Physical chemistry Module 3

Question 1

Enthalpy change definition?

Answer 1

ΔH, the heat energy transferred in a reaction at constant pressure
Units are kj mol^-1

Question 2

What are the standard conditions?

Answer 2

298 kelvin (25 degrees)
100kPa (1 atm)
Standard states

Question 3

Exothermic reaction definition?

Answer 3

Reaction gives out energy
Temperature of system goes up
ΔH is negative

Question 4

Endothermic reaction definition?

Answer 4

Reactions absorbs energy
Temperature of system goes down
ΔH is positive

Question 5

Activation energy definition?

Answer 5

The minimum amount of energy required to begin breaking reactant bonds and start a chemical reaction

Question 6

What’s the standard enthalpy change of reaction?

Answer 6

Δ(r)H, Enthalpy change when the reaction occurs in the molar quantities shown chemical equation under standard conditions

Question 7

What’s the standard enthalpy change of formation

Answer 7

Δ(f)H, enthalpy change when 1 mole of compound is formed from it’s elements in their standard states under standard conditions

Question 8

What’s the standard enthalpy change of combustion?

Answer 8

Δ(c)H, enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions

Question 9

What’s the standard enthalpy change of neutralisation?

Answer 9

Δ(neut)H, enthalpy change when an acid and an alkali react together under standard conditions to form 1 mole of water

Question 10

What is bond breaking?

Answer 10

Endothermic as requires energy

Question 11

What is bond forming?

Answer 11

Exothermic as releases energy

Question 12

What is enthalpy change in terms of bond forming and breaking?

Answer 12

The overall affect of both of them occuring

Average bond enthalpy of breaking bonds - average bond enthalpy of forming bonds

Question 13

How is enthalpy change found in the lab?

Answer 13

Calorimetry

Question 14

How do you calculate enthalpy changes using a formula?

Answer 14

Heat losss or gained in joules = mass of water in grams x 4.18 x change in temperature

Put all values into equation

Convert answer from joules to kJ by dividing by 1000

Work out how many moles of fuel produced this heat

Divide answer by the amount of moles to find standard enthalpy change (kJmol^-1) also make it negative

Question 15

How to work out enthalpy change using enthalpies of formation?

Answer 15

Write out the reactants equalling the products, then bellow write out all the elements in their natural states that you require

Draw arrows from the natural state elements to the reactants and products, then look how many moles are made and of which substance to work out the enthalpy changes of formation for reactants and products

(if element stay the same won’t be an enthalpy change, remember to multiply by the amount of moles)

Work out total distance from the reactants to products, bearing in mind the first arrow is going in the wrong direction

Question 16

How can enthalpy changes be worked out using the enthalpy change of combustion?

Answer 16

Same method before, however the direction of arrows have changed and so has what’s on the bottom

On the bottom have water and CO2 and both the lines will be going downwards with an amount of oxygen next to them to balance the equation

Work out total distance taking into account second line is going to wrong way

Question 17

What must occur in a particle collision for a reaction to happen?

Answer 17

Must collide in the right orientation

Must collide with at least the activation energy

Question 18

How to draw a Boltzman distribution?

Answer 18

Number of particles on y axis
Kinetic energy on x axis
Draw a line with a gradient of 1 going up into a smoth curve which then goes down into an asmyptote
Draw a line near the end to show the activation energy

Question 19

What happens to a boltzman distribution if you increase the temperature

Answer 19

Peak moves to the right and also a little bit downwards

Activation energy is in the same place but now there is a higher percent of particles past the activation energy

Question 20

How does increasing concentration speed up reactions?

Answer 20

The particles will be closer together, so they’ll collide more frequently. If there are more collisions more chances to react

Question 21

How does increasing pressure speed up reactions?

Answer 21

If any reactants are gases, the particles will be closer together, so will collide more frequently, so more chances to react

Question 22

How do catalysts speed up reactions?

Answer 22

They lower the activation energy by providing an alternative reaction pathway, if activation energy is lower more particles will have enough to react

They are chemically unchanged at the end of the reactiom

Question 23

How does temperature speed up the rate of reacriom?

Answer 23

Particles exposed to more thermal energy so have more kinetic energy, so will move faster meaning greater proportion of molecules will have more than activation energy, also there will be more frequent collisions

Question 24

What happens to the boltzman distribution if you introduce a catalyst?

Answer 24

The activation is lowered

Question 25

What’s a heterogenous catalyst?

Answer 25

In a different physical state to the reactants

Question 26

What’s a homogenous catalyst?

Answer 26

Catalyst which is in the same physical state as the reactants

Question 27

Rate of reaction formula?

Answer 27

Rate of reaction = amount of reactant used or product formed / time

Question 28

2 different ways to measure rate of reaction?

Answer 28

Change in mass

Volume of gas given off

Question 29

How do you work out reaction rate from the graph?

Answer 29

Gradient (change in y/change in x)

If it’s not a straight line find the gradient of the tangent at the certain points

Question 30

How to draw an endothermic enthalpy profile diagram?

Answer 30

Enthalpy, kJmol^-1 on y axis
Progress of reaction on x axis

The more stable Reactants are going to start at low energy, then there will be a big curve which goes up to less stable products

The distance from the reactants to the top of the curve is the activation energy

Question 31

How to dran an exothermic enthalpy diagram?

Answer 31

Enthalpy, kJmol^-1 on y axis
Progress of reaction on x axis

The less stable reactants are going to start at a high energy, and then will produce a small curve which goes all the way down to the bottom for the more stable products

The activation energy is the distance from the products to the top of the curve