Monitoring and Controlling Chemical Reactions Flashcards
how do you work out the rate of a reaction?
workout how fast the products are being formed or how fast the reactants are being used up
does the rate of reaction stay the same throughout the whole reaction?
no, it starts faster and slows
why does the rate of reaction decrease over time?
as more reactants are used up, less successful collisions take place.
What is collision theory?
in order for particles to react, they have to collide with each other with sufficient energy.
What is activation energy?
the energy required for particles to collide successfully and react
What are 4 factors that effect rate of reaction?
temperature
conc/pressure
surface area
catalyst
why does increased temperature increase the rate of reaction?
the particles have more energy so they collide successfully more often
Why does increasing the concentration or pressure increase the rate of reaction?
there are more reactant particles in the same volume so the frequency of successful collisions increases
Why does increasing the surface area increase the rate of reaction?
there are more particles exposed and therefore more particles are able to react, increasing the frequency of successful collisions
What are catalysts?
substances that speed up a reaction without being used up
How do catalysts increase rate of reaction?
they create an alternate pathway with less activation energy, meaning there are more successful collisions
What are 3 examples of catalysts?
enzymes
cobalt
nickel
Transition metals make good catalysts, true or false?
true
What is the symbol for a reversible reaction?
⇌
What is a reversible reaction?
one in which both the forward and backward reactions can take place.
What are 3 important things about systems in equilibrium?
The forward and backward reactions occur at the same rate
The concentrations of the reactants and products remain constant
It requires a closed system
At equilibrium, if the concentration of reactants is higher than the concentration of products, we say that the position of equilibrium lies to the:
left
In a reversible reaction, if the forward reaction is exothermic, the backward reaction is _____.
endothermic
If the position of equilibrium lies to the right, it means there are more:
products
In a reversible reaction, if the forward reaction is endothermic, what would be the affect of increasing the temperature?
Equilibrium would shift to the right
Yield of products would increase
In a reversible reaction, if the forward reaction is exothermic, what would be the effect of decreasing the temperature?
Yield of products would increase
Equilibrium would shift to the right
If the concentration of reactants is more than the concentration of products, where does the equilibrium lie?
to the left
What is le Chatelier’s principle?
the idea that if you change the conditions of a reversible reaction, the position of equilibrium will shift to try and counteract the change
If the pressure is increased, where will the equilibrium shift to?
it will shift to the side with the least amount of molecules
If you increase the concentration of a reactant, where will the equilibrium shift to?
the right
In a reversible reaction, changes in pressure will only affect substances that are in the:
gaseous state
H2(g) + I2(g) ⇋ 2HI(g)
In the above reaction, increasing the pressure will:
Have no effect on the position of equilibrium
What is the formula for finding concentration from mass and volume?
conc. = mass/volume
5.0 g of KNO3 is dissolved in 100 cm3 of water.
Calculate the concentration in g/dm3.
50
How do you carry out a titration?
1) Use the pipette to add 25 cm3 of alkali to a clean conical flask.
2) Add a few drops of indicator and put the conical flask on a white tile.
3) Fill the burette with acid and note the starting volume.
4) Slowly add the acid from the burette to the alkali in the conical flask, swirling to mix.
5) Stop adding the acid when the end-point is reached (this is when the acid has neutralised the alkali and the indicator changes colour).
Note the final volume reading, and calculate how much acid you added in total.
Repeat the titration until you get ‘concordant results’, which means volumes of acid that are within 0.10 cm3 of each other.
Use the concordant results to calculate the mean volume of acid required to neutralise the alkali.
Why is it important to swirl the conical flask in titration?
as you add the acid from the burette in order to evenly distribute it, and ensure that the colour change occurs as soon as neutralisation takes place.
Litmus is _____ in acidic solutions and _____ in alkaline solutions
red, blue
Phenolphthalein is _____ in acidic solutions and _____ in alkaline solutions
colourless, pink
Methyl orange in _____ in acidic solutions and _____ in alkaline solutions
red, yellow
How do you work out the volume for a gas at room temperature and pressure using moles?
volume (dm^3) = moles * 24
Calculate the volume of 6 g of helium (Ar = 4) at room temp. and pressure
moles = 6/4 = 1.5
1.5 * 24 =
36 dm^3
What is the atom economy of a reaction?
A measure of the proportion of starting materials that end up as useful products
2H2O2 → 2H2O + O2
Calculate the atom economy when forming oxygen from hydrogen peroxide. Give your answer to 3.s.f.
2H2O2 = 68
O2 = 32
32/68 * 100 = 47.1%
What are the problems with low atom economy?
There will be more waste products which will have to be disposed of
It will be less sustainable as it will use more resources
Identify 3 other factors, besides atom economy, that should be considered when choosing between two different methods of producing a useful product.
Percentage yield
Cost of raw materials
Position of equilibrium
Rate of reaction
Cost of maintaining the right conditions
Environmental impact of waste products
What is the percentage yield of a reaction?
The actual yield of a reaction compared to the maximum theoretical yield
Suggest 3 reasons why the percentage yield of a chemical reaction may be less than 100%.
The reactants may not all react (e.g. because the reaction is very slow or reaches equilibrium)
There may be side reactions, meaning that other products are produced instead
Some of the products may be lost (e.g. gases may float off, or some solids may be left on the filter paper)
