Chemistry C2.4 : Rates of reaction

What does the rate of reaction tell us?

How fast the reactants turn into products

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Why is it important in the industry that the reaction rate is fast?

If a product takes too long to produce, it will be hard to make a profit when it is sold, the rate of reaction must be fast enough to make the product quickly and safely. Any industrial process has to make money by producing useful products cheaply.

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What are the two ways we can find out the rate of reaction?

-measure how quickly the reactants are used up -find out how quickly the products of the reaction are made

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How can we measure rate of reaction?

Measure decreasing mass of a reaction mixture/Measuring the increasing volume of a gas given off/Measuring the deceasing light passing through a solution as some reactions in solution make an insoluble solid making a cloudy solution.

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What is the equation for working out the rate of reaction?

Rate of reaction = amount of reactant used or amount of product formed / time

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On a graph showing rate of reaction what does a steeper gradient represent?

The steeper the gradient the faster the rate of reaction.

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What are the four main factors that affect the rate of reaction?

-temperature -surface area -concentration of solutions or pressure of gases -presence of a catalyst

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What is the collision theory?

Reactions can only take place when particles of reactants come together, but reacting particles don't just have to bump into each other, they need enough energy to react when they collide

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What is activation energy?

The smallest amount of energy that particles must have before they can react, this is called the activation energy.

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Reactions are more likely to happen between reactant particles if we...

- increase the chance of reacting particles colliding with each other -increase the energy they have when they collide

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Why does an object with a smaller surface area and a large lump of solid have a slow reaction rate?

-The particles inside a large lump of solid are not in contact with the solution, so they can't react, the particles inside the solid have to wait for the particles on the surface to react first, decreasing the rate of reaction for the solid.

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Why does a solid with a larger surface area, in smaller lumps or in a powder have a faster reaction rate?

In smaller lumps or in a powder, each tiny piece of solid is surrounded by solution. More particles are exposed to the solution to attack, meaning that reactions can take place much more quickly.

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How does surface area affect the rate of reaction?

Only some collisions lead to reactions, increasing the number of collisions in a certain time and the energy of collisions produces faster rates. Larger surface area increases the frequency of collisions and does not mean collisions with more energy

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How does temperature affect rate of reaction?

When we increase the temperature, it always increases the rate of reaction

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How does a fridge stop food going off?

Fridges and freezers reduce the temperature and slow down the rate of reaction, when food goes off it is because of chemical reactions, reducing the temperature, reducing the temperature slows down these reactions.

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Why does increasing the temperature increase the rate of reaction?

-increase in particle collisions -particles collide with more energy

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How does an increase in particle collisions increase the rate of reaction?

When we heat up a substance, the particles gain kinetic energy, in liquids and gases, the particles move faster, resulting in more frequent successful collisions, increasing reaction rate

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How does an increase in particles colliding increase rate of reaction?

If there are more frequent collisions, there is a greater chance of successful collision and therefore a greater chance of a reaction between the particles

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How does increasing temperature result in an increase in the rate of reaction in term of the energy of the particles?

By increasing temperature, a higher proportion of the collisions have more energy greater than activation energy, when particles have more energy, it has a greater affect on reaction rate than the number of collisions.

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Summarise how increasing temperature can increase rate of reaction?

increased temperature = increased rate of reaction = particles moving energetically / more frequent collisions. More collisions result in a reaction as high proportion of particles have energy greater than activation energy.

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What is the effect of an increased concentration in a liquid?

Increasing concentration of reactants in a solution increases the rate of reaction, there are more particles of the reactants moving around the same volume of the solution.Particles are more likely to collide, more frequent collisions=faster reaction

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What is the effect of an increased pressure in a gas?

Increasing pressure of a gas, increases the particles in a given space. This increases the chance they will collide, increasing the pressure speeds up the rate of reaction.

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Summarise what is the effect of increasing the concentration and pressure?

Increasing concentration and pressure both mean that particles are closer together. This increases the frequency of collisions between particles, so the reaction rate increases.

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Does increasing concentration or volume increase particle energy?

No, by increasing concentration or pressure we increase the frequency of successful collisions, but does not increase the energy at which the particles collide.

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How can we investigate the effect of concentration on rate of reaction?

Changing the concentration by reacting marble chips with different concentrations of hydrochloric acid, you would measure the decrease in mass as carbon dioxide is given off in the reaction.

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What is a catalyst?

A catalyst is a substance which increases the rate of a reaction, it is not changed chemically itself at the end of the reaction, it is not used up and can be used over and over again.

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What are the usual forms of a catalyst and why?

Normally we use a catalyst in the form of powders, pallets or fine gauzes, this gives them the biggest possible surface area.

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What is an advantage of using a catalyst?

A catalyst speeds up a reaction, but does not get used up in the reaction. We can use a tiny amount of catalyst to speed up a reaction over and over again.

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How can we investigate the effect of a catalyst?

We can look at hydrogen peroxide solution decomposing, the reaction produces oxygen gas that can be collected in a gas syringe. We can measure the effect of many different substances on the rate of reaction.

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How can we investigate the effect of surface area?

By measuring the mass against time for different sizes of marble calcium carbonate chips.

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How can we measure the effect of temperature?

When we react sodium thiosulfate solution and hydrochloric acid it makes sulfur, which is insoluble in water, making a cloudy solution. We can record the length of time it takes for the solution to go cloudy at different temperatures.

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What is a disadvantage of using a catalyst?

Catalysts are expensive and are usually expensive metals like gold, platinum and palladium.

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Why would we use a catalyst instead of not using one?

Using a catalyst is cheaper than paying for the extra energy needed without one, to get the same rate of reaction without a catalyst would require higher temperatures and or/pressures.

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Why are catalysts good for the environment?

Without a catalyst we would have to use high temperatures and pressures that involve burning fossil fuels, so operating at lower temperatures and pressures conserves these non-renewable resources and reduces excess CO2 released into the atmosphere

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Why are some catalysts not good for the environment?

Many catalysts are transition metals or their compounds, these are often toxic, if they escape into the environment, they build up inside living things that eventually poison them eg. platinum/palladium in catalytic converters escape from exhaust

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How can we use nanoparticles for catalysts?

Nanoparticles are being used to develop new catalysts, scientists can arrange atoms into the best shapes for catalysing a particular reaction they have studied, a small mass of these catalysts has a huge surface area

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How can catalysts be used in medicine?

Before catalysts used in making new drugs contained precious metal compounds, bonded in an organic molecule. Now chemists can make these catalysts without the metal, making cheaper catalysts and taking away the risk of contaminating the drug.

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What are biological washing powders and how do they relate to catalysts?

Biological washing powders contain enzymes that help break apart stain molecules such as proteins at low temperatures, as enzymes are catalysts found in living things. That save energy as they do not need high temperatures

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Why is energy always involved in a chemical reaction?

Energy is always transferred as chemical bonds are broken and new ones are made.

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What is an exothermic reaction?

A reaction that transfers energy from the reacting chemicals to their surroundings. The energy transferred often heats the surroundings, meaning we can measure the rise in temperature as the reaction happens.

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What is an endothermic reaction?

A reaction that transfers energy from the surroundings to the reacting chemicals, as they take energy from their surroundings, these reactions cause a drop in temperature as they happen.

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What are some examples of exothermic reactions?

Fuels burning eg. when methane burns and is oxidised / respiration / Neutralisation reaction between acids and alkalis

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What are some examples of endothermic reactions?

Thermal decomposition of calcium carbonate, when heated it forms calcium oxide and carbon dioxide and only occurs if we keep heating the calcium carbonate strongly. / photosynthesis - allows plants to use sunlight to produce food and energy

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Can energy be created or destroyed?

No, energy cannot be created or destroyed in a chemical reaction

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What happens to energy in a reversible reaction?

The amount of energy released when we go in one direction in a reversible reaction must be exactly the same as the energy absorbed when we go in the opposite direction.

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Give an example of a reversible reaction?

When we heat hydrated blue copper sulfate crystals

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Describe the endothermic reaction in hydrated copper sulfate?

The crystals contain water as part of the lattice formed when the copper sulfate crystallised. It is hydrated, heating the copper sulfate drives off the water from the crystals producing white anhydrous copper sulfate,this is an endothermic reaction

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What does anhydrous mean?

without water

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Describe the exothermic reaction in anhydrous copper sulfate?

When we add water to anhydrous copper sulfate(white) we form hydrated copper sulfate, the substance turns blue and is exothermic. So much energy is produced we may see steam rising as the water boils.

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What happens when we soak filter paper in cobalt chloride solution?

When we soak filter paper in cobalt chloride solution and allow it to dry in an oven, the blue paper produced is cobalt chloride paper, the paper tuns pale pink when water is added to the paper.

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How can be exothermic reactions be used as body warmers?

Exothermic reactions release heat energy so they can be used as hand warmers.

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Give an example of a disposable hand warmer and the products used in the reaction?

The oxidation of iron to release energy, iron tuns into hydrated iron oxide in an exothermic reaction, where sodium chloride is used as a catalyst. This hand warmer is disposable and can be used only once but it lasts for hours.

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How are exothermic reaction used in self-heating cans?

calcium oxide + water --> calcium hydroxide. When you press the button on the can the seal breaks and lets calcium oxide an water mix , coffee is available in these self-heating cans.

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How can the endothermic reaction of ammonium nitrate be used as a cool pack?

When ammonium nitrate dissolves it takes in energy from its surroundings, making them colder. They are used as a treatment for sports injuries.

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What is a disadvantage of using ammonium nitrate and water as a cool pack?

It can only be used once, as once the water and ammonium nitrate have reacted, the reaction can not occur again.

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What is used in reusable warmers?

The crystallisation of a supersaturated solution is used in reusable warmers.

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Chemistry C2.4 : Rates of reaction

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