TOPIC 7 - Rates Of Reaction And Energy Processes
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Rate of reaction
A measurement of how fast the reaction happens.
How can rate of reaction be observed?
By measuring how quickly the reactants are used up/how quickly the products are formed.
Rate of reaction formula
Amount of reaction used up or product formed/ Time
How do you measure precipitation?
• mix 2 reactant solutions and put flask on paper that has a mark on it. • Observe the mark through the mixture and measure how long it takes for mark to be obscured. Faster disappears= faster reaction.
Why is the observation of precipitation subjective?
As different people may not agree on exactly when mark disappears
How do you measure change in mass in a reaction?
• Measure solution using a mass balance . Quicker the solution drops in mass = faster rate of reaction • Finished when mass stops changing
Safety precautions when measuring change in mass of a reaction.
Can put in fume cupboard
How do you observe the volume of gas given off?
Use a gas syringe, more gas given off during a set time interval= faster rate of reaction Is finished when no more gas is produced
Safety precautions when observing volume of gas given off?
Use right size gas syringe
How do you measure how surface area affects rate?
Use gas syringe, stopwatch, marble chips and hydrochloric acid. Measure gas given off at set time intervals and record in table Repeat w same volume/concentration of acid as well as same marble mass but w more crushed marble. Repeat w powder
Reaction to test for temp’s effect on reaction rate
Sodium thiosulfate and hydrochloric acid are heated in a water bath before mixing and placing over black mark. Time how long it takes to disappear. (Turns yellow) Can be repeated at different temps. Quicker disappears= faster rate
Gradient
Change in y / change in x
What does the rate of a chemical reaction depend on?
Collision frequency (more successful= faster reaction ) Energy transferred during a collision (need to pass activation energy)
3 factors leading to an increased rate of reaction
TEMPERATURE (+temp = +rate) CONCENTRATION (+concentration = +rate) SURFACE AREA (+surface area = +rate)
Why does increasing temp increase rate of reaction?
Particles move faster so in turn, have more collisions. Higher temp= increased energy of collisions
Why does increasing concentration increase rate?
If a solution is more concentrated, there’s more particles of a reactant in same volume so collisions are more likely. In a gas, increased pressure crowds particles so collision frequency will increase
Why does increasing surface area contribute to a higher rate of reaction?
Increased surface area to volume ratio means the reactants have more area to work on/ collide w.
Catalyst
A substance which increases rate of reaction while remaining chemically unchanged.
How do catalysts work?
They decrease activation energy by providing an alternative reaction pathway w a lower activation energy.
Enzymes
Biological catalysts which catalyse chemical reactions in living cells. Eg Respiration, photosynthesis, protein synthesis
Exothermic reaction
A reaction which gives out energy to the surroundings, usually in form of heat and is shown by a rise in temp of surroundings
Endothermic reaction
One which takes in energy from surroundings, usually in form of heat. Is shown by fall in temp of surroundings
Reaction profile of an exothermic reaction
Products are at a lower energy than reactants
Reaction profile of an endothermic reaction
Products are at higher energy than reactants