SC14/15/16: Quantitative Analysis, Chemical & Fuel Cells Flashcards
SC14a Yields
Formula to find the percentage yield of a reaction.
- Percentage yield = actual yield/theoretical yield x 100
SC14a Yields
Explain the 3 main reasons why the actual yield is less than the theoretical yield.
- The reaction may be incomplete so not all of the reactants are used up, possibly because the reaction has not been left for long enough or the reaction reached equilibrium.
- Some of the product is lost, for example, when a liquid is transferred from one container to another, some of it will be left behind on the walls of the first container.
- There may be other unwanted side reactions taking place, for example, some of the reactants may react to make a different product. These side reactions compete with the main reaction.
SC14b Atom economy
A) Describe what is ment by Atom economy
B) Formula to find Atom economy
A) Atom economy is a method of showing how effienctly a particular reaction makes use of the atoms in the reactants.
B) Atom economy = ( Mr of the useful product/sum of the Mr of all the reactants ) x 100
SC14b Atom economy
Describe the factors that effect the choice of a reaction pathway while also providing examples.
- One way to improve atom economy is to find uses for the waste products. Other factors chemists must consider are; energy consumption, rate of reaction, raw materials and conditions needed to produce a high yield if the reaction reaches equilibrium.
Fermentation:
- Raw materials: carbohydrates - these are renewable
- Temperature: 30-40C
- Pressure: Atmospheric pressure
- Catalyst: Enzymes in yeast
- Rate of reaction: Slow
- Purity: Impure - must be fractionally distilled
Reaction of ethene with steam:
- Raw materials: ethene obtained from crude oils - unrenewable
- Temperature: 300C
- Pressure: High ( 60-70atm )
- Catalyst: Concentrated phosphoric acid
- Rate of reaction: fast
- Purity: Pure
SC14c Concentrations
Provide the formula;
A) The concentration of a solution in g dm-3
B) The concentration of a solution in mol dm-3
C) Linking the 2 different concentrations
A) Concentration in g dm-3 = mass of solute in g / volume of solution in dm3
B) Concentration in dm-3 = number of moles of solute / volume of solution in dm3
C) Concentration in mol dm-3 = Concentration in g dm-3 / relative formula mass of solute
SC14e Molar volume of gases
A) What is Avogadro’s law?
B) What is the Molar gas volume?
A) Avogadro’s law states that, if the temperature and pressure are the same, equal volumes of different gases contain an equal volume of molecules.
B) The Molar gas volume is the volume occupied by one mole of molecules of any gas.
- Volume of gas = amoung of gas (mol) x molar volume
SC15a Fertilisers and the Harber process
A) What is the Harber process?
Describe the formation of;
B1) Ammonium nitrate
B2) Ammonium sulfate
A) The Harber process is a reversible reaction between hydrogen and nitrogen to form ammonia.
B1) Nitrogenous fertilisers are a source of soluble nitrogen compounds. Ammonium nitrate is a salt manufactured by reacting ammonia solution with dilute nitric acid.
- NH3(aq) + HNO3(aq) -> NH4NO3(aq)
B2) Ammonia sulfate is also a nitrogenous fertiliser. It is made in a laboratory by reacting ammonia solution with dilute sulfuric acid.
SC15a Fertilisers and the Harber process
Compare the production of Ammonia sulfate in a laboratory and industrially
Laboratory preparation:
- Scale of production: Small scale
- Starting materials: Ammonia solution and dilute sulfuric acid
- Stages: Titration, then crystalisation
- Type of process: Batch
Industrial production:
- Scale of production: Large scale
- Starting materials: raw materials for making ammonia and sulfuric acid
- Stages: Several stages
- Type of process: Continuous
Conditions used are related to;
- The availability and cost of raw materials and energy supplies
- The control of temperature, pressure and catalyst used produce an acceptable yield in an acceptable time.
SC15b Factors affecting equilibrium
Describe the different factors that effect dynamic equilibrium
1. Temperature increases
- Position of equilibrium: moves in the direction of the endothermic reaction
- Time taken to reach equilibrium: decreases
2. Pressure increases
- Position of equilibrium: moves towards the side with fewer molecules
- Time taken to reach equilibrium: decreases
3. Concentration increases
- Position of equilibrium: moves away from the reacting substance in the balanced equation
- Time taken to reach equilibrium: decreases
4. Catalyst added
- Position of equilibrium: no change
- Time taken to reach equilibrium: decreases
SC16a Chemical cells and fuel cells
Describe the components of a chemical cell and how chemical cells work
1. A simple chemical cell has these components;
- 2 different metals, each dipped into a solution of one of their salts.
- a ‘salt bridge’ to allow dissolved ions to pass from one solution to another.
2. A voltage is produced between the 2 metals. In general, the further apart in the reactivity series the 2 metals are, the greater the voltage.
- In a Daniel cells, the reaction is exothermic and when one of the reactants are used up, the reaction stops and voltage is no longer produced.
SC16a Chemical cells and fuels cells
Describe what fuel cells are.
- Fuel cells are supplied with fuel and oxygen from outside. No burning takes place inside fuel cells, just like chemical cells.
- Fuel cells do not go ‘flat’ - they produce a voltage for as long as reactants are supplied. Hydrogen-oxygen fuel cells use hydrogen and oxygen, and water is the only product.
- At the left hand electrode, hydrogen atoms lose electrons and form hydrogen ions.
- Electrons flow through the external circuit to the positive electrode. Hydrogen ions pass through a membrane to the right-hand electrode, where they gain electrons and react with oxygen to form water.
SC16a Chemical cells and fuel cells
Evaluate the strength and weaknesses of fuel cells.
- A hydrogen-oxygen fuel cells and electric motor are much quieter and need less maintance than a petrol or diesel engine, but the hydrogen still needs to be stored in a tank.
- Hydrogen-oxygen fuel cells do not produce CO2. However, most hydrogen is manufactured by the reaction of steam with coal or natural gas. These processes do release CO2 as a by-product.
