Physical Chemistry

Question 1

What is enthalpy?

Answer 1

It is the thermal energy that is stored in a chemical system. It is impossible to measure the enthalpy of the reactants or products but you can measure the energy absorbed or released to the surroudings during a chemical change

Question 2

What is the law of conservation of energy?

Answer 2

That energy cant be created or destroyed only moved from one to place to another

Question 3

What is the enthalpy change?

Answer 3

~The heat exchange with the surroundings during a chemical reaction at constant pressure
~The difference between the enthalpy of the products and the enthalpy of the reactants

Question 4

What happens in an exothermic reaction?

Answer 4

When the enthalpy of the products is smaller than the enthalpy of the reactants as there is a heat loss from the chemical system to the surroundings. The enthalpy change has a negative sign as heat has been lost by the chemical system

Question 5

What happens in an endothermic reaction?

Answer 5

When the enthalpy of the products is greater than the enthalpy of the reactants as there is a heat gain to the chemical system from the surroudnings. The enthalpy change has a positive sign becuase heat has been gained by the chemical system

Question 6

What do enthalpy profile diagrams show?

Answer 6

They show what happens to the enthalpies of the reactants and products during the course of the reaction

Question 7

What is the activation energy?

Answer 7

The minimum energy required to start a reaction by breaking bonds in the reactants

Question 8

What are the standard conditions?

Answer 8

Pressure: 100000 Pa
Temperature: 273K

Question 9

What is the enthalpy change of reaction?

Answer 9

The energy change associated with a given reaction

Question 10

What is the enthalpy change of formation?

Answer 10

The energy change that takes place when 1 mole of a compound is formed from its contituent elements in their standard state under standard condtions

Question 11

What is the standsrd enthalpy change of formation for an element in its standard state?

Answer 11

Is 0kJ/mol as according to the definition there is no change as no compound is formed so no energy is released or taken in

Question 12

What is the enthalpy change of combustion?

Answer 12

The energy change that takes place when 1 mole of a substance is completely combusted

Question 13

What is the enthalpy change of neutralisation?

Answer 13

The energy change associated with the formation of 1 mole of water from a neutralisation reaction

Question 14

What is calorimetry?

Answer 14

The quantitative study of energy in a chemical reaction which uses a mathematical relationship to calculate the enthalpy change from experimental quantitative data

Question 15

What is the equation for energy change?

Answer 15

Q=mc(change in temperature)
where:
Q=energy change
m= is the mass of the substance heated or cooled usually expressed in grams
c= is the specific heat capacity of the substance that is heated or cooled

Question 16

What is the specific heat capacity of water?

Answer 16

4.18 J/g/K

Question 17

What are bond enthalpies?

Answer 17

They tell you how much energy is needed to break each different chemical bond

Question 18

What is the reaction called if energy is needed to break bonds?

Answer 18

Endothermic

Question 19

What is the reaction called when bonds form an energy is released?

Answer 19

Exothermic

Question 20

What are average bond enthalpies?

Answer 20

The mean energy neede for 1 mole of a given type of gaseous bonds to undergo homolytic fission

Question 21

What does Hess’ law state?

Answer 21

That the enthalpy change in a chemical reaction is independent of the route it takes

Question 22

What is an enthalpy cycle?

Answer 22

A pictorial representation showing alternative routes between reactants and products

Question 23

What is the equation for the rate of reaction?

Answer 23

RoR= change in concentration/time

Units: mol dm^-3/s

Question 24

What factors affect the rate or reaction?

Answer 24

~temperate
~pressure
~concentration
~surface area
~catalyst

Question 25

What is the collision theory?

Answer 25

For 2 molecules to react they must first collide. This collision must have sufficient energy to overcome the activation energy of the reaction and the collision must have correct direction

Question 26

How does the concentration effect the rate of reaction?

Answer 26

The RoR increases with increased concentration as:
~Increased concentration gives more molecules in the same volume
~The molecules will be closer together and so there is a greater chance of them colliding with sufficient energy to overcome the activation energy
~Collisions will be more freuqent more collisions will occur in a cetain length of time

Question 27

How does the pressure effect the rate of reaction?

Answer 27

The RoR increase with increased pressure as:
~when the pressure of a gas is increased the molecules are pushed together
~the same number of molecules occupies a smaller volume
~more collisions are likely to occur as they are closer together to overcome the activation energy

Question 28

How can you calculate the rate of reaction?

Answer 28

By monitoring changed in physical quantities during a reaction like:
~concentration of a reactant or product (e.g. using a titration)
~gas volume of products
~mass of substnces formed, or decreasing mass of reactants
There values are plotted against time on a graph and the rate is equal to the gradient

Question 29

How does the temprature affect the rate of reaction?

Answer 29

The RoR increase with the temperature as:
~there is increased kinetic energy in the particels so they are more likely to collied and meet the activation energy so the rate of reaction increases