3.2 Physical Chemistry

Question 1

what is enthalpy change

Answer 1

All chemical reactions involve a change in energy.

The exchange of energy between the reaction mixture and the surroundings is enthalpy change ΔH in joules.

Question 2

what is an exothermic reaction

Answer 2

Exothermic- energy is transferred from reaction to surroundings. Chemical energy is released and temperature of surroundings increase. ΔH is negative as energy is lost.

Question 3

what is an endothermic reaction

Answer 3

Endothermic- ΔH is positive as products have higher energy than the reactants and heat is taken in from surroundings.

Question 4

how to test for enthalpy change experimentally

Answer 4

Reactions in solution 
Result obtained is an approximate as heat is lost 
Energy released or absorbed when the reaction takes place, q, can be calculated using q=mcΔT 
m-mass of reaction mixture 
C- specific heat capacity 
ΔT- change in temperature. 
Solvent used mostly is water. 
ΔH=   q/1000  or   (mcΔT)/1000n

Question 5

link between enthalpy change and temperature

Answer 5

If ΔT increases, the reaction is exothermic so ΔH is negative
If ΔT decreases the reaction is endothermic, so ΔH is positive

Question 6

what is bond enthalpy

Answer 6

The enthalpy change required to break and separate 1 mol of bonds in the molecule of gas so the resulting gaseous particles exert no forces upon each other.
X-Y(g) → X(g) + Y(g)
ΔH= sum of reactant bond enthalpy – sum of product bond enthalpy

Question 7

standard enthalpy change of combustion

Answer 7

the enthalpy change which occurs when 1 mole of the compound is burned completely in oxygen under standard conditions, and with everything in its standard state

Question 8

standard enthalpy change of reaction

Answer 8

the enthalpy change which occurs when equation quantities of materials react under standard conditions

Question 9

standard enthalpy change of formation

Answer 9

the enthalpy change which occurs when 1 mole of the compound is formed from its elements under standard conditions and with everything in it’s standard state

Question 10

standard enthalpy change of neutralisation

Answer 10

the enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water

Question 11

what is Hess’ law

Answer 11

States that if a reaction can take place by more than 1 route, the enthalpy change for the reaction is the same irrespective of the route taken, provided the initial and final conditions are the same
[for mechanism see physical flashcard]

Question 12

how to measure the rate of reaction

Answer 12

Change in mass 
Change in volume 
Change in concentration 
Change in acidity 
Change in color

Question 13

factors affecting the rate of reaction

Answer 13

The frequency of collisions can be increased by:

• Increasing the concentration
• Increasing the surface area of a solid
• Increasing the temperature
• Increasing the pressure in a gaseous reaction

Question 14

what is collision theory

Answer 14

A reaction cannot take place unless a collision occurs between the reacting particles.
For a reaction to occur:
Colliding particles must have the correct orientation
Energy needs to exceed activation energy

Question 15

what is the effect of concentration on rate of reaction

Answer 15

increases the chance of a collision occurring between the reacting particles. The more collisions there are, the faster the rate of reaction.
Increasing the pressure has the same effect for gaseous molecules as it compresses the gas and increases the concentration

Question 16

what is the effect of surface area on rate of reaction

Answer 16

If a reactant is solid, its surface area will affect the rate at which it is able to react.
Breaking a surface into smaller pieces will increase its surface area and thus more of its surface is exposed to a reagent and the reaction will proceed faster

Question 17

what is the effect of a catalyst on rate of reaction

Answer 17

Alter the rate of reaction without getting used up by the reaction.
Provide an alternate route, or mechanism, for the reaction that has a lower activation energy.
The enthalpy change and distribution of energies remain the same

Question 18

what is heterogenous catalysts

Answer 18

In a different phase to the reactants

Question 19

what are homogenous catalysts

Answer 19

In the same phase as the reactants

Question 20

what does a Boltzmann distribution curve show

Answer 20

The curve going through the origin shows there are no particles with 0 energy
At high energy the curve never touches the horizontal axis showing there is no theortetical maximum energy
Area under the curve represents the total number of particles

Question 21

what must happen for a reaction to occur and how does temperature influence this

Answer 21

A reaction can only occur if the colliding particles have an energy greater than, or equal, to the activation energy. Increasing the temperature increases the energy of the particles and therefore lowers the activation energy.

Question 22

what are reversible reactions

Answer 22

A reaction that can go both backwards and forwards

Question 23

what is a dynamic equilibrium

Answer 23

When the forward and the backward reaction are equal.
the system is in equilibrium because the amount of each chemical stays constant
The equilibrium is dynamic because the reactants and products are both constantly interacting
Can only be achieved in a closed system

Question 24

what is Le Chatelier’s principle

Answer 24

States that if a closed system at equilibrium is subject to a change, the system will move to minimise the effect of the change

Question 25

how does Le Chatelier’s principle work

Answer 25

If the concentration is increased, the system will move to decrease the concentration of that component
If the pressure is increased, the system will move to decrease the pressure
If the temperature is increased, the system will move to decrease the temperature

Question 26

what is the effect of changing pressure on equilibrium

Answer 26

The greater the number of gas molecules, the greater the pressure is. If pressure is increased in a system, the equilibrium position alters to resist the increased pressure by reducing the total number of gas molecules. This is achieved by moving to the side where there are less gas molecules

Question 27

what is the effect of heat on equilibrium

Answer 27

If a temperature increases, the mixture will respond by attempting to lower the temperature again by moving the equilibrium in the exothermic direction

Question 28

what is the effect of catalysts on equilibrium

Answer 28

A catalyst doesn’t alter the amount of product.
Speeds up the forward and backward reactions equally.
Reduce time taken to reach equilibrium

Question 29

what is the rate of reaction equation

Answer 29

change in concentration/ time

Question 30

what is the equilibrium constant equation

Answer 30

aA + bB ⇌ cC + dD, so Kc= [products]/[reactants]

is to the power of moles

Question 31

what do Kc values tell us

Answer 31

=1 means equilibrium is halfway
>1 equilibrium is towards the products
<1 equilibrium is towards the reactants