The rate and extent of chemical change Flashcards
How do you find the mean rate of reaction?
- amount of reactant used or product formed divided by time taken
How do you measure the rate of reaction from the amounts of reactants used?
Measuring the amounts of reactants used:
* If one of the products is a gas, measure the mass in grams (g) of the reaction before and after the reaction takes place and the time it takes for the reaction to happen.
* The mass of the mixture will decrease.
* The units for the rate of reaction may be given as g/s.
Higher tier:
* The amount of the reactant can also be measured in moles (mol).
* As the reaction takes place the reactant is used up, so the amount of reactant remaining decreases.
* The concentration of the reactant is calculated as the amount (mol) divided by the volume of the reaction mixture (dm^3). It is measured in mol/dm^3.
How do you measure the rate of reaction from the amount of product formed?
Measuring the amount of product formed
* If one of the products is a gas, measure the total volume of gas produced in cubic centimetres (cm^3)
* The units for rate of reaction may be given as cm^3/s.
How do you measure the rate of reaction from the time it takes for a reaction mixture to become opaque or change colour?
Measuring the time it takes for a reaction mixture to become opaque or change colour:
* Time how long it takes for the mixture to change colour.
* Rate of reaction approximates to 1 divided by the time taken for solution to change colour.
What is collision theory?
- Chemical reactions only occur when reacting particles collide with each other with sufficient energy.
- The minimum amount of energy required to cause a reaction is called the activation energy. There are 4 important factors that affect the rate of reaction: temperature, concentration, surface area and catalysts.
Temperature:
* In a hot mixture the particles move more quickly – they collide more often and with greater energy, so collisions are more successful.
Concentration:
* At higher concentrations, the particles are crowded closer together – they collide more often, so there are more successful collisions.
* Increasing the pressure of reacting gases also increases the frequency of collisions.
Surface area
* Small pieces of a solid reactant have a large surface area in relation to their volume.
* More particles are exposed and available to collisions, so there are more collisions and a faster reaction.
How do you investigate how changes in concentration affect the rates of reactions by methods involving the production of gas or a colour change?
This investigation uses the reaction between sodium thiosulfate and hydrochloric acid.
Sample method:
1. Set up the equipment as shown.
2. Add the hydrochloric acid to the flask and swirl to mix the reactants.
3. Start the timer.
4. Watch the cross through the flask.
5. When the cross is no longer to visible to stop the timer.
6. Repeat the experiment using hydrochloric acid of a different concentration.
Variables
* The independent variable is concentration of the acid.
* The dependent variable is the time it takes for the cross to ‘disappear’.
* The control variables are the volume of acid and the concentration and volume of sodium thiosulfate.
Hazards and risks
* Corrosive acid can damage eyes, so eye protection must be used.
* Sulfur dioxide gas can trigger an asthma attack, so the temperature must always be kept below 50 degrees celsius.
What are the 3 key things to remember when plotting graphs to show the progress of a chemical reaction?
- The steeper the line, the faster the reaction.
- When one of the reactants is used up, the reaction stops (the line becomes horizontal).
- The same amount of product is formed from the same amount of reactants, regardless of rate.
How do you find the rate of reaction at a particular time?
- Draw a tangent to the curve at that time.
- Find the gradient of the tangent.
- The gradient is equal to the rate of reaction at that time.
What is a catalyst?
- A catalyst is a substance that increases the rate of a chemical reaction without being used up in the process.
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Catalysts are not included in the chemical equation for the reaction.
A catalyst: - reduces the amount of energy needed for a successful collision.
- makes more collisions more successful.
- speeds up the reaction.
- provides a surface for the molecules to attach to, which increases their chances of bumping into each other.
What is a catalyst?
- A catalyst is a substance that increases the rate of a chemical reaction without being used up in the process.
- Catalysts are not included in the chemical equation for the reaction.
- Increasing the rate of chemical reactions is important in industry, because it helps to reducee costs.
A catalyst:
* reduces the amount of energy needed for a successful collision.
* makes more collisions more successful.
* speeds up the reaction.
* provides a surface for the molecules to attach to, which increases their chances of bumping into each other.
Different reactions need different catalysts, e.g.
* the cracking of hydrocarbons uses broken pottery.
* the manufacture of ammonia uses iron.
What are enzymes?
- Enzymes act as biological catalysts.
What is a reversible reaction?
- Some chemical reactions are reversible, they can go forward or backwards.
- In a reversible reaction, the products can react to produce the original reactants.
- The direction of reversible reactions can be changed by changing the conditions.
What is a closed system?
- In a closed system, no reactants are added and no products are removed.
- When a reversible reaction occurs in a closed system, an equilibrium is achieved when the rate of the forward reaction is equal to the rate of the backward reaction.
- The relative amounts of all the reacting substances at equilibrium depend on the conditions of the reaction.
What is Le Chatelier’s Principle?
- Le Chatelier’s Principle states that if a system in equilibrium is subjected to a change in conditions, then the system shiifts to resist the change.
What happens to the yield if the temperature is increased/decreased in an exothermic reaction?
In an exothermic reaction:
* If the temperature is raised, the yield decreases.
* If the temperature is lowered, the yield increases.
