R1.1 What drives chemical reactions?

Question 1

what do chemical reactions involve?

Answer 1

involve a transfer of energy between the system and the surroundings, while total energy is conserved

Question 2

system

Answer 2

inside the reaction vessel
open, closed, isolated

Question 3

open system

Answer 3

energy exchange
matter exchange

Question 4

closed system

Answer 4

energy exchange

Question 5

isolated system

Answer 5

can’t exchange matter or energy

Question 6

surroundings

Answer 6

outside the reaction vessel

Question 7

energy

Answer 7

measure of the ability to do work e.g. heat

Question 8

heat

Answer 8

transfer of energy from one place to another (how it gets there)

Question 9

temperature

Answer 9

a measure of the average kinetic energy in a system (place)

Question 10

enthalpy

Answer 10

a measure of the chemical energy in a system (place)

Question 11

what do all chemical reactions required what does this allow atoms to so

Answer 11

• all chemical reactions require bond breaking and forming
• allows atoms to rearrange to form new substances
• when atoms are bonded to other atoms, they are often more energetically favourable

Question 12

exothermic reaction

Answer 12

• Negative enthalpy change, products have less enthalpy (chemical energy) than reactants
• Bonds are made/formed
• Energy is released
• Heat is given off by the system to the surroundings
• System temperature = decreased
• Surroundings temperature = increased
• Enthalpy –heat→ temperature
• Enthalpy (chemical energy) of system transferred in the form of heat to the temperature of surroundings
• Thermodynamically possible because the enthalpy of the reactants is higher than the enthalpy of products
• If the rate is too slow, the reaction may not occur and be kinetically controlled
• Reactions are self-sustaining, meaning that they only stop when all reactants are used up
• The products are more energetically stable than the reactants

Question 13

endothermic reaction

Answer 13

• Positive enthalpy change, products have more enthalpy (chemical energy) than reactants
• Bonds are broken
• Energy is required
• Heat is absorbed by the system from the surroundings
• System temperature = increased
• Surrounding temperature = decreased
• Temperature –heat→ enthalpy
• Temperature of surroundings is transferred in the form of heat to the enthalpy (chemical energy) of system
• Generally self-sustaining
• The reactants are more energetically stable than the products

Question 14

what do energy profiles show

Answer 14

show the energies of reactants, transition states and products of the reaction with time

Question 15

transition state

Answer 15

• a stage during the reaction at which chemical bonds are partially broken and formed
• it is very unstable and cannot be isolated
• it is higher in energy than reactants and products

Question 16

activation energy

Answer 16

• the minimum amount of energy required for reactant molecules to have a successful collision and start the reaction
• this is the energy needed to reach the transition state

Question 17

exothermic energy profile

Answer 17

• Reactants higher in energy than products
• Reactants are closer to the transition state than products
• Lower activation energy

Question 18

endothermic energy profile

Answer 18

• Reactants lower in energy than products
• Reactants are further away from the transition state than products
• Higher activation energy

Question 19

standard enthalpy change for a chemical reaction

Answer 19

• the heat transferred at a constant pressure, under standard conditions and states - can determine from the change in temperature of a pure substance

Question 20

calculating enthalpy change of a reaction, what 2 equations are used

Answer 20

Q = mcΔT
ΔH = -Q/n

Question 21

standard conditions

Answer 21

• 100 kPa pressure
• 1 mol dm^3 concentration for all solutions
• Each substance in its standard state (s/l/g)
  Temp- not part of definition but usually 298 K

Question 22

can enthalpy be measured directly? how is enthalpy changed found?

Answer 22

• enthalpy cannot be directly measured
• we can only find enthalpy change by the effect the energy transfers have on the system

Question 23

standard enthalpy change of reaction

Answer 23

The enthalpy change when the reactants in the stoichiometric equation react to give the products under standard conditions and states
Exothermic or endothermic

Question 24

standard enthalpy change of formation

Answer 24

The enthalpy change when 1 mole of a compound is formed from its elements under standard conditions and states
Exothermic or endothermic

Question 25

standard enthalpy change of combustion

Answer 25

The enthalpy change when 1 mole of a substance burns in excess oxygen under standard conditions and states
Exothermic

Question 26

standard enthalpy change of neutralisation

Answer 26

The enthalpy change when 1 mole of water is formed by reacting an acid and an alkali under standard conditions and states
Exothermic