5,6,7 Enthalpy, Kinetics, Equilibrium

Question 1

enthalpy

Answer 1

chemical energy inside a substance

Question 2

bond enthalpy

Answer 2

the energy needed to break one mole of bonds in a gaseous molecule

Question 3

what is the enthalpy change for an exothermic reaction?

Answer 3

release energy to surrounding → product have less enthalpy (energy) than reactant → ∆H negative

Question 4

what happens to the temperature of surrounding during an endothermic reaction?

Answer 4

absorb heat energy from surrounding → temperature of surrounding decrease

Question 5

standard conditions means

Answer 5

• pressure = 100kpa
• concentration = 1mol/dm^3
• Each substance involved in the reaction is in its standard state (solid, gas or liquid)

Question 6

standard enthalpy change of reaction

Answer 6

the enthalpy change when molar quantities of reactants react to form products under standard conditions

Question 7

standard enthalpy change of formation

Answer 7

the enthalpy change when 1 mole of compound is formed from its elements under standard conditions

Question 8

standard enthalpy change of combustion

Answer 8

the enthalpy change when 1 mole of a substance is completely burnt in excess oxygen under standard conditions

Question 9

standard enthalpy change of neutralization

Answer 9

the enthalpy change when 1 mole of water is formed reacting an acid and base under standard conditions

exothermic！！

Question 10

specific heat capacity

Answer 10

the energy needed to raise the temperature of 1kg of a substance by 1K

Question 11

observations for exothermic reactions

Answer 11

1. surrounding gets warm
2. pop sound

Question 12

activation energy (Ea)

Answer 12

minimum energy needed for molecules to have successful collisions (reaction)

Question 13

catalyst

Answer 13

increases reaction rate by providing an alternative pathway of lower Ea; without being chemically changed by the reaction

Question 14

rate of reaction

Answer 14

change in the concentration of a reactant or product / unit time

Question 15

Explain, using the Maxwell–Boltzmann distribution, why increasing the temperature increases the rate of a chemical reaction

Answer 15

1. higher temperature = higher average kinetic energy of molecules → more molecules have Ek > Ea → more successful collisions per unit time → higher rate of reaction
2. higher average kinetic energy of molecules → molecules move faster → more frequent collisions → more successful collisions per unit time

Question 16

A state of dynamic equilibrium is reached in a closed system when

Answer 16

the rates of forward and backward reactions are equal

Question 17

equilibrium constant, Kc =

Answer 17

[product] / [reactant]

[C]^c [D]^d / [A]^a [B]^b

where [ ] is concentration, raised to the power of the coefficients (moles)

Question 18

Does a catalyst have effect on the position of equilibrium or Kc?

Answer 18

NO! Catalysts only helps the reaction to reach equilibrium faster.

Question 19

Kc is only affected by

Answer 19

temperature

Question 20

What happens to Kc if you double the moles of all reactants and products?

Answer 20

Kc^2

Question 21

What happens to Kc if you reverse the reactants and products?

Answer 21

1/Kc

Question 22

reaction quotient, Q

Answer 22

the ratio of products and reactants for a reaction that has NOT yet reached equilibrium; calculated in the same way as Kc

Question 23

Le Chatelier’s principle

Answer 23

if a change is made to a system at dynamic equilibrium, the equilibrium position moves to oppose the change

Question 24

Explain what happens to the equilibrium position when the concentration of reactant (LHS) increases

Answer 24

equilibrium moves to the product side (RHS) and favors forward reaction

Question 25

Explain what happens to the equilibrium position when the pressure increases

Answer 25

equilibrium moves to the side with less molecules (number of moles) to decrease the pressure again

*this is only for gas molecules!

Question 26

Explain what happens to the equilibrium position when the temperature increases

Answer 26

equilibrium moves to the endothermic direction to reverse the change