C6 (rate and extent of chemical change)

Question 1

what is the rate of reaction?

Answer 1

how quickly the reactants turn into the products.

Question 2

what units could quantity of product created / quantity of reactant used be measured in?

Answer 2

• mols
• grams
• cm^3
• dm^3

Question 3

how do you tell the rate of a reaction from a graph?

Answer 3

• 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.

Question 4

what is another method of representing the rate of a reaction on a graph?

Answer 4

• 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.

Question 5

what are the two ways you can calculate the mean rate of reaction for a gas?

Answer 5

• measure how the mass of the reaction changes
• measure the volume of gas given off by the reaction

Question 6

how do you calculate the volume of gas produced?

Answer 6

the reaction mixture is connected to a gas syringe or an upside down measuring cyclinder. as the reaction proceeds, the gas is collected.

rate = volume of gas produced / time taken

Question 7

how do you calculate the change in mass of a reaction?

(GAS)

Answer 7

the reaction mixture is placed on a mass balance. as the reaction proceeds and the gaseous product is given off, the mass of the flask will decrease.

rate = change in mass / time taken

Question 8

how do you calculate the mean rate of a reaction?

Answer 8

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

Question 9

how do you measure the rate of reaction at a specific point on a rate of reaction graph?

Answer 9

• 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.

Question 10

what is collision theory?

Answer 10

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.

Question 11

an increased rate of reaction is fundamentally based on two factors:

Answer 11

• 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.

Question 12

what does the rate of reaction at any particular point depend on?

Answer 12

1. amount of energy the particles have (the more they have, the more they can transfer during collision, meaning they’re more likely to pass the minimum activation energy)
2. frequency of the collisions (even though not all collisions are successful)

Question 13

how does increasing the concentration of reactant molecules increase the rate of reaction?

Answer 13

• 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.

Question 14

how does the pressure inside a container affect the rate of reaction?

Answer 14

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.

Question 15

how does increasing the temperature affect the rate of reaction?

Answer 15

• 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’. higher rate of successful collisions and therefore reaction

Question 16

how does a higher surface area increase the rate of reaction?

Answer 16

• 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.

Question 17

how does the presence of a catalyst in the reaction increase its rate of reaction?

Answer 17

• 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.

Question 18

what is a common catalyst?

Answer 18

transition metals, e.g. cobalt, nickel

Question 19

what do arrows that point both ways mean in a reaction equation?

Answer 19

• 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).

Question 20

use the reversible reaction of:

hydrated copper sulfate = anhydrous copper sulfate + water

to describe endothermic and exothermic directions in reversible reactions.

Answer 20

• 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.

Question 21

what are the key facts about reversible reactions?

Answer 21

• if it’s endothermic in one direction, it’s exothermic in the other.
• the same amount of energy is transferred in each case.

Question 22

when is equilibrium reached in a reversible reaction?

Answer 22

• 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.

Question 23

what is the position of the equilibrium?

Answer 23

if there are more products than reactants, we would say the equilibrium lies to the right (the side of the products on the reaction equation). vice versa in the case of reactants.

• this can change depending on the conditions, e.g. heat. higher heat means forward reaction is quicker, so position of equilibrium is on the right.

Question 24

what is Le Chatelier’s principle?

Answer 24

if you change the conditions of a reversible reaction, the position of equilibrium will shift to try and counteract that change.

Question 25

describe Le Chatelier’s principle using the equation:

N2 + 3H2 = 2NH3 (-92 kJ/MOL)

how would changing the temperature affect the position of equilibrium (assume it starts in the middle).

Answer 25

• (-92 kJ/MOL) is the overall energy change of the forward reaction. as it’s negative, we can tell that the forward reaction is exothermic, as it releases energy to the surroundings. therefore the backward reaction is endothermic.
• decreasing the temperature means the equilibrium would move in the exothermic direction, in order to release heat energy and counteract the change we just made. this means the position would move to the right, so we’d have more ammonia particles, and fewer nitrogen and hydrogen particles. and vice versa.

Question 26

describe Le Chatelier’s principle using the equation:

N2 + 3H2 = 2NH3 (-92 kJ/MOL)

how would changing the pressure affect the position of equilibrium (assume it starts in the middle).

Answer 26

• increasing the pressure means that the position would move to whichever side reduces the pressure back down.
• to lower the pressure, the equilibrium will move to whichever side has the least number of molecules. in this case, it would move to the right, as ammonia only has two molecules, and nitrogen and hydrogen has four. and vice versa.

Question 27

describe Le Chatelier’s principle using the equation:

N2 + 3H2 = 2NH3 (-92 kJ/MOL)

how would changing the concentration of nitrogen affect the position of equilibrium (assume it starts in the middle).

Answer 27

• adding more nitrogen to the container would increase the concentration of nitrogen.
• to oppose this change, the equilibrium shifts to the opposite side, in this case, to the right, and forms more ammonia.