3) Physical Chemistry Flashcards
Exothermic reaction
When energy is transferred to the surroundings
-temperature of the surroundings increase
-combustion, many oxidation, most neutralisation reactions
Endothermic reaction
When energy is taken in from the surroundings
-temperature of the surroundings decrease
-thermal decomposition
-citric acid + sodium hydrogencarbonate
Calorimetry experiments
Technique used to measure changes in enthalpy of chemical reactions
-combustion
-displacement
-dissolving
-neutralisation
-a calorimeter can be a polystyrene cup, a vacuum flask, metal can
Calorimetry experiments - dissolving/ displacement reactions
- Take temperature of reactants to make sure they are equal
- Mix in a polystyrene cup
- Measure the temperature of the solution at the end of the reaction
-to decrease energy lost to surroundings:
–put polystyrene cup into a beaker of cotton wool
–put a lid on the cup to reduce energy/heat lost by evaporation/ radiation
Calorimetry experiments - neutralisation
- Put 25cm2 of acid and base in separate beakers
- Place beakers in a water bath set to 25C until they are both the same temperature (25C)
- Add both solutions to a polystyrene cup with a lid
- Take the temperature of the mixture every 30 seconds, record the highest temperature
Calorimetry experiments - combustion
- Place 50g of water in a copper can and record its temperature - copper conducts heat well
- Weigh the spirit burner
- Place and light spirit burner
- Heat water, stirring constantly until temperatures reach 50C
- Put out flame, measure final temperature of water
- Weight the spirit burner
Calculate heat energy change
Heat transferred = mass of water (g) x specific heat capacity x temperature change (C)
-specific heat capacity - energy needed to raise the temperature of 1g of a substance by 1C
Calculate molar enthalpy change
-heat energy change per mole of substance
ΔH (kJ/mol) = heat energy change/ number of moles
Energy level diagram - exothermic
-energy of products < energy of reactants
-change in energy is negative
-downwards-arrow
-draw it
Energy level diagram - endothermic
-energy of products > energy of the reactants
-change in energy is positive
-upwards-arrow
-draw it
Bond energy - endothermic/ exothermic
Exothermic - forming bonds as it releases energy back to the surroundings
Endothermic - energy is needed to break bonds which is absorbed from the reactions surroundings
-whether a reaction is endo/ exo depends on the difference between exothermic and endothermic
Bonds - endothermic
-more energy absorbed than released
-more energy is required to break the bonds than gained from making the new bonds
-change in energy is positive
Bonds - exothermic
-more energy released than absorbed
-more energy is released when new bonds are formed than energy required to break the bonds in the reactants
-change in energy is negative - reactants have less energy than the products
Investigating: surface area of a solid on the rate of reaction
- Add dilute hydrochloric acid to the conical flask
- Use a delivery tube to connect this flask to an inverted measuring cylinder upside down in a water trough
- Add calcium carbonate chips into the conical flask and close the bung
- Measure the volume of gas produced in a fixed time using the measuring cylinder
- Repeat with different sizes of calcium carbonate chips
-increased surface area, increased rate of reaction
Investigating: effect of concentration of a solution on the rate of reaction
- Measure 50 cm3 of sodium thiosulfate solution into a flask
- Measure 5 cm3 of dilute hydrochloric acid into a measuring cylinder
- Draw a cross on a piece of paper and put it underneath the flask
- Add the acid into the flask and immediately start the stopwatch
- Look down at the cross from above and stop the stopwatch when the cross can no longer be seen
- Repeat using different concentrations of sodium thiosulfate solution (mix different volumes of sodium thiosulfate solution with water to dilute it)
-Increase concentration, rate of reaction will increase
Investigating: effect of temperature on the rate of reaction
- Dilute hydrochloric acid is heated to a set temperature using a water bath
- Add the dilute hydrochloric acid into a conical flask
- Add a strip of magnesium and start the stopwatch
- Stop the time when the magnesium fully dissolves
- Repeat at different temperatures and compare results
-increase in temperature, rate of reaction will increase
Investigating: effect of a catalyst on the rate of reaction
- Add hydrogen peroxide into a conical flask
- Use a delivery tube to connect this flask to a measuring cylinder upside down in water trough
- Add the catalyst manganese(IV) oxide into the conical flask and close the bung
- Measure the volume of gas produced in a fixed time using the measuring cylinder
- Repeat experiment without the catalyst of manganese(IV) oxide and compare results
Factors affecting rates of reaction
-concentration of the reactants in solution
-temperature at which the reaction is carried out
-surface area of solid reactants
-use of a catalyst
Rates of reaction - effect of increased concentration
In solution:
-more particles of reactant in the same volume increases the rate of successful collisions
Gas (increasing pressure):
-particles are more crowded
-frequency of collisions between particles will increase
-rate of reaction increases
Rate of reaction - effect of increased temperature
-particles move faster, therefore have more collisions
-increase the energy of the collisions, as particles are moving faster
-there are more successful collisions
-increasing rate of reaction
Rate of reaction - effect of a catalyst
-decrease the activation energy needed for a reaction to occur
-provides an alternative reaction pathway that has a lower activation energy
-more of the particles have at least the maximum amount of energy needed for a reaction to occur when particles collide
Rate of reaction - effect of increased surface area
-if a reactant is a solid, breaking it into smaller pieces will increase its surface area to volume ratio
-particles around it will have more area to react with, so the frequency of collisions will increase
Collision theory
The rate of a chemical reaction depends on:
-collision frequency of reacting particles - how often they collide. More collisions –> faster rate
-energy transferred during a collision - particles have to collide with enough energy for the collision to be successful
Reversible reactions
-product molecules can react with each other or decompose to form the reactant molecules again
-reaction can occur in both directions:
–forward reaction - forms the products
–reverse reaction - forms the reactants
-represented by opposing arrows used to indicate forward and reverse reaction occurring at the same time: ⇌
-dehydration of hydrated copper (II) sulfate
-thermal decomposition of ammonium chloride
Dehydration of hydrated copper (II) sulfate
-When anhydrous copper(II) sulfate is added to water, it turns blue and heat is given off so the reaction is exothermic
-When hydrated copper(II) sulfate crystals are heated in a test tube, the blue crystals turn into a white powder and a clear, colourless liquid (water) collects at the top of the test tube
CuSO4.5H2O (s) ⇌ CuSO4 (s) + 5H2O (l)
Water of crystallisation
The water that is included in the structure of some salts during the crystallisation process
-indicated with a dot written between the salt and surrounding water molecules
Thermal decomposition of ammonium chloride
-Heating ammonium chloride produces ammonia and hydrogen chloride gases:
NH4Cl (s) → NH3 (g) + HCl (g)
-As the hot gases cool down they recombine to form solid ammonium chloride
NH3 (g) + HCl (g) → NH4Cl (s)
Reversible reaction:
NH4Cl (s) ⇌ NH3 (g) + HCl (g)
Equilibrium in a sealed container
-no substances can be added to or removed
1. at the start, no products so rate of forward reaction is zero
2. reaction occurs, rate of forward reaction decreases as reactant used up
3. products increase, chance of reverse reaction occurring increases, rate increases
4. eventually, rate of forward reaction equal to reverse - dynamic equilibrium reached
Dynamic equilibrium
-reaction occurs in a sealed container
-forward and reverse reactions occurring at the same time
-rate of forward reactions = rate of reverse reaction
-concentration of products and reactants are constant
Position of equilibrium
If more reactant than product, position of equilibrium lies to the left
-equilibrium does not mean 50-50 concentrations of products and reactants
Conditions that can change position of equilibrium
-pressure
-temperature
-catalyst
Reaction will always try to counteract change
Equilibrium - changing pressure
-increase pressure, reaction will respond by reducing it
-shift equilibrium to direction that produces fewer gas molecules
-only for gases
Equilibrium - changing temperature
-increase temperature, reaction responds by reducing it
-shift equilibrium in direction that will reduce temperature (endothermic) - reverse reaction
-forward reaction is exothermic
