equilibrium will move to counteract that change

Question 24

What happens to equilibrium position when you increase the temperature of a reaction?

Answer 24

It will shift in the endothermic direction, to absorb the heat

The opposite occurs for the opposite situation

Question 25

What happens to equilibrium position when you increase the pressure of a reaction?

Answer 25

Equilibrium shifts to the side where there are fewest moles of gas, thus decreasing the pressure

If you decreased the pressure, it would shift to where there are more moles of gas

Question 26

What do catalysts do to equilibrium?

Answer 26

They increase the rate of both the forward and reverse reactions by the same amount

Question 27

Look up equilibrium constant, Kc

Answer 27

Textbook

Question 28

What is the general formula for calculating the equilibrium constant?

Answer 28

Concentration of products divided by the concentrations of reactants

If in the word equation there is more than 1 mole of something, you put the concentration of that compound to the power of that ratio

E.g. if it were 2H2O, you’d do [H2O]^2

Question 29

How do you use the the equilibrium constant, Kc?

Answer 29

You can use it to estimate the position of equilibrium

The larger Kc is, the further to the right equilibrium is, and the more products there are than reactants

The lower Kc is, the further to the left equilibrium is, and the more reactants there are than products