Energy Changes Flashcards
Exothermic
Transfers energy to surroundings
Shown by a rise in temperature.
Energy of products is higher than the energy of the reactants
Examples:
Combustion-gives out lots of energy
Neutralisation- acid+alkali
Oxidation- sodium+water releases energy and sodium is oxidised
Everyday uses:
Hand Warmers- oxidation of iron+air (salt solution catalyst) to release energy.
Self heating cans- between chemicals in their bases
Endothermic
takes in energy from surroundings
Shown by fall in temperature
Products have lower energy than the reactants
Examples:
Citric acid+sodium hydrogencarbonate- reaction in sherbet
Thermal Decomposition- CaCO3 (+heat)-> CO2+CaO
Everyday Uses:
Sport Injury Packs- allows injury pack to become instantly cooler without using freezer
Reaction Profiles
Show relative energy of reactants and products in a reaction and how the energy changes over the course of the reactions.
Exothermic- products lower than reactants
Endothermic- products higher than reactants
Activation Energy
Minimum amount of energy for reactants to start colliding and react.
Greater the activation energy, the more energy needed to start reactions.
Energy Transfer (Required Practical)
Exothermic/endothermic
1) Pour 50cm3 of 2M dilute of HCl into Polystyrene cup using 50cm3 measuring beaker
2) Stand the cup inside of a beaker of cotton wool for more insulation,
3) Measure the temperature of acid and record
4) Pour 5cm3 of 2M NaOH into a 10cm3 measuring cylinder
5) Put NaOH into cup, fit lid and stir using thermometer through hole
6) Record temperature when it stops changing
7) Repeat 4-6 until you’ve added 40cm3
NaOH.
8)Repeat everything for a second trial- calculate average
9) Plot a graph with mean temperature (y-axis) and volume of NaOH added (x- axis).
10) Line of best fit
Bond Energies
Energy Supplied to break bonds- endothermic
Energy released when forming bonds- Exothermic
Bond Energies Calculations
Each chemical bond has a particular bond energy associated with it- used to calculate overall energy change of reaction
Endothermic- energy change is positive as it’s taken energy from surrounding
Exothermic- energy change is negative as it’s released to its surroundings
Example:
H2+C2 -> 2HCl
Energy required to break bonds- (1x H-H) + (1x Cl-Cl)=678kJ/mol
Energy released from formation- (2xH-Cl)= 862kJ/mol
Overall change: 678-862= 184kJ/mol (exothermic)
Chemical reactions in cells
Electrochemical cell- 2 different electrodes in contact with an electrolyte:
2 electrodes must conduct electricity (usually metals)
Electrolyte is a liquid which reacts with electrodes as it contains ions
Chemical reaction between the 2 electrodes and the electrolyte creates a charge difference between the electrodes.
The electrodes are connected by a wire so the charge is able to flow/ electricity is produced.
Voltmeter could be connected to measure the voltage of cell
Voltage in cells
Depends on the electrolyte and electrodes used
Different electrode- different reactivities
Different electrolyte= different ions in liquid
Bigger the difference in reactivity of electrodes, bigger the voltage of the cell
Batteries
Battery formed by connecting 2 or more cells in series
Voltages of the cells are combined to give larger overall voltage.
Overtime reacting particles used up, turned into product of reaction. Reaction stops, no electricity produced- irreversible (alkaline batteries)
If reaction is reversible, cell can be recharged by connecting it to external electric current
Fuel Cells
An electrical cell that’s supplied
with fuel+oxygen
Fuel that enters cell would become oxidised,
sets up potential difference in cell
Many types of fuel cells which use different types of electrolytes and fuels
i.e Hydrogen-Oxygen fuel cells
Hydrogen-Oxygen fuel cells
Electrolyte- usually potassium hydroxide solution
Electrodes- usually porous carbon with a catalyst
Anode- H2 loses electrons to produce H+ ions (oxidation)
2H+ + 2e-> H2
Cathode- O2 gains electrons, reacts with H+ ions to form H2O (reduction):
O2+ 4H+ + 4e -> 2H2O
Electrons flow through external circuit from anode to cathode- electric current
Overall reaction:
2H2 + O2-> 2H2O
Advantages of Hydrogen-Oxygen Fuel Cells
Less pollutants released: by-products are water+heat
Less expensive than batteries in electric vehicles
Less time-consuming: doesn’t need to be recharged as often
Burning hydrogen in oxygen will give water, calculate the energy change for this reaction
2H2 + O2 -> 2H2O
H-H H-H + O=O -> H-O-H H-O-H
H-H = 436x2 = 872
O=O 498
O-H= 464x4= 1856
1370 -> 1856
1370-1856 = -468 Kj/mol
Exothermic
