R1.1 What drives chemical reactions? Flashcards
what do chemical reactions involve?
involve a transfer of energy between the system and the surroundings, while total energy is conserved
system
inside the reaction vessel
open, closed, isolated
open system
energy exchange
matter exchange
closed system
energy exchange
isolated system
can’t exchange matter or energy
surroundings
outside the reaction vessel
energy
measure of the ability to do work e.g. heat
heat
transfer of energy from one place to another (how it gets there)
temperature
a measure of the average kinetic energy in a system (place)
enthalpy
a measure of the chemical energy in a system (place)
what do all chemical reactions required what does this allow atoms to so
- 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
exothermic reaction
- 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
endothermic reaction
- 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
what do energy profiles show
show the energies of reactants, transition states and products of the reaction with time
transition state
- 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
activation energy
- 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
exothermic energy profile
- Reactants higher in energy than products
- Reactants are closer to the transition state than products
- Lower activation energy
endothermic energy profile
- Reactants lower in energy than products
- Reactants are further away from the transition state than products
- Higher activation energy
standard enthalpy change for a chemical reaction
- the heat transferred at a constant pressure, under standard conditions and states - can determine from the change in temperature of a pure substance
calculating enthalpy change of a reaction, what 2 equations are used
Q = mcΔT
ΔH = -Q/n
standard conditions
- 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
can enthalpy be measured directly? how is enthalpy changed found?
- enthalpy cannot be directly measured
- we can only find enthalpy change by the effect the energy transfers have on the system
standard enthalpy change of reaction
The enthalpy change when the reactants in the stoichiometric equation react to give the products under standard conditions and states
Exothermic or endothermic
standard enthalpy change of formation
The enthalpy change when 1 mole of a compound is formed from its elements under standard conditions and states
Exothermic or endothermic
standard enthalpy change of combustion
The enthalpy change when 1 mole of a substance burns in excess oxygen under standard conditions and states
Exothermic
standard enthalpy change of neutralisation
The enthalpy change when 1 mole of water is formed by reacting an acid and an alkali under standard conditions and states
Exothermic
