3.2 PHYSICAL CHEMISTRY Flashcards
Define chemical energy
- The energy held within the bonds between atoms
Define enthalpy
- A measure of the heat/thermal energy content of a substance
Define enthalpy change ∆H
- The heat exchange with the surroundings during a chemical reaction
Define system and surroundings
System = the atoms and bonds in the chemical reaction
Surroundings = everything around
Describe the relationship between energy and system/surroundings
- Heat/energy loss in a chemical system = heat/energy gain to the surroundings
- Heat/energy gain in a chemical system = heat/energy loss to the surroundings
Draw a labled exothermic diagram for enthalpy change
Draw a labled endothermic diagram for enthalpy change
Describe the term exothermic reaction in three ways
1) Heat energy is given out to surroundings from chemical reaction
2) More energy is released when product bonds are formed than the energy absorbed when breaking the reactant bonds
3) The enthalpy of reactants higher than enthalpy of products (thus, negative value)
Is bond breaking an exothermic or endothermic process?
- Endothermic
Is bond forming an exothermic or endothermic process?
- Exothermic
State and lable the two equations needed to work out ∆H from heat energy
Define activation energy Eₐ
- The minimum amount of energy required for a reaction to take place
Define calorimetry
- Working out the enthalpy change of a reaction
State what is measured during calorimetry to determine enthalpy change
- The temperature change of a chemical system (the reaction)
Conclude the type of reaction if a temperature rise is detected during calorimetry
- The reaction is exothermic
- Heat energy has been released to the surroundings
Conclude the type of reaction if a temperature drop is detected during calorimetry
- The reaction is endothermic
- Heat energy has been taken in from the surroundings
State the equation used after calorimetry
State why a polystyrine cup is used in a beaker during calorimetry
- Polystyrine acts as an insulator to reduce heat loss by evapourisation
Draw and lable a graph for calorimetry
Describe the term copper calorimetry for combustion enthalpy
- When the heat energy from the combustion of a fuel is used to increase the temperature of a known mass of water
State the three reasons why experimental values differ from data values for combustion enthalpy
1) Non-standard conditions (298K 100kPa)
2) Incomplete combustion
3) Potential heat loss to surroundings
Define average bond enthalpy
- The energy needed to break 1 mole of gaseous bonds in GASEOUS MOLECULES to form gaseous atoms
State why bond enthalpy
values are always positive
- Because bond breaking is an endothermic process
Describe what a smaller bond enthalpy indicates about that bond
Smaller bond enthalpy = weaker bond = less energy to break = more reactive
State the equation for ∆Hᶠ in a chemical reaction
State the equation for ∆Hᶜ in a chemical reaction
Define standard enthalpy change of formation ∆Hᶠ
- The enthalpy change when 1 mole of a compound is formed from its constituent elements under standard conditions
Define standard enthalpy change of neutralisation ∆Hₙₑᵤₜ
- The enthalpy change when 1 mole of water is formed from a neutralisation reaction under standard conditions
Define standard enthalpy change of combustion ∆Hᶜ
- The enthalpy change when 1 mole of a substance is completely combusted under standard conditions
Draw an enthaply change of reaction from enthalpy change of combustion
Draw an enthaply change of reaction from enthalpy change of formation
State why initial rate (T=0) has the highest rate of reaction
- Because concentrations of reactants are highest at the start of a reaction
- Gradient of tangent to the curve is also steepest here
State and describe the five factors that affect the rate of reaction
1) Temperature (↑ temp = ↑ rate)
2) Pressure (for gases) (↑ pressure = ↑ rate)
3) Concentration (↑ conc = ↑ rate)
4) Surface area (↑ SA = ↑ rate)
5) Catalyst (+ cat = ↑ rate)
State in terms of collision theory the effect of these factors (except catalysts) on the rate of reaction
- They ensure that particles collide with sufficient energy to overcome activation energy frequently
- Thus, ROR increases
State the equation for rate of reaction
State the units for rate of reaction
mol dm⁻³ s⁻¹
State the three ways in which changing quantitys can be measured for rate of reaction
1) Concentration of a reactant or product (titration)
2) Gas volumes of products (gas syringe)
3) Mass of products forming/decreasing mass of reactants (balance)
State the equation for rate of reaction on a graph
Describe how to determine rate of reaction from a graph
1) Draw a tangent to the curve wherever required
2) Use the formula gradient=∆y/∆x
Define catalyst
- Increase the rate of a reaction without being used up by the overall reaction
- Allows the reaction to proceed via a different route by lowering activation energy
State and explain the two types of catalysts
1) Homogenous catalysts (where the catalyst and reactants are in the same phase/state)
2) Heterogenous catalysts (where the catalyst and reactants are in the different phase/state)
Describe the two benefits of using catalysts
1) Reduces the energy demand from combustion of fossil fuels
2) Increases economic sustainability because it lowers temperatures
Explain what the boltzmann distribution shows
- The distribution of energies of particles at a particular temperature
- The proportion of particles with sufficient energy to overcome activation energy
Draw and lable a generic boltzmann distribution and state the features
Explain the effect of temperature on reaction rate via a labled boltzmann distribution
Explain the effect of catalysts on reaction rate via a labled boltzmann distribution
State the three features required for an equilibrium to be dynamic
1) Closed system
2) Concentrations of reactants and products remaining constant
3) Rate of forward and backward reactions to be equal
State the three features that affect position of equilibrium
1) Temperature
2) Gaseous pressure
3) Concentration
Explain fully the effect of changing concenteration on equilibrium position
Explain fully the effect of changing gaseous pressure on equilibrium position
Explain fully the effect of changing temperature on equilibrium position
State why catalysts do not affect the position of equilibrium
- Because they speed up the rate of the foward and backward reaction by equal amounts
Describe industrial conditions
- They tend to be a comprimise between rate of reaction and yeild of production (equilibrium position)
- To ensure profits while still being safe
Draw a concentration-time graph identifying dynamic equilibrium of reactants and products when equilibrium lies to the LEFT
Draw a concentration-time graph identifying dynamic equilibrium of reactants and products when equilibrium lies to the RIGHT
Draw a concentration-time graph identifying dynamic equilibrium of reactants and products when equilibrium lies in the MIDDLE
Define homogenous equilibrium
- Where all species are in the same phase/state
State the equilibrium constant Kc expression
State what is meant is equilibrium constant Kc has a value of 1
- Equilibrium lies in the MIDDLE of reactants and products
State what is meant is equilibrium constant Kc has a value bigger than 1
- Equilibrium lies to the RIGHT
- (Products/forward is favoured)
State what is meant is equilibrium constant Kc has a smaller than 1
- Equilibrium lies to the LEFT
- (Reactants/backwards is favoured)
State the units of equilibrium constant Kc
mol dm⁻³
