Unit 3: Physical chemistry
What would it mean if a reversible reaction is exothermic in the forward direction
*It is endothermic in the backwards direction with with equal and opposite ΔH* · When we write a reversible reaction showing enthalpy change, the *H always shows the enthalpy change of forward reaction*
Practical: Investigate the effect of changing the concentration of acid on the rate of reaction between marble chips and dil. HCl
*Repeat the previous experiment* but with *different concentrations of HCl* but the *same amount of chips, SA, volume of acid and temperature* If the *concentration of HCl doubles, the rate should double*, if it halves the rate should also halve (directly proportional)
What do reversible reactions in equilibrium try do to a change
*Reversible reactions will always try and counteract any changes that you make by undoing your change* (Le Chatelier's principle)
Describe and explain the graph of H2 gas evolved over time in a reaction (3)
- The *reaction is fast at the beginning because the concentration of reactants is high* - *steep gradient* - It is *slower in the middle as the concentration of reactants reduces* - *shallower gradient* - It *stops at the end because all reactants used up* When the reaction is over, the *curve goes flat.*
Example: the reaction for ammonia is exothermic in the forward direction, what would happen to the amount of ammonium formed if the temperature is increased: N2 + 3H2 ⇌ 2NH3
- The backwards endothermic reaction would increase - The equilibrium shifts to the LHS - Less ammonium is produced (equilibrium moves to LHS)
What it the energy level diagram of an ENDOthermic reaction
- energy of reactants higher than products (energy is taken in) - Upward arrow with +ΔH - Energy in y axis (kJ/mol)
What is the energy level diagram of an EXOthermic reaction
- energy of reactants lower than products (energy is released) - Downward arrow with -ΔH - Energy in y axis (kJ/mol)
How can you investigate the temperature change of combustion reactions
1) *Measure 100cm3 of water using a measuring cylinder* and transfer to a *copper can* (*better conductor*, so can absorb more heat energy) 2) Take the *initial temperature* of the water (21.5C) 3) *Weight the spirit burner with ethanol in it and a lid* 4) Set up the *spirit burner directly under the water* can with a *insulating shield* (to *prevent heat loss to the surrounding*) and *light the burner wick* 5) *Heat the water to around 50C* & *stir the water* using the *thermometer* & *measure the maximum temperature* the water gets 6) *Weigh the final mass of the spirit burner* 7) *Repeat for reliability*
How do you determine if a chemical reaction is overall exothermic or endothermic using bond energies
1) Draw the *displayed formula* of the reaction 2) *Count how many of each reactant bond is broken and multiply by its bond energy value,* the *total* is the *overall endothermic & is +* 3) Count how many of each *product bond is formed *and *multiply by its bond energy value*, the *total is the overall exothermic & is -* 4) Be careful with *multiple of the same bonds* that need to be broken and double bonds being different to single bonds 5) *Add the values together *(Remember + and - signs) e.g. +654+-748 6) If *overall reaction is +, the reaction is endothermic*, *if overall reaction is -, the reaction is exothermic*
How can rates of reaction be increased (5)
1) Increasing temperature 2) Increasing pressure (gas reactants) 3) Increasing concentration (solutions) 4) Increasing surface area (solid reactants) 5) Using catalysts
Practical: Investigate the effect of changing the surface area of marble chips reacting with HCl on the rate of reaction: CaCO3 + 2HCl à CaCl2 + H2O + CO2
1) Place *large marble chips in the flask on a balance,* and *add dil. HCl* using a measuring cylinder 2) *Quickly plug the flask with cotton wool* (to *stop acid spitting out but allow CO2 to escape)* 3) Then *weigh it, starting the clock at the same time*. 4) *Note the mass at regular intervals (every 20s)* until the *reaction is complete (mass stays constant)* 5) *Repeat* experiment but with the *same amount* of *smaller marble chips (higher surface area)*
How can you investigate temperature changes for neutralisation reactions (Practical) If you know the concentration of an alkali but not the acid, you can use this method to find the concentration of the acid (like a titration)
1) Place a *polystyrene cup in a glass beaker and fill it with 25cm3 of 2 mol/dm3 KOH* solution (alkali) using a *measuring cylinder* 2) Record *initial temperature* 3) Fill a *burette with 50cm of HCl acid (unknown concentration*) 4) Using the burette *add 5cm3 of HCl to the KOH solution at a time* and *stir vigorously* 5) Record the *maximum temperature* 6) *Continue adding 5cm3 of HCl stirring and recording the temperature until you finish the acid *(50c3)
How can the temperature change of displacement reactions be investigated (Practical)
1) Place a *polystyrene cup in a glass beaker* and *fill it* with 50cm3 of 0.2mol/dm3 *CuSO4 solution* using a *measuring cylinder* 2) *Weigh* 1.2g of *zinc* using a balance 3) Record *initial temperature of CuSO4 solution* (17.0C) 4) *Add zinc and stir the solution* 5) Record the *maximum temperature* of the mixture (27.3C)
How can you investigate temperature changes in dissolving reactions (Practical)
1) Place a *polystyrene cup in a glass beaker* and fill it with *100cm3 of water using a measuring cylinder* 2) Record *initial temperature of water using a thermometer* 3) Weigh 5.2g of *ammonium chloride* using a balance 4) *Add the ammonium chloride to the water and stir until all has dissolved*, then *record the minimum final temperature.* (endothermic)
What are 2 assumptions made in the dissolving reaction
1) The SHC of the diluted solution (NH3Cl and water) is the same as pure water (4.18J/g) 2) The mass of the solution stays 100g even with the 5.2g of NH4Cl added
Practical: investigate the effect of different solids on the catalytic decomposition of Hydrogen peroxide solution
1) Use the following apparatus to show how *volume of oxygen evolved over time changes* *(gas syringe connected to conical flask)* 2) Measure *100cm3 of H2O2 (aq) and transfer to a conical flask* 3) Weigh *0.2g of MnO2 using a balance* 4) Transfer *MnO2 to H2O2 solution and quickly replace the bung* with the gas syringe to stop gases escaping and *swirl the reaction* 5) *Record the volume of oxygen produced every 30s* for 3 mins and *plot a graph of oxygen vs time* 6) *Repeat the expirement* and keep everything the same except use 0.2g of *lead(IV)Oxide and copper(II)oxide catalysts* If the *graph for the catalyst is steepest, the rate is the highest for that catalyst and is the best catalyst*
What are the conditions in dynamic equilibrium (2)
1. The *forward and reverse reactions still happen at the same rate* 2. The *concentrations of the reactants and the products remain constant* (*not equal)
Why does rate of a reaction increase with a catalyst
A catalyst is a chemical that *speeds up a reaction by providing an alternate route with a lower activation energy* (and aren't used up in the reaction)
What does dynamic equilibrium mean?
A state where no net change is taking place (equilibrium), despite the fact that both reactions are still occurring (dynamic)
Describe heating ammonium chloride as a reversible reaction
Ammonium chloride is a *white solid*. It *breaks down when heated*, forming *ammonia and hydrogen chloride*. When these *2 gases are cool enough, they react together to form ammonium chloride again.* Ammonium chloride ⇌ ammonia + hydrogen chloride NH4Cl(s) ⇌ NH3(g) + HCl(g)
What happens to bonds in an exothermic reaction
Bonds are made releasing energy
What does Manganese(IV)Oxide (MnO2) do to H2O2
Catalases the decomposition of H2O2 into H2O and O2
Why does the rate of reaction increase with pressure (same as concentration)
Changing the pressure of gaseous reactants increases the rate of reaction because the *particles are closer, so collide more frequently* (successfully)
How can you find the molar enthalpy change of the combustion of ethanol reaction
Heat energy change = 100g x 4.18 x (62.8C-21.5C) = 17260J = 17.26kJ · Mass of ethanol burned = decrease in mass of the burner = 0.78g · Moles of ethanol (C2H5OH)= 0.78/46 = 0.01696 mol Molar enthalpy change for combustion of ethanol (ΔH): · 17.26kJ/0.01696mol = 1020kJ/mol. (ΔH) = -1020kJ (exothermic)
How can you find the molar enthalpy change from your results of the neutralisation reaction
Heat energy change: Q = (28+25) x 4.18 x (31.8-19.3C) = 2769.3J = 2.7693kJ Molar enthalpy change (ΔH) = 2.7693/0.05 = -55.4kJ/mol (exothermic)
Hydrogen can be made by the reaction of methane with steam. What would happen to the amount of hydrogen formed if the pressure was increased?: CH4(g) + H2O(g) ⇌ 3H2(g) + CO(g)
If pressure was increased: - Position of equilibrium would shift to LHS with fewer gas molecules to decrease the pressure - Less hydrogen produced as reaction moves to LHS (reactants)
Hydrogen gas reacts with iodine gas to form hydrogen iodide. What would happen to the amount of hydrogen iodide formed if the pressure was increased?: H2(g) + I2(g) ⇌ 2HI(g)
If pressure was increased: - The equilibrium position does not move as there are the same number of gas molecules on both sides - The amount of hydrogen iodide remains the same
Describe the dehydration of CuSO4 as a reversible reaction
If you *heat Hydrated CuSO4, the blue crystals* turn into white powder (anhydrous CuSO4) and water is driven off. If you add water again to the *white powder, it turns blue (hydrous CuSO4)* *CuSO4.5H2O (s) ⇌ CuSO4(s) + 5H2O (l)*
What would happen to the position of equilibrium if you increase or decrease the pressure of this reaction: A(g) ⇌ B(g)
Increase/decrease: The position of equilibrium does not move Because there are the same number of gas molecules of reactants and products
What would happen to the position of equilibrium if you increase or decrease the pressure of this reaction: 2A(g) ⇌ B(g)
Increase: Equilibrium position moves to right because fewer gas molecules on right (1:2) - decrease pressure Decrease: Equilibrium position moves to left because more gas molecules on left (1:2) - increase pressure
What would happen if the position of equilibrium in a reversible reaction lay on the left (LHS), middle or right (RHS) IN EQUILIBRIUM
LHS - *more reactants than products* middle - *similar amount of reactants and products* RHS - *more products than reactants*
Why does rate increase with surface area (2)
Mg powder reacts faster with HCl(aq) than Mg strips because: · In the ribbon, the *acid can only collide with the surface Mg atoms* · In the powder, *more Mg atoms exposed, so higher frequency of collisions*
What are some examples of exothermic reactions (4)
Neutralization reactions e.g. NaOH + HCl Displacement e.g. Thermit reaction Dissolving e.g. dissolving NaCl in water Combustion e.g. combustion of ethanol
How can you determine the molar enthalpy change of the displacement reaction from your results
Q = 50 x 4.18 x (27.3-17) = 2152.7J = 2.1527 kJ Number of moles of Zn = 1.2/65 = 0.0185 mol Molar enthalpy change (ΔH)= 2.1527kJ/0.0185 mol = -215kJ/mol
What is the equation to find heat energy
Q = mcΔT Q = heat energy change m = mass (usually of water where 1cm3 = 1g) c = specific heat capacity (for water, 4.2J/g) ΔT = Change in temperature (of water)
What are reversible reactions
Reversible reactions are reactions where a product can turn back into the reactants, and are expressed with a *⇌ symbol*
What are bond energies?
The energy needed to break/make 1 mole of a type of bond is called bond energy. It is given in kJ/mol. · You don't need to memorize the bond energies. · When breaking bonds, bond values are -, when making, values +
What does the position of equilibrium depend on (2)
The position of equilibrium depends on: 1) Temperature 2) Pressure 3) Concentration* not in syllabus
What is the effect of increasing the surface area of the marble chips in the reactions
The reaction for the small marble chips is *much faster (steeper gradient) and finishes faster* (mass becomes faster after only 4 minutes as opposed to 6 mins)
What are the conditions for a reversible reaction to be in dynamic equilibrium
The reversible reaction is in a *sealed container (closed system)* and the *conditions are kept constant*
What is specific heat capacity
The specific heat capacity (c) of a substance is the amount of heat energy needed to raise the temperature of *1g of substance 1oC* · The c of water is 4.2 J/g. The more the mass of water, the higher the amount of energy is needed to raise the temp 1C
How can you find the molar enthalpy change from your results of the dissolving reaction
This is an endothermic reaction because the temperature of water falls · Q = mcΔT, so Q = 100 x 4.18 x (18.3-15.1) = *1.3376kJ* · Mr of NH4Cl = 53.5; n = 5.2/53.5 = 0.0972mol Molar enthalpy change (ΔH)= *1.3376kJ*/0.0972mol = +13.8kJ/mol (amount of thermal energy absorbed by 1 mol of dissolving NH4Cl)
What are the 2 assumptions we make in the neutralisation reaction
We make 2 assumptions in this experiment: 1) The density of the mixture is the same as water (1cm3 = 1g) 2) The SHC of the mixture is the same as water
What happens to bonds in an endothermic reaction
bonds break which takes in energy (less stable)
What is activation energy
minimum amount of energy in a collision for a reaction to occur
What would happen if a reaction is exothermic in the forward direction, what would happen if you decrease the temperature
the reaction will try and *increase the temperature by increasing the forward exothermic reaction* and *equilibrium position moves to the right*
Why may the theoretical value of the molar enthalpy of ethanol may not be obtained practically (2)
· *Heat loss to surroundings* · *Incomplete combustion of alcohol* (releases less energy), if the combustion is complete, the flame should be blue which it is not
What does a catalyst do to the position of equilibrium
· A catalyst overall *increases the rate of reaction, it increases the rate of a reaction* of the *forward AND backwards EQUALLY*. This means that the *system reaches equilibrium faster.* · Therefore, it helps a reversible reaction reach *equilibrium quicker and DOES NOT have an effect on the position of equilibrium*
What can you do with the results of the exothermic neutralisation reaction
· At first, the *temperature increases* as you continue adding HCl, *but then decreases.* · This is because *all the alkali reacts with the acid after a certain point* (neutralization point), so there is *no more heat change* · You can plot a graph with *2 lines of best fit* following the points, where the 2 *lines intersect* is the neutralization point (*max temperature* and shows the *amount of acid added*)
Why does the rate of reaction decrease over time (increases at a decreasing rate)
· At the start there are *many particles to react*, but they all *get used up* in successful collisions · So the *solution becomes less concentrated* (fewer atoms) and a *lower chance/frequency of successful collisions *(rate slows to a stop)
Describe an energy profile diagram
· Energy profile diagrams show a curve between reactant and product energy levels which is activation energy curve · The peak of the curve to the reactant level is the activation energy to initiate the reaction · The overall energy change (ΔH) is the difference between the product level and the reactant level. Catalysts don't affect the ΔH but lower the activation energy
What is an exothermic and endothermic reaction
· Exothermic - a reaction that gives out heat energy · Endothermic - a reaction that takes in heat energy
How can a reaction be overall exothermic or endothermic
· For a reaction to be overall *exothermic*, *more energy must be released by bond making than taken in by bond breaking* · For a reaction to be overall *endothermic, more energy must be taken in by bond breaking than released by bond making*
Why does doubling the concentration produce 2x the product
· If one reactant is in *excess, doubling the concentration means that it can react with more of the other reactant to produce double the gas*
What is the effect of changing pressure (gases) on the position of equilibrium (2)
· If you *increase pressure*, the equilibrium tries to *reduce it by by moving in the (direction) with fewer molecules of gas (lower pressure)* · If you *decrease pressure*, the equilibrium tries to *increase it by by moving in the (direction) with more molecules of gas (higher pressure)*
Why does rate increase with concentration
· In concentrated solutions, there are *more particles for the same volume* · So there is a *higher chance/frequency of a successful collisions* and faster rate
What is the molar enthalpy change (ΔH) in exothermic and endothermic reactions
· In exothermic reactions ΔH is negative (release energy) · In endothermic reactions ΔH is positive (absorb energy)
What is molar enthalpy change
· The heat energy change taken in/released is called molar enthalpy change or ΔH measured in kJ/mol
Describe the rate of reaction during a reaction (3)
· The rate changes all through the reaction. It is greatest at the start, but decreases as the reaction proceeds. · The faster the reaction, the steeper the curve. · When the reaction is over, the curve goes flat.
Why does rate increase with temperature (3)
· When the solution is heated, the *particles gain KE* (move faster) · This makes the particles *move faster & have more frequent successful collisions.* *Collisions are more energetic (more than activation energy)*
Why do catalysts help in industry
· catalysts are used in industry to help substances *react at lower temperatures and pressures which saves money (less energy)*
What is collision theory requirements for particles to react/collide successfully
· the *particles must collide in the correct orientation* · the *collision must have enough energy to be successful.*
How can the rate (change over time) of a chemical reaction be measured (2)
· the amount of a reactant used up per unit of time or · the amount of a product formed per unit of time. E.g. vol of gas, time taken for precipitaiton to form
