quantative chemistry Flashcards
Relative atomic mass
It is the average mass of the isotopes of an element compared to 1/12th of the mass of a carbon atom - top number in periodic table
Law of Conservation of Mass
- The Law of Conservation of Mass states that no matter is lost or gained during a chemical reaction.
- Mass is always conserved, therefore the sum of the relative atomic/molecular masses of the reactants will be the same as the sum of the relative atomic/molecular masses of the products.
By adding up the relative formula masses of the substances on each side of a balanced symbol equation, you can see that mass is conserved. The total M of the reactants equals the total M, of the products.
Reactions where mass seems to change
For some types of reaction, if you carry them out in a container that isn’t sealed, you might find that the mass of stuff inside the reaction container has either increased or decreased during the reaction.
Reactions where the mass seems to increase
- If the mass increases, it’s probably because one or more of the reactants is a gas that’s found in air (e.g. oxygen) and all the products are solids, liquids or aqueous.
- The particles in a gas move around and fill the space they’re in. So before the reaction, the gas is floating around in the air. It’s there, but it’s not contained in the reaction vessel, so you can’t account for its mass.
- When the gas reacts to form part of the product, the particles become contained inside the reaction vessel -so the total mass of the stuff inside the reaction vessel increases.
metal + oxygen→ metal oxide
Reactions where the mass seems to decrease
- If the mass decreases, it’s probably because one of the products is a gas and all the reactants are solids, liquids or aqueous.
- Before the reaction, all the reactants are contained in the reaction vessel.
- If the vessel isn’t enclosed, then the gas that’s produced can escape from the reaction vessel as it’s formed. It’s no longer contained in the reaction vessel, so you can’t account for its mass - the total mass of the stuff inside the reaction vessel decreases.
metal carbonate → metal oxide + carbon dioxide
Limiting reactants
A reaction stops when all of one of the reactants is used up. Any other reactants are in excess. The reactant that’s used up is called the limiting reactant (because it limits the amount of product that’s formed).
The amount of product formed is directly proportional to the amount of the limiting reactant used. This is because if you add more of the limiting reactant there will be more reactant particles to take part in the reaction, which means more product particles are made (as long as the other reactants are in excess).
What is a titration?
A titration is an experiment that lets you see what volume of a reactant is needed to react completely with a certain volume of another reactant. For example, you can use a titration to find out exactly how much acid is needed to neutralise a certain quantity of alkali (or vice versa).
Carrying out a titration
- Use a pipette with a pipette filler to measure out a set volume of alkali.
- Put the alkali in a flask with a few drops of indicator.
- Using a funnel, fill a burette with some acid of a known concentration. Run a small amount through the tap, then turn the tap off. Place the flask containing the alkali under the burette.
- Take a reading of the volume of acid in the burette by reading off the value where the bottom of the meniscus touches the scale.
- The first titration you do should be a rough titration. This helps to make your final results more accurate by giving you an approximate idea of how much acid is needed to neutralise the alkali. Add the acid to the alkali a bit at a time, giving the flask a regular swirl.
- The indicator changes colour when all the alkali has been neutralised.
This is the end-point of the reaction. 7. Record the volume of acid left in the burette. - Calculate the amount of acid that was needed to neutralise the alkali by subtracting the final reading of acid in the burette from the initial reading of acid in the burette. This is your rough titre.
- Now do an accurate titration. Take an initial reading of how much acid in the burette. Then run the acid in to within 2 cm³ of the end point. Continue to add the acid drop by drop- -you need to spot exactly when the colour of the indicator changes for your result to be accurate.
- Record the amount of acid left in the burette at the end-point of the titration and use this, along with the initial reading, to calculate the volume of acid used to neutralise the alkali.
- Repeat the accurate titration a few times, until you have at least three results that are within 0.10 cm³ of each other (they are concordant).
- Calculate the mean volume of acid that was needed to neutralise the alkali using the concordant results (ignoring the rough titre and any anomalous results).
Why percentage yields are never 100%
- The reaction is incomplete
- Product is lost during practical procedures
- Unexpected side reactions may be happening
Why percentage yields are never 100% - The reaction is incomplete
In an incomplete reaction, not all of the reactants are converted to products, and the yield will be lower than expected. For example, if a reaction mixture is not heated strongly enough, not all of the reactants will have the energy to react and form products.
Why percentage yields are never 100% - Product is lost during practical procedures
You always lose a bit of material when you transfer it from one container to another - even if you manage not to spill it. Some of it always gets left behind on the inside surface of the old container.
You’ll also lose some product when separating it from the reaction mixture. For example, when you filter a liquid to remove solid particles, you nearly always lose a bit of liquid or a bit of solid.
* If you want to keep the liquid, you’ll lose the bit that remains with the solid and filter paper (as they always stay a bit wet).
* If you want to keep the solid, some of it’ll get left behind when you scrape it off the filter paper.
Why percentage yields are never 100% - Unexpected reactions may be happening
Sometimes there can be other unexpected reactions happening, known as side-reactions. For example, the reactants may react with gases in the air, or impurities in the reaction mixture, rather than reacting to form the product you want. Side-reactions can also be caused by changes to the reaction conditions. Again, this means a lower yield.
Advantages of high atom economy
Reactions with low atom economies use up resources very quickly. They also make lots of waste materials that have to be disposed of somehow. That tends to make these reactions unsustainable- the raw materials will run out and the waste has to go somewhere.
For the same reasons, reactions with low atom economies usually aren’t very profitable. Raw materials can be expensive to buy and waste products can be expensive to remove and dispose of responsibly.
One way around the problem is to find a use for the waste products rather than just throwing them away. Since there’s often more than one way to make the product you want, you could find a reaction that has a similar atom economy but gives useful ‘by-products’ rather than useless ones.
Examples
* Bitumen is a by-product of crude oil distillation and refining (see p.254). It is a sticky, thick black liquid that is used to pave roads.
* One of the reactions used in the production of nylon gives ammonium sulphate as a by-product. Ammonium sulphate can be used as a fertiliser, so even though it’s not the desired product of the reaction, it’s still a useful by-product, rather than waste.
What is atom economy?
A lot of reactions make more than one product. Some of them will be useful, but others will be waste. The atom economy of a reaction tells you what percentage of the mass of the reactants ends up as desired products when manufacturing a chemical. From this you can also work out what percentage is wasted.
100% atom economy means that all the atoms in the reactants have been turned into desired products. The higher the atom economy the ‘greener the process.
Choosing reactions in industry
In industry, atom economy isn’t the only factor to consider when choosing which reaction pathway to use to make a certain product. These factors also need to be considered:
* The percentage yield of the reaction affects how much useful product is made. The higher the yield, the less reactants are wasted, so the cheaper the reaction.
* The rate of the reaction must be fast enough to produce the amount of product that is required in a sensible amount of time.
* If the reaction is reversible, the position of equilibrium must also be considered. The number of products formed relative to the amount of reactants depends on the equilibrium position, so it may be necessary to alter the position of equilibrium to favour the products. This may be done by changing the reaction conditions, which can be expensive.
