F322: Enthalphy Changes, Equilibria And Rates

Question 0

Bond enthalpy

Answer 0

The energy required to break 1 mole of a given type of bond in the gaseous state.

Always +ve numbers as bond breaking is a endothermic process.

Question 1

Standard enthalpy change of reaction

Answer 1

The enthalpy change that accompanies a reaction. In the molar quantities expressed in a chemical equation under standard conditions (1atm, 25 degrees, conc of 1mol/dm3)

Question 2

Equation to calculate enthalpy chane of reaction

Answer 2

Sum of bond energies of reactants - sum of bond energies of products

Question 3

Exothermic reaction

Answer 3

A reaction that gives out energy to its surroundings. -ve enthalpy change of reaction. More energy released when bonds are formed in products, than is required to break the bonds in the reactants.

Question 4

Endothermic

Answer 4

A reaction that takes energy to its surroundings. +ve enthalpy change of reaction. Less energy released when bonds are formed in products, than is required to break the bonds in the reactants.

Question 5

Hess’s law

Answer 5

The total energy change of a reaction is the same, regardless of the route taken, provided the initial and final conditions are the same.

Question 6

Standard conditions

Answer 6

25 degrees C (298K) and 1atm (100KPa)

Question 7

Standard enthalpy change of formation

Answer 7

The enthalpy change when 1 mole of a compound is formed from it’s elements under standard conditions.

Question 8

Standard enthalpy change of combustion

Answer 8

The enthalpy change when 1 mole of a substance completely combusts under standard conditions.

Question 9

Collision theory

Answer 9

Particles must collide with a minimum about of energy for a reaction to occur

Question 10

Activation energy

Answer 10

Minimum amount of energy required in a collision to break bonds in order for a reaction to occur.

Question 11

Effect of increased concentration on rate of reaction

Answer 11

More particles per unit volume, increased frequency of collisions, increased rate

Question 12

Effect of increased pressure (gas) on rate of reaction

Answer 12

Same number of particles in a smaller volume, increased frequency of collisions, increased rate

Question 13

Effect of increasing temperature on rate

Answer 13

Particles now have more energy, move around faster (greater KE), collide more frequently, more molecules have >Ea so more collisions are successful

Question 14

Catalyst

Answer 14

Speeds up the rate of reaction without being consumed in the overall reaction. It provides an alternative pathway/route with a lower Ea, so more molecules have >Ea.

Question 15

Industrial importance of catalysts (examples)

Answer 15

Iron in ammonia production

Platinum/Palladium/Rhodium in catalytic converters

Question 16

Industrial benefits of using catalysts

Answer 16

1) Lower temperatures can be used, reducing production costs and CO2 emissions from burning hydrocarbons to produce energy.
2) Enable different reactions to be used with better atom economy to reduce waste.
3) some catalysts such as enzymes function effectively at room temperature.

Question 17

Boltzmann distribution

Answer 17

Shows the distribution of energies for a sample of molecules

Question 18

Dynamic equilibrium

Answer 18

Achieved when the rate of the forward reaction is equal to the rate of the backward reaction. Requires a closed system, and no overall change in macroscopic properties.

Question 19

Closed system

Answer 19

A system where no reactants or products can escape (energy can exchange with surroundings)

Question 20

Equilibrium yield

Answer 20

The amount of product in the equilibrium mixture

Question 21

Le Chatelier’s Principle

Answer 21

A change in the conditions to a system at equilibrium will cause a shift in the position of the equilibrium in the direction that MINIMISES the effect of the change.

Question 22

Effect of increasing on the equilibrium position

Answer 22

Equilibrium shifts in the endothermic direction

Question 23

Effect of increasing pressure on the equilibrium position

Answer 23

Equilibrium shifts to the side with FEWER moles of gas

Question 24

Effect of changing concentration on the equilibrium position

Answer 24

Shifts right if: products removed, reactants added

Shifts left if: reactants removed, products added

Question 25

Effect of adding a catalyst on the equilibrium position

Answer 25

No effect on equilibrium position
As it speeds up the rate of forward and backward reaction by the same amount.

Note: particularly useful in reactions where increasing the temp lowers the yield.