A chemical mechanism is a theoretical conjecture that tries to
describe in detail what takes place at each stage of an overall chemical
reaction. The detailed steps of a reaction are not observable in most
cases. The conjectured mechanism is chosen because it is
thermodynamically feasible, and has experimental support in isolated
intermediates (see next section) or other quantitative and qualitative
characteristics of the reaction. It also describes each reactive intermediate, activated complex, and transition state,
and which bonds are broken (and in what order), and which bonds are
formed (and in what order). A complete mechanism must also explain the
reason for the reactants and catalyst used, the stereochemistry observed in reactants and products, all products formed and the amount of each.
SN2 reaction mechanism. Note the negatively charged transition state in brackets in which the central carbon atom in question shows five bonds, an unstable condition.
The electron or arrow pushing method is often used in illustrating a reaction mechanism; for example, see the illustration of the mechanism for benzoin condensation in the following examples section.
A reaction mechanism must also account for the order in which
molecules react. Often what appears to be a single-step conversion is in
fact a multistep reaction.
Reaction Intermediates
Reaction
intermediates are chemical species, often unstable and short-lived
(however sometimes can be isolated), which are not reactants or products
of the overall chemical reaction, but are temporary products and/or
reactants in the mechanism's reaction steps. Reaction intermediates are
often free radicals or ions.
The kinetics (relative rates of the reaction steps and the rate equation
for the overall reaction) are explained in terms of the energy needed
for the conversion of the reactants to the proposed transition states
(molecular states that corresponds to maxima on the reaction coordinates, and to saddle points on the potential energy surface for the reaction).
In this case, experiments have determined that this reaction takes place according to the rate law. This form suggests that the rate-determining step is a reaction between two molecules of NO2. A possible mechanism for the overall reaction that explains the rate law is:
2 NO2 → NO3 + NO (slow)
NO3 + CO → NO2 + CO2 (fast)
Each step is called an elementary step, and each has its own rate law and molecularity.
The elementary steps should add up to the original reaction. (Meaning,
if we were to cancel out all the molecules that appear on both sides of
the reaction, we would be left with the original reaction.)
When determining the overall rate law for a reaction, the slowest
step is the step that determines the reaction rate. Because the first
step (in the above reaction) is the slowest step, it is the rate-determining step. Because it involves the collision of two NO2 molecules, it is a bimolecular reaction with a rate law of .
Other reactions may have mechanisms of several consecutive steps. In organic chemistry, the reaction mechanism for the benzoin condensation, put forward in 1903 by A. J. Lapworth, was one of the first proposed reaction mechanisms.
Benzoin condensationreaction mechanism. Cyanide ion (CN−) acts as a catalyst here, entering at the first step and leaving in the last step. Proton (H+) transfers occur at (i) and (ii). The arrow pushing method is used in some of the steps to show where electron pairs go.
A chain reaction is an example of a complex mechanism, in which the propagation steps form a closed cycle.
Other experimental methods to determine mechanism
Many experiments that suggest the possible sequence of steps in a reaction mechanism have been designed, including:
for reactions in solution, measurement of the effect of pressure on
the reaction rate to determine the volume change on formation of the
activated complex
for reactions of ions in solution, measurement of the effect of ionic strength on the reaction rate
A correct reaction mechanism is an important part of accurate predictive modeling. For many combustion and plasma systems, detailed mechanisms are not available or require development.
Even when information is available, identifying and assembling
the relevant data from a variety of sources, reconciling discrepant
values and extrapolating to different conditions can be a difficult
process without expert help. Rate constants or thermochemical data are
often not available in the literature, so computational chemistry techniques or group additivity methods must be used to obtain the required parameters.
Computational chemistry methods can also be used to calculate potential energy surfaces for reactions and determine probable mechanisms.
A reaction step involving one molecular entity is called unimolecular.
A reaction step involving two molecular entities is called bimolecular.
A reaction step involving three molecular entities is called trimolecular or termolecular.
In general, reaction steps involving more than three molecular
entities do not occur, because is statistically improbable in terms of
Maxwell distribution to find such transition state.
Access to health care may vary across countries, communities, and
individuals, largely influenced by social and economic conditions as
well as health policies.
Providing health care services means "the timely use of personal health
services to achieve the best possible health outcomes".
Factors to consider in terms of healthcare access include financial
limitations (such as insurance coverage), geographic barriers (such as
additional transportation costs, possibility to take paid time off of
work to use such services), and personal limitations (lack of ability to
communicate with healthcare providers, poor health literacy, low
income).
Limitations to health care services affects negatively the use of
medical services, efficacy of treatments, and overall outcome
(well-being, mortality rates).
Health care systems are organizations established to meet the health needs of targeted populations. According to the World Health Organization (WHO), a well-functioning health care system requires a financing mechanism, a well-trained and adequately paid workforce, reliable information on which to base decisions and policies, and well maintained health facilities to deliver quality medicines and technologies.
An efficient health care system can contribute to a significant part of a country's economy,
development and industrialization. Health care is conventionally
regarded as an important determinant in promoting the general physical and mental health and well-being of people around the world. An example of this was the worldwide eradication of smallpox in 1980, declared by the WHO as the first disease in human history to be completely eliminated by deliberate health care interventions.
Delivery
Primary care may be provided in community health centers.
While the definitions of the various types of health care vary
depending on the different cultural, political, organizational and
disciplinary perspectives, there appears to be some consensus that
primary care constitutes the first element of a continuing health care
process and may also include the provision of secondary and tertiary
levels of care. Health care can be defined as either public or private.
The emergency room is often a frontline venue for the delivery of primary medical care.
Primary care is often used as the term for the health care
services that play a role in the local community. It can be provided in
different settings, such as Urgent care centers which provide same day appointments or services on a walk-in basis.
Primary care involves the widest scope of health care, including all ages of patients, patients of all socioeconomic and geographic origins, patients seeking to maintain optimal health, and patients with all types of acute and chronic physical, mental and social health issues, including multiple chronic diseases. Consequently, a primary care practitioner must possess a wide breadth of knowledge in many areas. Continuity is a key characteristic of primary care, as patients usually prefer to consult the same practitioner for routine check-ups and preventive care, health education, and every time they require an initial consultation about a new health problem. The International Classification of Primary Care
(ICPC) is a standardized tool for understanding and analyzing
information on interventions in primary care based on the reason for the
patient's visit.
Common chronic illnesses usually treated in primary care may include, for example: hypertension, diabetes, asthma, COPD, depression and anxiety, back pain, arthritis or thyroid dysfunction. Primary care also includes many basic maternal and child health care services, such as family planning services and vaccinations. In the United States, the 2013 National Health Interview Survey
found that skin disorders (42.7%), osteoarthritis and joint disorders
(33.6%), back problems (23.9%), disorders of lipid metabolism (22.4%),
and upper respiratory tract disease (22.1%, excluding asthma) were the
most common reasons for accessing a physician.
In the United States, primary care physicians have begun to
deliver primary care outside of the managed care (insurance-billing)
system through direct primary care which is a subset of the more familiar concierge medicine.
Physicians in this model bill patients directly for services, either on
a pre-paid monthly, quarterly, or annual basis, or bill for each
service in the office. Examples of direct primary care practices include
Foundation Health in Colorado and Qliance in Washington.
In context of global population aging, with increasing numbers of older adults at greater risk of chronic non-communicable diseases, rapidly increasing demand for primary care services is expected in both developed and developing countries. The World Health Organization attributes the provision of essential primary care as an integral component of an inclusive primary health care strategy.
Secondary care
Secondary care includes acute care:
necessary treatment for a short period of time for a brief but serious
illness, injury, or other health condition. This care is often found in a
hospitalemergency department. Secondary care also includes skilled attendance during childbirth, intensive care, and medical imaging services.
The term "secondary care" is sometimes used synonymously with "hospital care". However, many secondary care providers, such as psychiatrists, clinical psychologists, occupational therapists, most dental specialties or physiotherapists,
do not necessarily work in hospitals. Some primary care services are
delivered within hospitals. Depending on the organization and policies
of the national health system, patients may be required to see a primary
care provider for a referral before they can access secondary care.
In countries which operate under a mixed market health care system, some physicians
limit their practice to secondary care by requiring patients to see a
primary care provider first. This restriction may be imposed under the
terms of the payment agreements in private or group health insurance
plans. In other cases, medical specialists may see patients without a
referral, and patients may decide whether self-referral is preferred.
In other countries patient self-referral to a medical specialist
for secondary care is rare as prior referral from another physician
(either a primary care physician or another specialist) is considered
necessary, regardless of whether the funding is from private insurance schemes or national health insurance.
Tertiary care is specialized consultative health care, usually for inpatients and on referral from a primary or secondary health professional, in a facility that has personnel and facilities for advanced medical investigation and treatment, such as a tertiary referral hospital.
The term quaternary care is sometimes used as an extension of tertiary care in reference to advanced levels of medicine which are highly specialized and not widely accessed. Experimental medicine and some types of uncommon diagnostic or surgical
procedures are considered quaternary care. These services are usually
only offered in a limited number of regional or national health care
centers. Quaternary care is more prevalent in the United Kingdom.
Home and community care
Many types of health care interventions are delivered outside of health facilities. They include many interventions of public health interest, such as food safety surveillance, distribution of condoms and needle-exchange programs for the prevention of transmissible diseases.
Many countries, especially in the west, are dealing with aging
populations, so one of the priorities of the health care system is to
help seniors live full, independent lives in the comfort of their own
homes. There is an entire section of health care geared to providing
seniors with help in day-to-day activities at home such as
transportation to and from doctor's appointments along with many other
activities that are essential for their health and well-being. Although
they provide home care for older adults in cooperation, family members
and care workers may harbor diverging attitudes and values towards their
joint efforts. This state of affairs presents a challenge for the
design of ICT (information and communication technology) for home care.
Because statistics show that over 80 million Americans have taken time off of their primary employment to care for a loved one,
many countries have begun offering programs such as Consumer Directed
Personal Assistant Program to allow family members to take care of their
loved ones without giving up their entire income.
With obesity in children rapidly becoming a major concern, health
services often set up programs in schools aimed at educating children
about nutritional eating habits, making physical education a requirement
and teaching young adolescents to have positive self-image.
Ratings
Health care ratings are ratings or evaluations
of health care used to evaluate the process of care and health care
structures and/or outcomes of health care services. This information is
translated into report cards that are generated by quality
organizations, nonprofit, consumer groups and media. This evaluation of
quality is based on measures of:
Health
care extends beyond the delivery of services to patients, encompassing
many related sectors, and is set within a bigger picture of financing
and governance structures.
Health system
A health system, also sometimes referred to as health care system or healthcare system is the organization of people, institutions, and resources that deliver health care services to populations in need.
Health care industry
A group of Chilean 'Damas de Rojo' volunteering at their local hospital
The health care industry
incorporates several sectors that are dedicated to providing health
care services and products. As a basic framework for defining the
sector, the United Nations' International Standard Industrial Classification
categorizes health care as generally consisting of hospital activities,
medical and dental practice activities, and "other human health
activities." The last class involves activities of, or under the
supervision of, nurses, midwives, physiotherapists, scientific or
diagnostic laboratories, pathology clinics, residential health
facilities, patient advocates or other allied health professions.
For example, pharmaceuticals and other medical devices are the leading high technology exports of Europe and the United States. The United States dominates the biopharmaceutical field, accounting for three-quarters of the world's biotechnology revenues.
Health services research can lead to greater efficiency and equitable delivery of health care interventions, as advanced through the social model of health and disability, which emphasizes the societal changes that can be made to make populations healthier. Results from health services research often form the basis of evidence-based policy in health care systems. Health services research
is also aided by initiatives in the field of artificial intelligence
for the development of systems of health assessment that are clinically
useful, timely, sensitive to change, culturally sensitive, low burden,
low cost, built into standard procedures, and involve the patient.
In most countries there is a mix of all five models, but this varies
across countries and over time within countries. Aside from financing
mechanisms, an important question should always be how much to spend on
healthcare. For the purposes of comparison, this is often expressed as
the percentage of GDP spent on healthcare. In OECD countries for every extra $1000 spent on healthcare, life expectancy falls by 0.4 years. A similar correlation is seen from analysis carried out each year by Bloomberg.
Clearly this kind of analysis is flawed in that life expectancy is only
one measure of a health system's performance, but equally, the notion
that more funding is better is not supported.
In 2011, the health care industry consumed an average of 9.3 percent of the GDP or US$ 3,322 (PPP-adjusted) per capita across the 34 members of OECD countries. The US (17.7%, or US$ PPP 8,508), the Netherlands (11.9%, 5,099), France (11.6%, 4,118), Germany (11.3%, 4,495), Canada (11.2%, 5669), and Switzerland (11%, 5,634) were the top spenders, however life expectancy in total population at birth was highest in Switzerland (82.8 years), Japan and Italy (82.7), Spain and Iceland (82.4), France (82.2) and Australia
(82.0), while OECD's average exceeds 80 years for the first time ever
in 2011: 80.1 years, a gain of 10 years since 1970. The US (78.7 years)
ranges only on place 26 among the 34 OECD member countries, but has the
highest costs by far. All OECD countries have achieved universal (or
almost universal) health coverage, except the US and Mexico.
In the United States, where around 18% of GDP is spent on health care, the Commonwealth Fund analysis of spend and quality shows a clear correlation between worse quality and higher spending.
Health information technology (HIT) is "the application of
information processing involving both computer hardware and software
that deals with the storage, retrieval, sharing, and use of health care
information, data, and knowledge for communication and decision making."
Health information technology components:
Electronic Health Record (EHR) - An EHR contains a patient's comprehensive medical history, and may include records from multiple providers.
Electronic Medical Record (EMR) - An EMR contains the standard medical and clinical data gathered in one's provider’s office.
Personal Health Record (PHR) - A PHR is a patient's medical history that is maintained privately, for personal use.
Medical Practice Management
software (MPM) - is designed to streamline the day-to-day tasks of
operating a medical facility. Also known as practice management software
or practice management system (PMS).
Health Information Exchange
(HIE) - Health Information Exchange allows health care professionals
and patients to appropriately access and securely share a patient’s
vital medical information electronically.
In chemical kinetics, the overall rate of a reaction is often approximately determined by the slowest step, known as the rate-determining step (RDS) or rate-limiting step. For a given reaction mechanism, the prediction of the corresponding rate equation (for comparison with the experimental rate law) is often simplified by using this approximation of the rate-determining step.
In principle, the time evolution of the reactant and product
concentrations can be determined from the set of simultaneous rate
equations for the individual steps of the mechanism, one for each step.
However, the analytical solution of these differential equations is not always easy, and in some cases numerical integration may even be required.
The hypothesis of a single rate-determining step can greatly simplify
the mathematics. In the simplest case the initial step is the slowest,
and the overall rate is just the rate of the first step.
Also, the rate equations for mechanisms with a single
rate-determining step are usually in a simple mathematical form, whose
relation to the mechanism and choice of rate-determining step is clear.
The correct rate-determining step can be identified by predicting the
rate law for each possible choice and comparing the different
predictions with the experimental law, as for the example of NO 2 and CO below.
The concept of the rate-determining step is very important to the
optimization and understanding of many chemical processes such as catalysis and combustion.
Example reaction: NO 2 + CO
As an example, consider the gas-phase reaction NO 2 + CO → NO + CO 2. If this reaction occurred in a single step, its reaction rate (r) would be proportional to the rate of collisions between NO 2 and CO molecules: r = k[NO 2 ][CO], where k is the reaction rate constant, and square brackets indicate a molar concentration. Another typical example is the Zel'dovich mechanism.
First step rate-determining
In fact, however, the observed reaction rate is second-order in NO 2 and zero-order in CO, with rate equation r = k[NO 2 ]2. This suggests that the rate is determined by a step in which two NO 2
molecules react, with the CO molecule entering at another, faster,
step. A possible mechanism in two elementary steps that explains the
rate equation is:
NO 2 + NO 2 → NO + NO 3 (slow step, rate-determining)
NO 3 + CO → NO 2 + CO 2(fast step)
In this mechanism the reactive intermediate species NO 3 is formed in the first step with rate r1 and reacts with CO in the second step with rate r2. However NO 3 can also react with NO if the first step occurs in the reverse direction (NO + NO 3 → 2 NO 2 ) with rate r−1, where the minus sign indicates the rate of a reverse reaction.
The concentration of a reactive intermediate such as [NO 3 ] remains low and almost constant. It may therefore be estimated by the steady-state
approximation, which specifies that the rate at which it is formed
equals the (total) rate at which it is consumed. In this example NO 3 is formed in one step and reacts in two, so that
The statement that the first step is the slow step actually means that the first step in the reverse direction is slower than the second step in the forward direction, so that almost all NO 3 is consumed by reaction with CO and not with NO. That is, r−1 ≪ r2, so that r1 − r2 ≈ 0. But the overall rate of reaction is the rate of formation of final product (here CO 2), so that r = r2 ≈ r1.
That is, the overall rate is determined by the rate of the first step,
and (almost) all molecules that react at the first step continue to the
fast second step.
Pre-equilibrium: if the second step were rate-determining
The
other possible case would be that the second step is slow and
rate-determining, meaning that it is slower than the first step in the
reverse direction: r2 ≪ r−1. In this hypothesis, r1 − r−1 ≈ 0, so that the first step is (almost) at equilibrium. The overall rate is determined by the second step: r = r2 ≪ r1,
as very few molecules that react at the first step continue to the
second step, which is much slower. Such a situation in which an
intermediate (here NO 3 ) forms an equilibrium with reactants prior to the rate-determining step is described as a pre-equilibrium For the reaction of NO 2 and CO, this hypothesis can be rejected, since it implies a rate equation that disagrees with experiment.
If the first step were at equilibrium, then its equilibrium constant expression permits calculation of the concentration of the intermediate NO 3 in terms of more stable (and more easily measured) reactant and product species:
The overall reaction rate would then be
which disagrees with the experimental rate law given above, and so
disproves the hypothesis that the second step is rate-determining for
this reaction. However, some other reactions are believed to involve
rapid pre-equilibria prior to the rate-determining step, as shown below.
Nucleophilic attack by one water molecule R+ + OH− → ROH.
This reaction is found to be first-order with r = k[R−Br], which indicates that the first step is slow and determines the rate. The second step with OH− is much faster, so the overall rate is independent of the concentration of OH−.
A
useful rule in the determination of mechanism is that the concentration
factors in the rate law indicate the composition and charge of the activated complex or transition state. For the NO 2 –CO reaction above, the rate depends on [NO 2 ]2, so that the activated complex has composition N 2O 4, with 2 NO 2 entering the reaction before the transition state, and CO reacting after the transition state.
A multistep example is the reaction between oxalic acid and chlorine in aqueous solution: H 2C 2O 4 + Cl 2 → 2 CO 2 + 2 H+ + 2 Cl−.
The observed rate law is
which implies an activated complex in which the reactants lose 2H+ + Cl− before the rate-determining step. The formula of the activated complex is Cl 2 + H 2C 2O 4 − 2 H+ − Cl− + xH 2O, or C 2O 4Cl(H 2O)– x (an unknown number of water molecules are added because the possible dependence of the reaction rate on H 2O was not studied, since the data were obtained in water solvent at a large and essentially unvarying concentration).
One possible mechanism in which the preliminary steps are assumed
to be rapid pre-equilibria occurring prior to the transition state is
In a multistep reaction, the rate-determining step does not necessarily correspond to the highest Gibbs energy on the reaction coordinate diagram. If there is a reaction intermediate whose energy is lower than the initial reactants, then the activation energy needed to pass through any subsequent transition state
depends on the Gibbs energy of that state relative to the lower-energy
intermediate. The rate-determining step is then the step with the
largest Gibbs energy difference relative either to the starting material
or to any previous intermediate on the diagram.
Also, for reaction steps that are not first-order, concentration terms must be considered in choosing the rate-determining step.
Chain reactions
Not all reactions have a single rate-determining step. In particular, the rate of a chain reaction is usually not controlled by any single step.
Diffusion control
In
the previous examples the rate determining step was one of the
sequential chemical reactions leading to a product. The rate-determining
step can also be the transport of reactants to where they can interact
and form the product. This case is referred to as diffusion control
and, in general, occurs when the formation of product from the
activated complex is very rapid and thus the provision of the supply of
reactants is rate-determining.
![r=k[NO_{2}]^{2}](https://wikimedia.org/api/rest_v1/media/math/render/svg/7c34576960b3342cceed6c0b51b1764beb2dfcf4)
![{\displaystyle {\frac {d{\ce {[NO3]}}}{dt}}=r_{1}-r_{2}-r_{-1}\approx 0.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/7ecf92288f5a82cd3e1109f4bcf95c175b0304d5)
![{\displaystyle K_{1}={\frac {{\ce {[NO][NO3]}}}{{\ce {[NO2]^2}}}},}](https://wikimedia.org/api/rest_v1/media/math/render/svg/80999e5d22d393cfb0b436e351a14dbcc7ea1ffe)
![{\displaystyle [{\ce {NO3}}]=K_{1}{\frac {{\ce {[NO2]^2}}}{{\ce {[NO]}}}}.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/657b6abb8fc1acb20fbcda4fd85dd5d01cc84657)
![{\displaystyle r=r_{2}=k_{2}{\ce {[NO3][CO]}}=k_{2}K_{1}{\frac {{\ce {[NO2]^2 [CO]}}}{{\ce {[NO]}}}},}](https://wikimedia.org/api/rest_v1/media/math/render/svg/469e8efed24cec0fda01438c293b926abf7ea94d)
![{\displaystyle v=k{\frac {{\ce {[Cl2][H2C2O4]}}}{[{\ce {H+}}]^{2}[{\ce {Cl^-}}]}},}](https://wikimedia.org/api/rest_v1/media/math/render/svg/f393df90aea1ad148e41b5c616546f5204a71e28)
