C6 Rate of and Extent of Chemical Change Flashcards
what is the rate of a chemical reaction?
how fast the reactants are changed into products
what is an example of a slow reaction?
the rusting of iron
chemical weathering (like acid rain damaging limestone buildings)
what is an example of a moderate speed reaction?
magnesium reacting with an acid to produce a gentle steam of bubbles
what is an example of a fast reaction?
burning
what is an example of a really fast reaction?
explosions, they release a lot of gas and are over in a fraction of a second
the …………. the line on the graph, the …………… the rate of reaction.
the …steeper………. the line on the graph, the …faster………… the rate of reaction.
why does the graph become less steep over time?
because the reactants are used up
what does a graph of a quick reaction look like?
steep lines and flat really quickly
the rate of a chemical reaction depends on:
the collision frequency of reacting particles (how often they collide). the more collisions there are the faster the reaction is eg doubling the frequency off collisions doubles
the energy transferred during a collision. particles have to collide with enough energy for the collision to be successful.
what is a successful collision?
a collision that ends in the particles reacting to form products
what is the activation energy?
the minimum amount of energy that particles need to react.
what do particles need to do to start the reaction?
particles need enough energy to break the bonds in the reactants and start the reaction.
factors that increase the number of collisions or the amount of energy particles need to collide with will…………
increase the rate of reaction
what are factors that increases the rate of reaction?
1) temperature
2) the concentration of a solution or pressure of a gas
3) surface area
4) the presence of a catalysts
how does increasing the temperature increase the rate of reaction?
1) when the temperature is increased the particles all move faster
2) if they move faster they collide more frequently
3) also the faster they move the more energy they have, so more of the collisions will have enough energy to make the reaction happen.
how does increasing the concentration or pressure increase the rate of reaction?
1) if a solution is made more concentrated, it means there are more particles knocking about in the same volume of water or other solvent
2) similarly, when the pressure of a gas is increased, it means that the same number of particles occupies a smaller space.
3) this makes collisions between the reactant particles more frequent
how does increasing the surface area increase the rate of reaction?
1) if one of the reactants is a solid then breaking it up into smaller pieces will increase its surface are to volume ratio
2) this means that for the same volume of the solid, the particles around it will have more area to work on - so there will be collisions more frequently.
how does using a catalyst increase the rate of reaction?
1) a catalyst is a substance that speeds up a reaction, without being used up in the reaction itself. this means its not part of the overall reaction equation.
2) different catalysts are need for different reactions, but they all work by decreasing the activation energy needed for the reaction to occur. they do this by providing an alternative pathway with a lower activation energy.
3) enzymes are biological catalysts - they catalyse reactions in living things
what is the equation for rate of reaction?
rate of reaction = amount of reactant used up or amount of product formed / time
what are the three ways of measuring the rate of reaction?
1) precipitation and colour change
2) change in mass
3) the volume of gas given off
how do you measure precipitation and colour change?
1) transparent initial solution and product that is a precipitate (which clouds the solution)
2) observe a mark through the solution and measure how long it takes for it to disappear - the faster the mark disappears the quicker the reaction
3) if the reactants are coloured and the products are colourless ( or vice versa) you can time how long it takes for the solution to lose (or gain) its colour
4) the results are subjective - different people might not agree over the exact point when the mark disappears or the solution changes colour. you can plot a rate of reaction graph from the results.
what is another word for cloudiness?
turbidity
how do you measure change in mass?
1) measuring the speed of a reaction that produces a gas can be carries our using a mass balance.
2) as the gas is released, the mass disappearing is measured on the balance.
3) the quicker the reading on the balance drops, the faster the reaction
4) if you take measurements at regular intervals you can plot a rate of reaction graph and find the rate quite easily.
5) this is the most accurate of the three methods because the mass balance is very accurate. but it has the disadvantage of releasing the gas straight into the room.
why is cotton wall used when measuring the change in mass?
it lets the gas escape out of the flask but stops the acid spitting out
how do you measure the volume of gas given off?
1) this involves the use of a gas syringe to measure the volume of gas given off.
2) the more gas given off during a given time interval, the faster the reaction
3) gas syringes usually give volumes accurate to the nearest cm cubed, so there quite accurate. you can take measure, at regular intervals and plot a rate of reaction graph using this method too. you have to be quite careful though - is the reaction is too vigorous, you can easily blow the plunger out of the end of the syringe
why is the mass balance method only accurate when the flask inst too hot?
because the loss in mass that you see might be partly due to evaporation of liquid as well as being due to the loss of gas formed during the reaction.
why is precipitation and colour change not very accurate?
its subjective
how do you use magnesium and HCL to produce hydrogen gas?
1) add a set volume of dilute hydrochloric acid to a chronical flask.
2)add some mg ribbon to the acid and quickly attach an empty gas syringe to the flask.
3) start the stopwatch. take readings of the volume of gas in the gas syringe at regular intervals
4) plot the results in a table
5) plot a graph with time on the x axis and volume of gas produced on the y axis
how can you investigate the effect of using different acid concentrations?
1) add a set volume of dilute hydrochloric acid to a chronical flask.
2)add some mg ribbon to the acid and quickly attach an empty gas syringe to the flask.
3) start the stopwatch. take readings of the volume of gas in the gas syringe at regular intervals
4) plot the results in a table
5) repeat with different concentrations of acid. but the amount of mg ribbon and the volume of acid should be kept the same, only change the concentration
5) plot a graph all the different concentrations on the same graph. time on the x axis and volume of gas produced on the y axis
how do you investigate a reaction that doesn’t produce a gas?
1) add a set volume of dilute sodium thiosulfate to a chronical flask.
2) place the flask on a piece of paper with a black cross drawn oin it.
3) add some dilute HCL to the flask and start the stopwatch
4) not watch the black cross disappear through the cloudy sulfur and time how long it takes to go
what should you do when reacting sodium thiosulfate and HCL?
carry it our in a well ventilated place, because sulfur dioxide is released
how do you work out the mean rate for the whole reaction?
work out the overall change in the y value and the divide this by the total time taken for the reaction
how do you find the reaction rate at a particular point
draw a tangent, then pick two points on the line that are easy to read, use them to calculate the gradient on the tangent in order to find the rate. change in y divided by change in x
what is a reversible reaction?
when the products of the reaction can react together to produce the original reactants
how do reversible reactions work?
1) as the reactants react their concentrations fall the forward reactions slows down. but as more products are made and their concentrations rise the backwqards reaction will speed up.
2) after a while the forwards reaction will be going at the exact same rate as the backward one - the system is at an equilibrium
3) at equilibrium, both reactions are still happening but there is no overall effect (its a dynamic equilibrium) this means the concentrations of reactants and products have reached a balance and wont change.
4) equilibrium is only reached if the reversible reaction takes place in a closed ssystem.
what is dynamic equilibrium?
the concentrations of recreants and products have reached a balance and wont change
what is a closed system?
non of the reactants or products can escape and nothing else can get in.
if the equilibrium lies to the right…………
the concentration of products is greater than that of the reactants
if the equilibrium lies to the left………….
the concentration of reactants is greater than that of the products
the position of the equilibrium depends on the following conditions;
temperature
the pressure (only gasses)
the concentration of the reactants and products
if the reaction is endothermic in one way………
it will be exothermic the other way
hydrated copper sulfate = anhydrous copper sulfate + water
is an endothermic reaction
anhydrous copper sulfate + water = hydrated copper sulfate
is an exothermic reaction
hydrated copper sulfate <=> anhydrous copper sulfate + water
is a reversible reaction
how does the copper sulfate experiment work (exo and endo)
if you heat blue hydrated copper sulfate crystals it drives the water off and leaves white anhydrous copper sulfate power this is endothermic
if you then add a couple of drops of water to the white powder you get the blue crystals back again this is exothermic
what is Le Chateliers principle?
if you change the conditions of a reversible reaction at an equilibrium the system will try and counteract that change.
what happens if you decrease the temp in a reversible reaction
the equilibrium will move in the exothermic direction to produce more heat. this means you’ll get fewer products for the endothermic reaction, and more products for the exothermic reaction.
what happens if you increase the temp in a reversible reaction
the equilibrium will move in the endothermic direction to try and decrease it. youll get more products for the endo reaction and less products for the exo reaction
what happens if you increase the pressure in a reversible reaction
the equilibrium tries to reduce it - it moves in the direction where there are fewer molecules of gas
what happens if you decrease the pressure in a reversible reaction
the equilibrium tries to increase it - it moves in the direction where there are more molecules of gas
what happens if you change the concentration of the reactants or products of a reversible reaction
the system will no longer be at an equilibrium, so the system responds to bring itself back to equilibrium again
what happens if you increase the concentration of the reactants?
the system tries to decrease it by making more products
what happens if you decrease the concentration of products
the system tries to increase it again by reducing the amount of reactants
