Energetics, Equilibria and Kinetics Flashcards
Exothermic
Reaction that releases more energy during bond making than it takes on for bond breaking
Endothermic
Reaction that requires more energy for bond breaking than it releases during bond making
Standard conditions
100kPa
298K
Specific Heat Capacity
Amount of heat needed to raise the temperature of 1g of a substance by 1K
Standard Enthalpy Change of Neutralisation
Enthalpy change when one mole of water is formed in a reaction between acid and alkali under standard conditions
Hess’ Law
Enthalpy change for a reaction is independent of the route taken
First Law of Thermodynamics
Energy can be neither made nor destroyed only converted from one form into another
Factors affecting bond dissociation enthalpy
Directly proportional to bond strength and inversely proportional to bond length
State why standard enthalpy of formation for oxygen is quoted as zero
All elements in their standard states have a standard enthalpy of formation of zero, as there is no change involved in their formation
Bond dissociation enthalpy
Enthalpy change required to break a given covalent bond in a particular molecule in the gaseous state
Mean bond enthalpy
Enthalpy change required to break a given covalent bond averaged over a range of different molecules all in the gaseous state
Describe steps to determine an accurate minimum/maximum temperature for an enthalpy change
- start timer
- record temperature every 30 seconds
- add reactant
- plot a graph of temperature against time
- extrapolate minimum temperature to the point reactant is added
Explain why the value for enthalpy of formation obtained using mean bond enthalpies is different to the actual value
Mean bond enthalpies are not SAME as actual as they are averaged over a different molecules and bond dissociation enthalpy varies between molecules
Explain how enthalpy change will differ if referred to the formation of water in liquid state as oppose to gaseous
Value will be more exothermic since energy condensation is an exothermic process/energy is released to condense water vapour
Enthalpy of Formation
Enthalpy change when one mole of a compound is formed from its constituent elements with reactants in products in their standard states under standard conditions
Enthalpy of Combustion
Enthalpy change when one mole of a substance completely burns in oxygen under standard conditions
Suggest why calculated value for enthalpy change of combustion differs from the actual value
- Heat loss to surroundings
- Incomplete combustion
- Heat capacity of apparatus
Equilibrium
- Concentration of reactants and products does not change
- Rate of forwards reaction = Rate of backward reaction
- Closed system
- Conditions do not change
Le Châtelier’s Principle
For any factor that changes a system at equilibrium the position of equilibrium will shift to oppose that change
Describe how increasing temperature affects the position of equilibrium
Position of equilibrium shifts to favour endothermic reaction to decrease temperature
Describe how increasing pressure affects the position of equilibrium
Position of equilibrium will shift to the side with fewer moles of gas to decrease pressure
Describe how catalysts affect position of equilibrium
Have no effect because they increase the rate of the forward and backward reaction by the same amount
State why volume does not need to be known to calculate Kc for certain reactions
Volume cancels in Kc equation / same number of moles on either side of the equation
Dynamic
Ongoing/Continuos
Homogenous
All reactants are in the same phase
Describe how to calculate Kc value for the reverse reaction if Kc for forward reaction is known
Reciprocal of Kc value for forward reaction
Suggest how you can tell from equations that a substance is behaving as a catalyst
- substance is unchanged
- offers alternative route (forms intermediate species)
Activation energy
Minimum energy (1) required to break bonds for a reaction to occur (1)
Suggest why at a constant temperature most molecules eventually react
Some particles always have energies higher than the activation energy (1) and molecules gain energy due to collisions (1)
Catalyst
A substance that increases the rate of a chemical reaction (1) without being permanently changed or used up (1)
Describe how a catalyst works
Catalyst lowers the activation energy (1) by providing an alternative route for the reaction (1) so that more particles have energies higher than the activation energy (1)
Suggest a way collision frequency between particles can be increased without changing temperature
Increase concentration/ pressure
NOT catalyst because they ONLY cause more energetic collisions not a higher frequency
Suggest why a small increase in temperature leads to a large increase in reaction rate
Many more particles have energies higher than the activation energy
Explain why reactions occur faster at high temperatures
More molecules have energies higher than the activation energy (1) causing more energetic collisions (1) so there is a higher frequency of successful collisions (1)
Give two requirements for a reaction to occur between gaseous molecules
Collision
Sufficient energy (higher than activation energy)
Correct orientation collision (last option)
Disadvantages of high pressure/temperatures
Expensive to generate
High cost of plant (to resist high temperature /pressure, i.e. strong pressure vessel and equipment)
Suggest why operating temperature increases without the use of a catalyst
Activation energy is higher than with catalyst (1) so temperature increased so more molecules have energies higher than activation energy (1)
Suggest how yield can be increased without altering temperature or pressure
Recycle reactants
Explain why rate of reactions decrease over time
Reactants are used up so lower frequency of collisions per unit time
Transition state
some bonds are in the process of being made while others are being broken
Suggest why Maxwell-Boltzmann distribution is asymptotic
No particles are zero energy and there is no maximum energy
Suggest why catalysts are used in the form of a gauze
large surface area
cheaper (since gauze is thin wire mesh)
Rate of reaction
Change in concentration over time
Reasons percentage yield is lower than expected
- reversible reaction
- product lost in isolation process
- product lost in other reactions
Explain why reactions occur slow at lower pressures
- molecules are spread further apart
- lower frequency of successful collisions between gas particles
How to write Kc expression for heterogenous reaction
Do not include solids in Kc expression since its concentration remains constant
How does changing conditions affect the value of Kc
- changing concentration or addition of catalyst has no effect
- temperature changes value of Kc
Explain why increasing temperature affects the value of Kc but increasing concentration of reactants or using a catalyst does not
- increasing temperature favours endothermic reaction (in order to decrease temperature) so concentration of products and reactants changes, hence value of Kc changes
- increasing concentration of reactants means concentration of products must increase to oppose change so ratio of products to reactants is unchanged, hence value of Kc remains constant
- catalyst increases rate of forward and backward reaction by same amount so concentration of products and reactants is unchanged, hence Kc remains constant
Enthalpy Change
- change in heat energy
- measured at constant pressure
Suggest why you cannot measure enthalpy change of salt to form hydrated salt directly
- difficult to measure temperature of solid
- difficult to prevent solid from dissolving
- impossible to add precise amount of water
Explain what happens to the most probable energy and number of molecules that have the most probable energy when temperature increases or decreases
temperature increases
- moves to right
- fewer molecules have most probably energy
temperature decreases
- move to left
- more molecules have most probable energy
Explain how to work out initial rate of reaction from measuring volume of product over regular intervals of time
- plot graph of volume against time
- draw tangent and work out gradient at beginning
Suggest why you cannot measure the enthalpy change of the following reaction directly C(s) + 1/2O2(g) -> CO(g)
difficult to prevent complete combustion
temperature of gases can be measured directly so ignore
