Section 3: Physical Chemistry
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What is the difference between an endothermic and exothermic reaction?
Exothermic - releases heat to surroundings, increases in temperature. Endothermic - absorbs heat from surroundings, decreases in temperature
Describe an experiment for a combustion reaction (calorimetry)
Put 50g of water in a copper can, then record its temperature. Weigh a spirit burner with lid, then place the burner under the copper can and light the wick. Place the burner under the water and let it heat up and constantly stir the water until it reaches 50 °C. Put the flame out by using the lid and measure the final tmeperature of the water. Then weigh the burner and lid again. Calculate enthalpy change
Describe an experiment for a dissolving reaction (calorimetry)
Place 25 cm3 of dilute hydrochlric acid in a polysyrene cup and record the temperature of the acid. Put 25cm3 of dilute sodium hydroxide solution in a measuring cylinder and record the temperature and record its temperature. Add the alkali to the acid and stir. Take the temperature every 30 seconds and record the highest temperature it reaches.
Describe a experiment for a neutralisation reaction (calorimetry)
Place 25 cm3 of dilute hydrochlric acid in a polystyrene cup and record the temperature of the acid. Put 25cm3 of dilute sodium hydroxide solution in a measuring cylinder and record the temperature and record its temperature. Add the alkali at 5cm3 intervals to the acid and stir. Record the maixmum temperature it reaches after each addition.
How is the molar enthalpy change calculated? What does each part of the equation represent
Step 1: Work out energy change: E = m x c x ΔT. Step 2 Work out moles of the limiting reagent Step 3 E/n = ΔH. E - energy, m - mass of surrounds (usually mass of water = volume of water), c - specific heat capacity - 4.2, n - number of moles of reactant
50cm3 of 0.10moldm-3 silver nitrate solution was put in a calorimeter, 0.2g of zinc powder was added. Temp of solution rose by 4.3 °C. Calculate energy change, moles of zinc reacted and molar ethalpy change
• 50 x 4.2 x 4.3 = 903/1000= 0.903 • n=0.2/65= 0.003mol • Δ H =0.903/0.003 = -307kJ/mol
Excess sodium hydrogencarbonate was added to 30.0 cm3 of 2moldm-3 hydrochloric acid. The temp fell 10.3°C. Calculate the energy change,moles of zinc reacted, molar enthalpy change
• 30 x 4.2 x 10.3=1297.8/1000=1.2978 • n=2/36.5=0.055 • ΔH = 1.298/0.0055= +23.6kJ/mol
The rate of reaction depends on what 5 things?
temperature, concentration, catalyst, surface area, pressure (for gases only)
On a rate graph what tells you about the rate of reaction
The steeper the line the faster the rate, it will go flat the when the reaction has stopped
Rate of reaction formula
Amount of reactant used or amount of product formed / Time
3 ways to measure the speed of a reaction
Precipitation, Change in mass, Volume of gas given off
What is Collision Theory
Rate of reaction depends on how often and how hard the reacting particles collide with each other. Particles need to collide with enough energy to be successful and react. The activation energy is required to break bonds.
What does a catalyst do
Lowers the activation energy, is not used up, provides an alternative reaction pathway
Complete the following equations: magnesium + hydrochloric acid ——> calcium carbonate + hydrochloric acid ——–> sodium thiosulfate + hydrochloric acid ———->
magnesium + hydrochloric acid ——> magnesum chloride + hydrogen calcium carbonate + hydrochloric acid ——–> calcium chloride + carbon dioxide + water sodium thiosulfate + hydrochloric acid ———-> sodium chloride + sulfur+ sulfur dioxide + water
Why does rate change with surface area?
As the particle size decreases the surface area increases. The increased surface area means there is more surface for collisions to take place. There are more frequent collisions. There are more frequent successful collisions. The rate increases.
Why does rate change with temperature?
The temperature increases. Particles have more energy and move faster. There are more frequent collisions and more frequent successful collisions. The rate increases. The rate also increases because….The particles have more energy so when they collide a greater proportion of the collisions are successful
Why does rate change with concentration?
As concentration increases…The frequency of collisions increases…The frequency of successful collisions increases, The rate increases
Why does rate change with a catalyst?
With a catalyst the proportion of collisions that are successful is increased, the frequenct of collisions is not affected
Describe the difference in the enthalpy profile diagrams for an endothermic and exothermic reaction
Exothermic - product line below reactant line ΔH line points downwards and is negative. Endothermic - product line above reactant line ΔH line points upwards and is positive
What is the energy change associated with bond breaking and bond making
Bond breaking - energy absorbed, endothermic process. Bond making - energy released, exothermic process
How do you calculate enthalpy change from bond energies? (4 steps)
Step 1: Draw out molecules based on the balanced equation. Step 2: Work out the number of each type of bond you are breaking and bond you are making. Step 3: Use the bond energies to work out the total energy absorbed to break the bonds and energy released from making bonds. Step 4: Bonds broken - Bonds made = Enthalpy change in kJ/mol
Sketch an energy level diagram to show activation energy. Add a line to show the activation energy if a catalyst was present also
Curved line going up and then down to reactants. Activation energy line goes from level with reactants to the top of the curve. Another line but with a lower peak for the catalysed reaction, activation arrow is therefore shorter
Calculate the enthalpy change for the following reaction: 2H2O2 -> 2H2O + O2. Bond energies (O-H = 464, O-O = 146, 0=0 = 498)
-206 kJ/mol
Calculate the enthalpy change for the following reaction: 2H2 + O2 -> 2H2O Bond energies (O-H = 464, H-H = 436, 0=0 = 498)
-486 kJ/mol