SEM 2 Chemical reactions Exam revision Flashcards
What is a subscript?
Subscripts tell you how many of the atoms are bonded in the compound. The subscript only applies to the element written just before it.
What is a co-efficient?
A co-efficient is a number in front of a chemical formula, that acts as a multiplier. Everything after that co-efficient is multiplied by the co-efficient.
Explain chemical formulas with parenthesis.
The subscript outside the parenthesis, tells you to multiply the elements in the parenthesis
what are the signs of a chemical reaction?
There are six signs that a chemical reaction may have occurred:–
a gas may be released;
a colour change may occur;
An odour is produced;
A temperature change may occur (either hotter or colder);
Light and/or sound may be produced; and,
A precipitate (often seen as cloudiness) may be formed (a precipitate is a solids that forms when two liquids are mixed).
Law of conservation of mass
“Law of conservation of mass” states that matter can be created nor destroyed in a chemical reaction only rearranged. The mass of the reactants equals the mass of the products.
why do we balance chemical equations?
When we write a chemical equation for a reaction, we need to balance the number of atoms on the reactant and product sides to ensure the conservation of mass.
What is a combustion reaction?
Reactions in which a substance reacts with oxygen and heat is released (exothermic).
E.g. CH4(g) + O2 (g) 2H2O (g) + CO2(g) + energy
complete combustion def
In complete combustion (when a lot of oxygen is available), the typical reaction is:
hydrocarbon + oxygen→carbon dioxide + water
incomplete combustion reaction
Incomplete combustionhappens when there is not enoughoxygento createcarbon dioxide.
In these cases, carbon and carbon monoxide are created instead (or as well as CO2).
What is an acid?
Have pH’s below 7 and the lower the pH the more acidic the solution. An acid is a molecule that is able to lose or donate a hydrogen cation.
What is a base?
Have pH’s above 7 and the higher the pH the more alkaline the solution.A base is a molecule that is able to gain or accept a hydrogen cation.Have pH’s above 7 and the higher the pH the more alkaline the solution.
what is the Ph scale?
The “pH scale” is used to describe how acidic or basic a substance is.
The pH represents the concentration of H+ ions in the solution.
pH = 7: neutral (e.g. pure water – neither acid nor base)
pH < 7: acid (the lower the pH, the stronger the acid)
pH > 7: basic (or alkaline) (the higher the pH, the stronger the base)
What are 3 different ways to measure the Ph?
There are many different ways that we can determine if something is acidic or basic.
Litmus paper – red for acids, blue for bases
pH meter – measure precise pH values
Indicators – an indicator changes colour in
weak base vs a strong base
A weak base is when Only a few hydrogen ions are accepted by the base molecules. eg. ammonia NH3
A strong base is where Almost all hydrogen ions accepted by the base molecules. eg Sodium hydroxide NaOH, potassium hydroxide KOH.
What is a neutralization reaction?
Neutralization is a chemical reaction between an acid and a base.
It is another type of double displacement (or double replacement) reaction.
This reaction produces water and a “salt” (or metal salt), which is an ionic compound.
Strong acid vs weak acid?
A strong acid is where Almost all acid molecules donate their hydrogen ions. Nitric acid HNO3, Sulphuric acid H2SO4
A weak acid is where Only a few acid molecules donate their hydrogen ions. Carbon acid H2CO3
Strong acid vs weak acid?
A strong acid is where Almost all acid molecules donate their hydrogen ions.
A weak acid is where Only a few acid molecules donate their hydrogen ions.
What are some uses of acids?
Some uses of acids including the role in digestion in animals and antacids to help indigestion.
What is acid rain?
Normal rain has pH of 5.6. Rain is considered to be acidic if the pH is ≤ 5.2.
Acid rain is formed when the sulfur dioxide (SO2) and nitrogen oxides (NOx) react with water, oxygen and other chemicals in the air.
These chemical are released into the air by fossil-fuel power plants, vehicles and oil refineries
Effects of acid rain
It’s not just plants and animals though that are affected. Acid rain can ruin buildings, bridges, roads and statues by stripping away the material and corroding metal that makes up these structures.
At higher elevations it is also damaging to forests and inhibit them from growing. This affects a range of species in an ecosystem.
two things required for a chemical reaction to occur?
break the bonds between the reactant atoms
form new bonds between atoms in the products
what is enthalpy?
Enthalpy (or the heat content of a system) is the energy required for a substance (the reactants) to undergo a change.
Enthalpy is shown by the symbol ΔH.
Δ means “difference” or “change in”.
Enthalpy is calculated by:
Products minus reactants
What is an endothermic reaction?
Endothermic chemical reactions are reactions that absorb energy.
Eg. Photosynthesis
What is an exothermic reaction?
Exothermic reactions release heat/energy to the surroundings. Eg. Cellular respiration
Is the enthalpy positive or negative in a exothermic reaction?
For exothermic reactions the enthalpy will always be negative because the reaction is producing heat energy.
Is the enthalpy positive or negative in a exothermic reaction?
For exothermic reactions the enthalpy will always be negative because the reaction is producing heat energy.
Is the enthalpy positive or negative in a endothermic reaction?
For endothermic reactions the enthalpy will always be positive because the reaction is using heat energy.
Incomplete combustion can make a flame appear sooty. Suggest what is making the flame appear sooty and explain your decision.
Incomplete combustion happens when there is not enough oxygen available to react resulting in the formation of carbon (soot) and or carbon monoxide gas.
Do all chemical reactions absorb energy?
So ALL reactions will initially absorb energy to break the bonds and activate the reaction.
Explain why an air hole on a bunsen burner produces a hotter flame.
opening the airhole allows the maximum amount of oxygen in to ensure the fuel gas burns.
what is the cellular respiration equation?
glucose + oxygen > carbon dioxide + water
what is the balanced chemical equation for a aerobic cellular respiration equation?
C6H1206 + 602 > 6C02 +6H20
