Rates of Reaction Flashcards
All 24 cards in this deck
In the marble chips and hydrochloric acid practical, what two pieces of apparatus are needed to collect and measure the gas?
A delivery tube with a bung in the flask, connected to a gas syringe (or leading into an upturned measuring cylinder of water in a trough).
Why does the mixture turn cloudy when sodium thiosulfate reacts with dilute hydrochloric acid?
A solid/precipitate of sulfur is formed.
In the 'disappearing cross' method (sodium thiosulfate + dilute hydrochloric acid), what is timed, and what does a shorter time show?
The time from mixing until the cross under the flask can no longer be seen (the sulfur precipitate hides it). A shorter time means a faster rate.
How can the conical flask and its contents be kept at a constant temperature such as 45°C during a rate investigation?
Stand the conical flask in a water bath.
According to collision theory, when does a reaction occur between two particles?
When the particles collide with sufficient energy — at least the activation energy.
Explain, in terms of particles, why increasing the temperature increases the rate of reaction.
Particles have more energy so move faster, giving more frequent collisions, and a higher proportion of collisions have at least the activation energy.
Explain, in terms of particles, why increasing the concentration of a solution increases the rate of reaction.
There are more particles in the same volume, so there are more frequent collisions.
Why does using a powdered solid instead of large lumps increase the rate of reaction?
Powder has a larger surface area to volume ratio, so more particles are available to react and collisions are more frequent.
How does increasing the pressure of a reaction involving gases affect the rate, and why?
The rate increases — the gas particles are closer together in the same volume, so collisions are more frequent.
On a graph of volume of gas against time, what does a flat (levelled-off) line show?
The reaction has stopped: no more gas is made because a reactant has been used up.
On a volume of gas against time graph, where is the rate of reaction fastest, and why does it slow down?
At the start, where the curve is steepest; the rate falls as reactants are used up, giving less frequent collisions.
What is the equation for the mean rate of reaction?
mean rate of reaction = amount of product formed (or reactant used) ÷ time taken
e.g. volume of gas in cm ÷ time in s gives cm s.
How is the rate at a particular time found from a curved volume–time graph?
Draw a tangent to the curve at that time and find its gradient (change in volume ÷ change in time), in cm s.
How does the curve for powdered magnesium compare with the curve for the same mass of magnesium ribbon on the same axes?
Steeper and to the left of the original curve, levelling off at the same final volume.
What is a catalyst?
A substance that increases the rate of a reaction without altering the products, and is itself chemically unchanged and unchanged in mass at the end.
How does a catalyst increase the rate of a reaction?
It provides an alternative route with a lower activation energy, so a greater proportion of collisions are successful.
On a reaction profile, what is different and what is the same when a catalyst is used?
The peak is lower (lower activation energy); the start and end energies are the same because the reactants and products are the same.
What are enzymes?
Biological catalysts.
Which process, catalysed by enzymes in yeast, is used to produce alcoholic drinks?
Fermentation.
True or false? Enzymes change the products of the reactions they catalyse.
False. Like all catalysts, enzymes speed up the reaction without altering the products.
In a rate investigation using calcium carbonate and hydrochloric acid at different temperatures, name three variables that must be controlled.
Particle size of the calcium carbonate, mass of calcium carbonate, and concentration (or volume) of acid.
Apart from collecting gas, state one other quantity that can be measured over time to follow the rate of a reaction that produces a gas.
The loss in mass of the reaction flask on a balance.
When following a rate by loss in mass, why is a loose plug of cotton wool put in the neck of the flask?
It lets the gas escape, so the mass falls, but stops liquid spraying out of the flask, which would also lose mass and give a false reading.
Why is only a small amount of catalyst needed, even for a large amount of reactant?
The catalyst is not used up, so the same catalyst can keep speeding up the reaction over and over again.