Monitoring and Controlling Chemical Reactions (OCR)

Define theoretical yield

The maximum mass it is possible to make from a given mass of reactants

What is the calculation for percentage yield?

percentage yield = actual yield/theoretical yield x 100

Why may a percentage yield be below 100%?

Reactants may react in a different way than expected, reaction may not go to completion, some of the product may be lost when you separate it from the reaction mixture to purify it

Define atom economy

A measure of how many atoms in the reactants form a desired product

What is the equation for atom economy?

Atom economy = RFM of desired product / sum of RFM for all products x 100

What factors should you consider when choosing a reaction pathway?

Yield of the product, atom economy of reaction, usefulness of by products, rate of reaction, equilibrium position and whether it's a reversible reaction

Define by product

A substance formed in a reaction in addition to the desired product

How do you convert cm^3 to dm^3?

Divide by 1000

What equation links concentration, mass, and volume?

concentration = mass/volume

What equation links concentration, moles, and volume?

Concentration = moles / volume

Define concentration

The amount of substance present in a given volume

What is a titration used for?

To find the concentration of an unknown solution

What is the long glass tube called in a titration?

Burette

Why should you use a single indicator in a titration?

So there is a sudden colour change and therefore producing a more accurate reading

What are some potential errors in a titration?

The burette not being clamped vertically, eye protection not being worn

What piece of equipment can you use to accurately measure a volume of liquid?

Volumetric Pipette

How much of the known solution should you use in a titration?

25cm^3

Define titre

The difference between the initial burette reading and the final burette reading

How should you take a burette reading?

Take the reading off the bottom of the meniscus

Define meniscus

Curved surface of liquid

What are concordant titre results?

Within 0.1cm^3 of each other

What volume does 1 mole of gas occupy?

24dm^3

What equation can be used to identify gas volume?

Volume = amount in mol x 24dm^3

Define rate of reaction

A measure of how quickly reactants are used or products are formed

What is the calculation for rate of reaction?

Rate = amount / time taken

How can you measure rate of reaction with gas?

Attach conical flask to gas syringe and react magnesium with HCl, measure how much hydrogen gas given off after set time

What factors increase rate of reaction?

Catalyst, higher temperature, increased concentration, larger surface area

Why does increasing temperature increase rate of reaction?

Particles have more kinetic energy, so move faster, so have a higher chance of successfully colliding, meaning they have a higher chance of reacting

When will a collision not be successful?

If particles have less energy than the activation energy

Define activation energy?

The minimum amount of energy required to start a reaction

What relationship does reaction time have with rate of reaction?

Inversely proportional

Why does increasing concentration increase rate of reaction?

There are more particles in a given volume, so they are closer together, meaning the chance of successful collisions are higher

When measuring rate of reaction using magnesium ribbons, what should be controlled?

The length of the magnesium ribbon

Why does a reaction go faster at a higher pressure?

Particles become more crowded so higher chance of successful collisons

Why does increasing surface area increase rate of reaction?

More reactant particles are available for collisions, therefore collisions are more likely, therefore more successful collisions

Define catalyst

A substance that speeds up the rate of chemical reaction but is unchanged itself in a reaction

Give an example of a catalyst in biological systems?

Enzymes

How do catalysts work?

They provide an alternative reaction pathway with a lower activation energy, so more particles can successfully collide and react

What is a reversible reaction?

A reaction where the products can react together to form the original reactants

What happens at equilibrium?

The concentrations of all reacting substances remain constant, forward and backward reactions still happen so it's a dynamic equilibrium

What conditions are needed for dynamic equilibrium?

Closed system and a constant temperature

Define dynamic equilibrium

When the rate of the forward reaction is equal to the rate of the reverse reaction and concentrations of reactants and products remain unchanged

When will the equilibrium be on the left? (Concentration)

When the concentration of reactants is greater than the concentration of products

What happens to the equilibrium position if pressure is increased?

Moves towards the direction of the least moles of gas

What happens to the equilibrium position if you increase concentration of a substance?

Position moves away from that that substance

What happens to the equilibrium position if temperature is increased?

Moves in the direction of the endothermic change

What does the equilibrium yield depend on?

Pressure and temperature

Define equilibrium yield?

The amount of desired product present in a reaction at equilibrium

State Le Chatelier's principle

If a system is at equilibrium and a change is made to any of these conditions, then that system will respond to counteract the change

What happens to the mass of a catalyst as the reaction occurs?

Remains unchanged

Why doesn't an increase in temperature affect the yield of a product?

Higher temperature favours endothermic reaction so equilibrium shifts to left hand side

Describe in detail an experiment on how concentration of acid affects the rate of reaction

Add 10cm^3 of sodium thiosulfate solution to a conical flask, then add 40cm^3 of water. Measure and record temperature of the solution. Place the conical flask on a piece of paper with a black cross drawn on it. Add another 10cm^3 to the flask and start a stopwatch. When the cross is no longer visible stop the stopwatch and record the time taken in a table. Repeat the experiment 5 times, using different concentrations each time. Calculate rate of reaction using volume/time.