3) Physical Chemistry Flashcards
(33 cards)
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
