Rates of Reactions Flashcards
What is reaction rate?
How fast a reaction happens
How is reaction rate observed?
Measuring how quickly reactants are turned into products or how quickly they are used up
Formula for rate of reaction
Amount of reactant used or amount of product formed / time
Benefits of fast reactions in industry
Making the required product as cheaply as possible maximises profit
Yield more product in a given time
Drawbacks of fast reactions in industry
Expensive to generate conditions
Safety concerns
What does the collision theory explain?
Why reactions occur at different rates and how they occur
What does the collision theory state must happen for chemical reactions to occur?
Particles must collide with enough energy (the activation energy)
What does the rate of a chemical reaction depend on?
Frequency of collisions between reacting particles
Amount of energy transferred in a collision
What does increasing the rate of reaction mean increasing?
The number of collisions and / or energy of reacting particles
How can we increase the rate of reaction?
Increase temperature
Increase concentration/ pressure in gases
Increase surface area to volume ratio
How can we calculate reaction rates from a graph?
Draw tangent and work out it’s gradient
What does a smaller gradient tell us?
Slower rate of reaction
How can we observe how concentration affects reaction rate? What is seen?
Precipitation reaction
A colour change
What solutions must we use for a precipitation reaction?
2 see through solutions that mix to form a precipitate which clouds the solution
What are 2 solutions we could use in a precipitation reaction?
Sodium thiosulfate and hydrochloric acid
How is a precipitation reaction carried out?
Put a conical flask on top of a sheet of paper with an X in the centre
Pour in solution that is not acid
Pour in acid with a known concentration and press start on stop watch
Stop when you can’t see X
What should we repeat precipitation reactions with?
Different acid concentrations
What are the independent and dependent variables for precipitation reactions?
Acid concentration
Time taken for cross to disappear
How can reaction rate be measured with a mass change?
A gas is released and lost mass is measured on a mass balance
What shows a faster reaction in a mass change experiment?
Quicker the reading on the mass balance drops
How do we show surface area affecting reaction rate?
Hydrochloric acid and marble chips
Experiment to show surface area affecting reaction rate
Place a known mass of marble chips in a measuring cylinder and pour in an excess volume of acid with a known concentration
Record volume of gas given off every second using a gas syringe
What are the independent and dependent variables in the surface area experiment?
Surface area of the marble chips
Volume of gas given off each second
When had the marble chip reaction finished?
When no more gas is produced
How should the surface area experiment be repeated?
Same volume and concentration of acid and the same mass of marble chips but with more crunched up (increased surface area)
Then repeat with powdered chalk
What chemicals are used to measure how temperature affects reaction rate?
Sodium thiosulfate
Hydrochloric acid
What do sodium thiosulfate and hydrochloric acid react to form?
Yellow precipitate of sulfur
In the experiment to measure how temperature affects reaction rate, what is the measure of this reaction?
The time taken for the precipitate to form
Steps for temperature experiment
Measure out fixed volumes of sodium thiosulfate and hydrochloric acid using a measuring cylinder and heat in a water bath to a desired temperature
Mix in a conical flask and place on white paper with a cross on it
Time how long it takes for black cross to disappear
How should the experiment to measure temperature affecting reaction rate be changed?
Different temperatures
Why does increasing temperature increase reaction rate?
Increases collision frequency
Makes reactants gain energy so a greater proportion will have more energy than activation energy needed
Why does increasing concentration/ pressure increase rate of reactions?
Higher collision frequency
More particles of the same reactant in the same volume
What do catalysts do?
Increase reaction rates
Why are catalysts used in industrial reactions?
Reduce energy costs
Why are catalysts efficient?
Aren’t chemically changed or used up in reactions
Small amount needed to catalyse large amounts of reactants
What will catalysts not change?
Products
Why do catalysts increase the likelihood of collisions?
Decrease activation energy needed for a reaction to occur
How do catalysts decrease activation energy?
Provide an alternative reaction pathway that has a lower activation energy
What show a reaction’s progress?
Reaction profiles
What happen during chemical reactions?
Energy changes
What does an increase / decrease in temperature depend on?
Which reagents are used in the reaction
What reactions cause changes in heat energy?
Salts dissolving in water
Neutralisation - most exothermic
Displacement - most exothermic
Precipitation - exothermic
Experiment to measure temperature change when dissolving salts in water
Add salt to polystyrene cup of water and measure temperature change when salt has dissolved
Exothermic reactions
Energy from reacting chemicals transferred to surroundings leading to temperature increase
Examples of exothermic reactions
Combustion
Neutralisation
Oxidation
Hand warmers
Endothermic reactions
Energy from surroundings transferred to reacting chemicals, decreasing temperature of the surroundings
Examples of endothermic reactions
Sports injury packs
Thermal decomposition
Citric acid + baking soda
What must be supplied to break bonds? What type of reaction is this?
Energy
Endothermic
What happens to bonds during a chemical reaction?
Old bonds are broken and new bonds are formed
What is released when new bonds are formed? What type of reaction is this?
Energy
Exothermic
Bond energy calculations
Overall energy = energy required to break bonds - energy released by forming bonds
What is overall energy change measured in?
KJ mol-1
Experiment to measure change in temperature of a reaction
Polystyrene cup into large beaker of cotton wood
Add known volume of reagent to cup and measure initial temperature of the solution
Add measured mass of second reagent and stir
Put a lid on cup
Record max/ min temperature that it reaches
Calculate temperature change
Why is a polystyrene cup used? What does it reduce?
Good thermal insulator
How much the external temperature affects the reaction
Why is cotton wool used? What does it reduce?
Gives insulation
Energy transfer to or from reaction mixture
