3.2.1 +3.2.2 Enthalpy Changes + Rates Of Reaction

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 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 is an endothermic reaction?

Answer 5

A reaction with an overall positive enthalpy change - enthalpy of products > enthalpy of reaction

Question 6

What is an exothermic reaction?

Answer 6

A reaction with an overall negative enthalpy change - enthalpy of products < enthalpy of reactants

Question 7

What is the meaning of activation energy?

Answer 7

The minimum energy required for a reaction to take place

Question 8

What are the standard conditions?

Answer 8

100 kPa
298K

Question 9

What does in standard state mean?

Answer 9

The state an element/compound exists at in standard conditions

Question 10

Define enthalpy change of formation

Answer 10

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

Question 11

Give an example of an equation which represents standard enthalpy formation

Answer 11

H2 (g) + 1/2 O2 (g) -> H2O (l)

Question 12

Define enthalpy change of combustion

Answer 12

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

Question 13

Give an example of an equation which represents standard enthalpy of combustion

Answer 13

C (s) + O2 (g) ->CO2 (g)

Question 14

Define enthalpy change of neutralisation

Answer 14

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

Question 15

How can you calculate enthalpy change from experimental data?

Answer 15

Use the equation Q=mcT where M is the mass of the substance being heated, c is the specific heat capacity of that substance and Delta T is the change in temperature

Question 16

What are the advantages of using a bomb calorimeter?

Answer 16

Minimise heat loss
pure oxygen used so it ensures complete combustion

Question 17

Why may experimental methods for enthalpy determination not be accurate?

Answer 17

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

Question 18

What does average bond enthalpy mean?

Answer 18

The main energy required to break one mole of bonds in gaseous molecules

Question 19

Why would using bond and enthalpies not be as accurate as using standard enthalpy of combustion/formation?

Answer 19

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

Question 20

How to calculate enthalpy change of reaction using average bond enthalpies?

Answer 20

DeltaH = sum of (bond enthalpies of reaction)- sum of (bond enthalpies of products)

Question 21

What is the equation used to calculate rate?

Answer 21

Rate equals change in concentration / time

Question 22

What is the unit for rate of reaction?

Answer 22

Mol dm-3s-1

Question 23

What most particles do in order to react?

Answer 23

Collide with sufficient energy (activation energy) and the correct orientation

Question 24

Do most collisions result in a reaction

Answer 24

No

Question 25

What are the factors that affect rate of reaction?

Answer 25

Temperature Pressure Concentration Surface area Catalyst

Question 26

What is the effect of increasing temperature on rate of reaction? why?

Answer 26

Increasing temperature increases the rate of reaction Much higher proportion of particles have energy greater than the activation energy Many more successful collisions per second Increased rate

Question 27

What is the effect of increasing concentration/pressure on rate of reaction and why?

Answer 27

Increased concentration/pressure Increased rate of reaction There are more particles in a given volume More frequent successful collisions Increased rate

Question 28

What are the variables in an experiment that can be monitored to calculate the rate of reaction?

Answer 28

Concentration of reactant or product Gas volume of products Mass of substances formed

Question 29

How to calculate rate from a concentration time graph

Answer 29

Draw a tangent Work out the gradient of the tangent using the equation Gradient equals change in Y / change in X

Question 30

What is a catalyst?

Answer 30

A substance which increases the rate of reaction but is not used up in the reaction

Question 31

How do catalysts work and how do they increase the rate of reaction?

Answer 31

They provide an alternate reaction pathway (with a lower activation energy) Due to lower activation energy more particles have energy so more frequent collisions so increased reaction rate

Question 32

What does homogeneous catalyst mean?

Answer 32

A catalyst that is in the same phase as the reactants, e.g. liquid catalyst mixed with liquid reactants

Question 33

What does heterogeneous catalyst mean?

Answer 33

Catalyst used in the reaction is in different phases to the reactants, e.g. gaseous reactants passed over solid catalyst

Question 34

What are catalytic converters?

Answer 34

They are present in vehicles to reduce toxic emissions and prevent photochemical smog

Question 35

Define activation energy

Answer 35

The minimum energy that particles must collide with for a reaction to occur

Question 36

Name some important features of boltzamann distribution

Answer 36

Area under the curve = total number of molecules Area under the curve does not change when conditions alter The curve starts at the origin Curve does not touch or cross the energy access Only the molecules with energy greater than activation energy can react

Question 37

What are the axis in Boltmann distribution?

Answer 37

X axis is energy Y axis is number of molecules with a given energy