C6 (rate and extent of chemical change) Flashcards
how do you tell the rate of a reaction from a graph?
- graph has ‘time’ on x axis and ‘quantity of product formed’, usually measured in grams, on y axis. if the product being formed is a gas, it will be measured in cm^3.
- the steeper the slope, the faster the reaction.
- if the line has plateaued, the reaction has stopped. all of the reactant molecules have already reacted.
- initially, most reaction graphs will have a steep slope, therefore an increased rate of reaction. this is because there is a large number of reactant molecules, so lots of them are reacting and forming the product.
what is another method of representing the rate of a reaction on a graph?
- having ‘quantity of reactant used’ on the y-axis, and ‘time’ on the x-axis.
- initially steep downward slope, as all the reactant molecules are ready to be reacted, then it gradually slopes, as the number of reactant molecules gradually decreases, then the reaction stops, with the line plateauing on the x-axis.
how do you calculate the mean rate of a reaction?
mean rate of reaction (g/s) = quantity of product formed / time taken
or
mean rate of reaction (g/s) = quantity of reactant used / time taken
how do you measure the rate of reaction at a specific point on a rate of reaction graph?
- the slope of the curve is constantly changing, as the rate of reaction is also constantly changing.
- draw a straight line which just touches the curve at the point we’re interested in. this is a tangent.
- the gradient of the tangent gives us an idea of the rate of reaction.
- use the tangent to construct a right-angle triangle, where the right-angle is beneath the line on the graph.
- find the lengths of the sides of the triangle. to find the rate of reaction, do rise/run.
what is collision theory?
chemical reactions can only take place when the reacting particles collide with each other. the collisions must have a minimum amount of energy (activation energy).
- if particles don’t collide with enough energy, nothing will happen, they’ll just bounce apart again.
- the rate of a chemical reaction is determined by the frequency of successful collisions.
an increased rate of reaction is fundamentally based on two factors:
- the amount of energy particles have - more energy, more energy transferred during the collision, more likely to reach activation energy.
- the frequency of collisions - not all collisions have to be successful (might not reach activation energy). having collisions more often however leads to a higher chance of more successful collisions.
how does increasing the concentration of reactant molecules increase the rate of reaction?
- increasing concentration of reactant molecules means that there are more particles in the same volume.
- therefore, more collisions will occur more frequently, and the reaction will be faster.
- concentration of reactant particles is directly proportional to the rate of reaction.
- increasing the concentration of reactant molecules results in a higher rate of reaction and more product at the end, as you started with more reactant molecules.
how does the pressure inside a container affect the rate of reaction?
increasing the pressure means that there are more collisions between particles as there are more particles per unit area.
- same principle as increasing concentration of particles, but that concept is based in water, and this is the gas version.
how does increasing the temperature affect the rate of reaction?
- increasing the temperature gives particles more kinetic energy, resulting in faster movement and more frequent collisions and higher energy collisions. therefore it increases the number of particles with the required ‘activation energy’.
how does a higher surface area increase the rate of reaction?
- a substance with a high surface area: volume ratio will have a much higher area over which collisions with other reactants can take place.
- the frequency of collisions will be higher, leading to a higher rate of reaction.
how does the presence of a catalyst in the reaction increase its rate of reaction?
- catalyst: substance that speeds up a reaction without being used up in the reaction itself (don’t include in reaction equation).
- allow us to carry out reactions quickly without needing to increase the temperature, saving money.
reduces the ‘activation energy’ required by producing an alternative pathway. this increases the rate as more particles will have enough energy to react. means there will be a higher proportion of successful collisions.
what do arrows that point both ways mean in a reaction equation?
- arrows that point one way means that the reactants come together to form the product, but the product cannot split back up and form the reactants again (or vice versa)
- arrows that point both ways in a chemical reaction show that the reactants can form the product, and the product can also react to reform the original reactants.
- this a reversible reaction.
- you can change the direction of a reversible reaction by changing the conditions (e.g. heating it up or cooling it down).
use the reversible reaction of:
hydrated copper sulfate = anhydrous copper sulfate + water
to describe endothermic and exothermic directions in reversible reactions.
- heating hydrated copper sulfate forms anhydrous copper sulfate and water. as we’re heating this, we’re putting energy in, meaning that the forward reaction is endothermic.
- taking anhydrous copper sulfate and adding water back into it, the reaction reverses. in this reaction, energy is released, and it gets hot. the reverse reaction is exothermic.
what are the key facts about reversible reactions?
- if it’s endothermic in one direction, it’s exothermic in the other.
- the same amount of energy is transferred in each case.
when is equilibrium reached in a reversible reaction?
- at the beginning, the two reactions will take place at different rates.
- equilibrium can only be reached in a closed system - a sealed environment. no reactants or products can escape.
- the forward reaction maybe really fast, lots of reactants, no products, however, this means that backward reaction won’t yet be able to start.
- after a while, rates even out, forward and backward reactions will go at same speed. concentrations of reactants and products won’t change anymore - they cancel each other out. equilibrium has been achieved.
- doesn’t necessarily mean that numbers of reactants and products are same. as long as forwards and backwards rates are same, the numbers will be CONSTANT, not the same.
