Chemistry 2b Flashcards
What is one of the slowest reactions?
The rusting of iron
What is an example of a moderate speed reaction?
A metal (like magnesium) reacting with acid to produce a gentle stream of bubbles
What is a really fast reaction?
An explosion where it is all over in a fraction of a second
What are the four things that the rate of a reaction depends on?
Temperature, concentration (or pressure for gasses), catalyst and surface area of solids (or size of solid pieces)
What would a graph for a slow reaction look like?
The line wouldn’t be very steep
What happens to graphs as the rate of reaction quickens?
The slope of the graphs gets steeper. The steepest and longest line would mean that there were more reactants as well
What can cause an increased rate if reaction?
Increase in temperature, Increase in concentration/pressure, if a catalyst is added or by a solid reactant being crushed up into smaller bits
What are the two different things you can measure to work out the rate of reaction?
How quickly the reactants are used up or how quickly the products are formed
What is the calculation to work out the rate of reaction?
Rate of reaction = Amount of reactant used or amount of product formed DIVIDED BY the time
How can precipitation be used to measure the rate of a reaction?
This is when the product is a precipitate and clouds the solution, you watch a mark through the solution (X on paper under the glass) and measure how long it takes to disappear. The quicker it disappears, the quicker the rate of reaction. Only works when the initial substance is see-through.
What is the issue with working out the rate of reaction by timing the precipitation?
The result is subjective-Different people would have different thoughts as to when the mark has disappeared
How can a change in mass (usually gas given off) be used as a way to measure the rate of reaction?
This is used when the reaction produces a gas. Use a mass balance. Weighs less as the gas leaves. The quicker the reading on the balance drops, the quicker the rate of reaction. Then a rate of reaction graph is used to plot the results from this method
Why is measuring the change in mass a very accurate way of testing the rate of reaction?
Because a mass balance is very accurate and you can easily see how much it has changed. Fact not oppinion
What is the disadvantage of measuring the change in mass for rate of reaction?
The gas is released straight into the room which can be dangerous, depending on the substance
How can the volume of gas being given off be used as a way to measure weight of reaction?
Use a gas syringe to measure the volume of gas given off. The more gas given off in a certain time interval, the faster the reaction. A graph of gas volume against time elapsed can be plotted to show the rate of reaction.
What are the good and bad points of measuring rate of reaction by the volume of gas given off?
Gas syringes give volumes accurate to the nearest millimetre usually so they are quite accurate however if the reaction is too vigorous, you can break the syringe
What is the reaction of hydrochloric acid and marble chips often used for?
To demonstrate the effect of breaking a solid into smaller bits
How do you complete the experiment of hydrochloric acid and marble chips?
Measure the volume of gas evolved with a gas syringe and take readings at regular intervals then make a table of readings and plot them as a graph. Repeat the experiment with the same volume of acid and exactly the same mass of marble chips but crunched up more and then repeat again but with the same mass of powdered chalk instead of marble chips
Why does the size of marble chip affect the rate of reaction?
Finer particles means the marble has a larger surface area which causes more frequent collisions so the rate of reaction is faster
What is the reaction of magnesium metal with dilute hydrochloric acid usually used to measure?
The effects of increased concentration ( same as the marble/acid reaction)
How do we use the magnesium metal/dilute HCl reaction to measure rate of reaction?
The reaction gives off hydrogen gas which can be measured with a mass balance and then repeat with more concentrated acid solutions but the same amount of magnesium and the same volume of acid. It can then be plotted as a graph to make the results easier to analyse
In a graph for the magnesium/HCl experiment, what would a steeper graph mean?
A higher concentration - a faster rate of reaction
What is the experiment for sodium thiosulfate and HCl?
The chemicals are both clear solutions, together they react to form a yellow precipitate (sulfur). Time how long it takes for a black mark under the glass to disappear. Can be repeated at different temperatures but can be hard to do accurately and safely. Best way is to use a water bath first. Keep depth of liquid the same. This reaction can also test the effects of concentration. Don’t get a graph, just results
What do the results for this experiment show?
The higher the temperature, the quicker the reaction and therefore the less time it takes for the mark to disappear
What is the decomposition of hydrogen peroxide usually used to measure?
It is usually used to show the effect of different catalysts. It can also be used to measure the effect of temperature or of concentration of the H2O2 solution
What is the equation for the decomposition of hydrogen peroxide?
2H2O2(aq) -> 22H2O(l) + O2(g)
The decomposition is often quite slow, how can it be sped up?
By adding a sprinkle of manganese(IV) oxide catalyst
Other catalysts which work are found in where?
Potato peel, and blood
How can the decomposition of hydrogen peroxide be measured?
Oxygen gas is given off which provides an ideal way to measure the rate of reaction using the gas syringe method.
What would the results of this method show?
Better catalysts give a quicker reaction which is shown by a steeper graph that levels off quickly
What do more collisions mean?
More collisions increases the rate of reaction
What are collisions?
How often the reacting particles collide successfully
How does a higher temperature increase collisions?
A higher temperature means the particles move around quicker, meaning they’re going collide more often
How does a higher concentration (or pressure) increase collisions?
A higher concentration means that there are more particles that can knock into each other and a higher pressure means that there are more particles squashed closer together so they will collide more often
How does a larger surface area increase collision?
Because the particles have more area to work on so there will be more collisions
What else does a higher temperature cause in collision theory?
Increasing the temperature causes faster collisions
What needs to happen for a reaction to take place?
The particles need to collide with enough energy, the minimum amount of energy needed is known as the activation energy. A higher temperature means that there will be more particles colliding with enough energy to make the reaction happen.
What is a catalyst?
A catalyst is a substance which speeds up a reaction, without being changed or used up in the reaction
How does a solid catalyst work?
By giving the reacting particles a surface to stick to. This increases the number of successful collisions and so speeds up the reaction
What do catalysts help to reduce?
Costs in industrial reactions
How do catalysts save money?
Because it means the plant doesn’t need to operate for as long to produce the same amount of stuff
How are catalysts good for sustainable development?
They mean that the reactions can take place at lower temperature. This reduces the energy used up in the reaction (energy cost)
What are the disadvantages of catalysts?
They are expensive to buy and often need to removed and cleaned, different use different catalysts so if you make more than one product you probably need more than one catalyst. Also they can be poisoned by impurities and stop working so you have to keep your reaction mixture clean
What is another advantage of catalysts?
They never run out so usually when you buy one, you don’t have to buy another one
What is an exothermic reaction?
A reaction that gives out heat
What is the full definition of an exothermic reaction?
An exothermic reaction is one which transfers energy to the surroundings, usually in the form of heat and usually shown by a rise in temperature
What is the best example of an exothermic reaction?
Burning fuels, also known as combustion, it gives out a lot of heat
What are other examples of exothermic reactions?
Neutralisation (acid+alkali) reactions. Also many oxidation reaction, eg sodium + water makes heat.
What are some everyday uses of exothermic reactions?
Hand warmers use the oxidation of iron in air to generate heat. Self heating cans of hot chocolate and coffee also rely on exothermic reactions between chemicals in their bases
What happens in an endothermic reaction?
Heat is taken out
What is the full definition of an endothermic reaction?
AN endothermic reaction is one which takes in energy from the surroundings, usually in the form of heat and is usually shown by a fall in temperature
Are endothermic reactions more or less common than exothermic reactions?
They are much less common
What is a good example of an ednothermic reaction?
Thermal decompositions. E.G CaCO3->CaO+CO2 Heat must be applied to make calcium carbonate decompose to make quicklime
What is an everyday use of an endothermic reaction?
Some sports injury packs use endothermic reactions-they take in heat and the pack becomes very cold, they are mire convenient than carrying ice around
What sort of reactions can reversible reactions be?
They can be endothermic and exothermic
What happens with a reversible reaction is it is endothermic in one direction?
It will be exothermic in the other direction. The energy absorbed by the endothermic reaction is equal to the energy released by the exothermic reaction
What is a good example of a reversible reaction?
The thermal decomposition of hydrated copper sulfate.
Hydrated copper sulgfat anhydrous copper sulfate + water
How do you complete the thermal composition of hydrated copper sulfate?
You heat the blue hydrated copper sulfate crystals and the water is driven off, leaving white anhydrous copper sulfate powder (endothermic). If you add a couple of drops of water to the white powder, the blue crystals come back again (exothermic)